Forums

Cute dpot hack for increased resolution

Started by Phil Hobbs July 22, 2020
On Friday, July 24, 2020 at 4:33:04 PM UTC+10, Phil Hobbs wrote:
> On 2020-07-23 22:16, Chris Jones wrote: > > On 24/07/2020 09:14, Phil Hobbs wrote: > >> On 2020-07-23 16:25, Ricketty C wrote: > >>> On Thursday, July 23, 2020 at 7:39:36 AM UTC-4, pcdh...@gmail.com > >>> wrote:
<snip>
> > By the way, I don't know if you've seen this, but google and skywater > > fab are open-sourcing the PDK for a 0.13um CMOS process, and google is > > paying for some free shuttle runs, subject to certain rules including > > that all designs be open-sourced too. > > > > If you can think of anything that you really wish you could get on a > > chip, and that can be made in CMOS, and that is useful to you but not > > secret enough to not want to publish the design, now is the time! > > Shuttle run in November and you get hundreds of parts back, so maybe > > enough for a low-volume product. Lots of people are playing with the > > tools so quite likely someone else would lay it out for fun if you > > didn't have time. > > > > There is a link to the slack workspace here, with many more details: > > > > https://groups.google.com/forum/#!topic/skywater-pdk-announce/zijPNEsqsx4 > > > > > > Sounds like a good opportunity for sure. If somebody would do that with > a decent low-noise analogue bipolar process (say 0.5 um) I'd be all over it.
Analog Devices has a gorgeous 20-stage process which even delivers good quality PNP bipolar transistors. The chances of getting open-source access to that can't be good, but I suppose we could dream. -- Bill Sloman, Sydney
On 24/07/2020 16:48, Bill Sloman wrote:
> On Friday, July 24, 2020 at 4:33:04 PM UTC+10, Phil Hobbs wrote: >> On 2020-07-23 22:16, Chris Jones wrote: >>> On 24/07/2020 09:14, Phil Hobbs wrote: >>>> On 2020-07-23 16:25, Ricketty C wrote: >>>>> On Thursday, July 23, 2020 at 7:39:36 AM UTC-4, pcdh...@gmail.com >>>>> wrote: > > <snip> > >>> By the way, I don't know if you've seen this, but google and skywater >>> fab are open-sourcing the PDK for a 0.13um CMOS process, and google is >>> paying for some free shuttle runs, subject to certain rules including >>> that all designs be open-sourced too. >>> >>> If you can think of anything that you really wish you could get on a >>> chip, and that can be made in CMOS, and that is useful to you but not >>> secret enough to not want to publish the design, now is the time! >>> Shuttle run in November and you get hundreds of parts back, so maybe >>> enough for a low-volume product. Lots of people are playing with the >>> tools so quite likely someone else would lay it out for fun if you >>> didn't have time. >>> >>> There is a link to the slack workspace here, with many more details: >>> >>> https://groups.google.com/forum/#!topic/skywater-pdk-announce/zijPNEsqsx4 >>> >>> >> >> Sounds like a good opportunity for sure. If somebody would do that with >> a decent low-noise analogue bipolar process (say 0.5 um) I'd be all over it. > > Analog Devices has a gorgeous 20-stage process which even delivers good quality PNP bipolar transistors. The chances of getting open-source access to that can't be good, but I suppose we could dream. >
It also has low tempco metal film resistors iirc, whereas polysilicon resistors are nothing like that. When I worked for them I never got to use it because the stuff I did had to be cheap to make so was on more common processes. But, it is surprising what can be achieved in bog standard CMOS with sufficient effort.
On 24/07/2020 16:32, Phil Hobbs wrote:
> On 2020-07-23 22:16, Chris Jones wrote: >> On 24/07/2020 09:14, Phil Hobbs wrote: >>> On 2020-07-23 16:25, Ricketty C wrote: >>>> On Thursday, July 23, 2020 at 7:39:36 AM UTC-4, pcdh...@gmail.com >>>> wrote: >>>>>> I guess I'm not following what this gains?&Acirc;&nbsp; You get lots more >>>>>> steps, but >you don't know any more about where they are.&Acirc;&nbsp; What >>>>>> am I missing? >>>>> >>>>> What it gets me is a combination of settability and bandwidth that >>>>> I can't get in any single part at any price. I'll dump in a bit of >>>>> pseudorandom DNL and see if that causes any issues. I don't expect >>>>> it to--it'll get homogenized much like the regular steps. >>>>> >>>>> For an iterative adjustment (think LC filter tuning) I can get >>>>> close by setting the fine pot to 0, adjusting the coarse pot, then >>>>> walking the two together to find the optimum. Near full scale the >>>>> possible search range is wider, but I don't think it'll take too >>>>> long provided that I'm willing to settle for "good enough" rather >>>>> than a global optimum. >>>>> >>>>> There are two mildly-interacting adjustments, but the response >>>>> surface is simple--no local minima. >>>>> >>>>> It's for taking out the effect of extrinsic emitter resistance in >>>>> the BJT diff pair that's the heart of the circuit. It works by >>>>> applying a signal to one of the bases that cancels out the >>>>> effective emitter degeneration and so restores the desired >>>>> Ebers-Moll behaviour at higher photocurrents. >>>>> >>>>> If I feel like getting fancy, once I've found the right >>>>> neighbourhood I can give the turd a final polish to get a bit >>>>> closer. ;) >>>>> >>>>> There are non-negligible effects such as the tempco of wiper >>>>> resistance that limit how good this approach can be. Sure beats a >>>>> trimpot on a motor though! >>> >>>> >>>> Ok, so this circuit is not a real time control loop as much as it is >>>> like a successive approximation.&Acirc;&nbsp; You don't mind the value varying >>>> unpredictably as long as it eventually reaches a setting. >>>> >>> >>> Right.&Acirc;&nbsp; It'll be done only occasionally, since R_E of a BJT is a weak >>> function of everything.&Acirc;&nbsp; In a diff pair, R_E produces degeneration, >>> which tends to make the current split by the ratios of the R_E >>> values, whereas we want it to do the Ebers-Moll thing and split >>> strictly as exp(delta V_BE/kT):1.&Acirc;&nbsp; R_E degeneration generally >>> dominates the error budget at higher photocurrents. >>> >>> If transistors Q1 and Q2 have resistances R_E1 and R_E2, then the >>> required voltage is >>> >>> dV_BEcor = I_E2 * R_E2&Acirc;&nbsp; - I_E1 * R_E1. >>> >>> Since V_B1 is controlled by a feedback loop trying to make the total >>> DC current from two or more photodiodes cancel out, the correction >>> voltage gets applied to the base of Q2.&Acirc;&nbsp; Since the resistances are >>> nearly constant, rapid adjustment of the correction law is not required. >>> >>> However, the unwanted degeneration applies at AC as well as DC, and >>> the attainable benefit is limited by phase shift and amplitude error >>> in the correction voltage.&Acirc;&nbsp; Thus&Acirc;&nbsp; I do need to form the correction in >>> analogue with as high a bandwidth as reasonably possible. >>> >>> There'll be a cal table in flash, which will probably rely on the >>> AD5273's decent DNL to interpolate relatively coarsely between >>> steps--aiming for, say, 8-bit accuracy.&Acirc;&nbsp; That's potentially enough to >>> reduce the degeneration effect by 50 dB in the spherical cow universe. >>> >>> Then there'll be a magic pushbutton on the box so that users in lab >>> settings can tell it that their experiment is all set up, so please >>> optimize the noise cancellation for these exact conditions.&Acirc;&nbsp; Nobody >>> will mind if that takes 5 seconds or so.&Acirc;&nbsp; (The command will be >>> available electronically as well--USB serial or maybe a dedicated >>> control line.) >>> >>> The magic BFP640, with its ~1-kV Early voltage, excellent saturation >>> behaviour, and ultralow C_CB, allows all sorts of cute tricks. I'm >>> dorking V_CE on one side of the diff pair in real time to force the >>> power dissipation of Q1 and Q2 to be equal irrespective of the >>> splitting ratio (within limits). >>> >>> With some routed slots to control local temperature gradients, it >>> looks like I can get performance equivalent to a 40-GHz monolithic >>> dual. (We'll see how spherical the cows are in real life--I've seen >>> some pretty fat ones.) ;) >>> >>> Fun. >>> >>> Cheers >>> >>> Phil Hobbs >>> >>> (Who's stocking up on BFP640s as he did on BF862s) >>> >> >> By the way, I don't know if you've seen this, but google and skywater >> fab are open-sourcing the PDK for a 0.13um CMOS process, and google is >> paying for some free shuttle runs, subject to certain rules including >> that all designs be open-sourced too. >> >> If you can think of anything that you really wish you could get on a >> chip, and that can be made in CMOS, and that is useful to you but not >> secret enough to not want to publish the design, now is the time! >> Shuttle run in November and you get hundreds of parts back, so maybe >> enough for a low-volume product. Lots of people are playing with the >> tools so quite likely someone else would lay it out for fun if you >> didn't have time. >> >> There is a link to the slack workspace here, with many more details: >> >> https://groups.google.com/forum/#!topic/skywater-pdk-announce/zijPNEsqsx4 >> >> > > Sounds like a good opportunity for sure.&Acirc;&nbsp; If somebody would do that with > a decent low-noise analogue bipolar process (say 0.5 um) I'd be all over > it. > > Cheers > > Phil Hobbs >
I'd welcome ideas anyway. Even if there is only a small chance of it working in CMOS, it might be worth trying, maybe even just on the corner of someone else's chip. I'm mostly interested in trying something that takes little effort but that could not be replicated easily using off-the-shelf parts. Maybe a trigger comparator + adjustable delay + sampler with several channels, for a sampling scope frontend. Even just a beefy inverter in 0.13um might make a nice fast edge generator for testing scope risetime, especially as it'll be a flipchip chipscale package thing so no bondwires. If there is a usable bipolar then I might do a bandgap reference / LDO and see what noise performance I can get from burning a few mA rather than the few uA that they usually allow themselves. I quite likely won't have time though.
On 2020-07-24 07:43, Chris Jones wrote:
> On 24/07/2020 16:32, Phil Hobbs wrote: >> On 2020-07-23 22:16, Chris Jones wrote: >>> On 24/07/2020 09:14, Phil Hobbs wrote: >>>> On 2020-07-23 16:25, Ricketty C wrote: >>>>> On Thursday, July 23, 2020 at 7:39:36 AM UTC-4, pcdh...@gmail.com >>>>> wrote: >>>>>>> I guess I'm not following what this gains?&Acirc;&nbsp; You get lots more >>>>>>> steps, but >you don't know any more about where they are.&Acirc;&nbsp; What >>>>>>> am I missing? >>>>>> >>>>>> What it gets me is a combination of settability and bandwidth that >>>>>> I can't get in any single part at any price. I'll dump in a bit of >>>>>> pseudorandom DNL and see if that causes any issues. I don't expect >>>>>> it to--it'll get homogenized much like the regular steps. >>>>>> >>>>>> For an iterative adjustment (think LC filter tuning) I can get >>>>>> close by setting the fine pot to 0, adjusting the coarse pot, then >>>>>> walking the two together to find the optimum. Near full scale the >>>>>> possible search range is wider, but I don't think it'll take too >>>>>> long provided that I'm willing to settle for "good enough" rather >>>>>> than a global optimum. >>>>>> >>>>>> There are two mildly-interacting adjustments, but the response >>>>>> surface is simple--no local minima. >>>>>> >>>>>> It's for taking out the effect of extrinsic emitter resistance in >>>>>> the BJT diff pair that's the heart of the circuit. It works by >>>>>> applying a signal to one of the bases that cancels out the >>>>>> effective emitter degeneration and so restores the desired >>>>>> Ebers-Moll behaviour at higher photocurrents. >>>>>> >>>>>> If I feel like getting fancy, once I've found the right >>>>>> neighbourhood I can give the turd a final polish to get a bit >>>>>> closer. ;) >>>>>> >>>>>> There are non-negligible effects such as the tempco of wiper >>>>>> resistance that limit how good this approach can be. Sure beats a >>>>>> trimpot on a motor though! >>>> >>>>> >>>>> Ok, so this circuit is not a real time control loop as much as it is >>>>> like a successive approximation.&Acirc;&nbsp; You don't mind the value varying >>>>> unpredictably as long as it eventually reaches a setting. >>>>> >>>> >>>> Right.&Acirc;&nbsp; It'll be done only occasionally, since R_E of a BJT is a >>>> weak function of everything.&Acirc;&nbsp; In a diff pair, R_E produces >>>> degeneration, which tends to make the current split by the ratios of >>>> the R_E values, whereas we want it to do the Ebers-Moll thing and >>>> split strictly as exp(delta V_BE/kT):1.&Acirc;&nbsp; R_E degeneration generally >>>> dominates the error budget at higher photocurrents. >>>> >>>> If transistors Q1 and Q2 have resistances R_E1 and R_E2, then the >>>> required voltage is >>>> >>>> dV_BEcor = I_E2 * R_E2&Acirc;&nbsp; - I_E1 * R_E1. >>>> >>>> Since V_B1 is controlled by a feedback loop trying to make the total >>>> DC current from two or more photodiodes cancel out, the correction >>>> voltage gets applied to the base of Q2.&Acirc;&nbsp; Since the resistances are >>>> nearly constant, rapid adjustment of the correction law is not >>>> required. >>>> >>>> However, the unwanted degeneration applies at AC as well as DC, and >>>> the attainable benefit is limited by phase shift and amplitude error >>>> in the correction voltage.&Acirc;&nbsp; Thus&Acirc;&nbsp; I do need to form the correction >>>> in analogue with as high a bandwidth as reasonably possible. >>>> >>>> There'll be a cal table in flash, which will probably rely on the >>>> AD5273's decent DNL to interpolate relatively coarsely between >>>> steps--aiming for, say, 8-bit accuracy.&Acirc;&nbsp; That's potentially enough >>>> to reduce the degeneration effect by 50 dB in the spherical cow >>>> universe. >>>> >>>> Then there'll be a magic pushbutton on the box so that users in lab >>>> settings can tell it that their experiment is all set up, so please >>>> optimize the noise cancellation for these exact conditions.&Acirc;&nbsp; Nobody >>>> will mind if that takes 5 seconds or so.&Acirc;&nbsp; (The command will be >>>> available electronically as well--USB serial or maybe a dedicated >>>> control line.) >>>> >>>> The magic BFP640, with its ~1-kV Early voltage, excellent saturation >>>> behaviour, and ultralow C_CB, allows all sorts of cute tricks. I'm >>>> dorking V_CE on one side of the diff pair in real time to force the >>>> power dissipation of Q1 and Q2 to be equal irrespective of the >>>> splitting ratio (within limits). >>>> >>>> With some routed slots to control local temperature gradients, it >>>> looks like I can get performance equivalent to a 40-GHz monolithic >>>> dual. (We'll see how spherical the cows are in real life--I've seen >>>> some pretty fat ones.) ;) >>>> >>>> Fun. >>>> >>>> Cheers >>>> >>>> Phil Hobbs >>>> >>>> (Who's stocking up on BFP640s as he did on BF862s) >>>> >>> >>> By the way, I don't know if you've seen this, but google and skywater >>> fab are open-sourcing the PDK for a 0.13um CMOS process, and google >>> is paying for some free shuttle runs, subject to certain rules >>> including that all designs be open-sourced too. >>> >>> If you can think of anything that you really wish you could get on a >>> chip, and that can be made in CMOS, and that is useful to you but not >>> secret enough to not want to publish the design, now is the time! >>> Shuttle run in November and you get hundreds of parts back, so maybe >>> enough for a low-volume product. Lots of people are playing with the >>> tools so quite likely someone else would lay it out for fun if you >>> didn't have time. >>> >>> There is a link to the slack workspace here, with many more details: >>> >>> https://groups.google.com/forum/#!topic/skywater-pdk-announce/zijPNEsqsx4 >>> >>> >>> >> >> Sounds like a good opportunity for sure.&Acirc;&nbsp; If somebody would do that >> with a decent low-noise analogue bipolar process (say 0.5 um) I'd be >> all over it. >> >> Cheers >> >> Phil Hobbs >> > > > I'd welcome ideas anyway. Even if there is only a small chance of it > working in CMOS, it might be worth trying, maybe even just on the corner > of someone else's chip. > > I'm mostly interested in trying something that takes little effort but > that could not be replicated easily using off-the-shelf parts. Maybe a > trigger comparator + adjustable delay + sampler with several channels, > for a sampling scope frontend. Even just a beefy inverter in 0.13um > might make a nice fast edge generator for testing scope risetime, > especially as it'll be a flipchip chipscale package thing so no > bondwires. If there is a usable bipolar then I might do a bandgap > reference / LDO and see what noise performance I can get from burning a > few mA rather than the few uA that they usually allow themselves. I > quite likely won't have time though. >
Well, a high-bandwidth dpot/attenuator comes to mind. Something around 1-2k and 8 bits, maybe an R-2R thing. If the switches were as good as the TMUX1511's, it could be pretty swoopy. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics Briarcliff Manor NY 10510 http://electrooptical.net http://hobbs-eo.com
On 24/07/2020 23:10, Phil Hobbs wrote:
> On 2020-07-24 07:43, Chris Jones wrote: >> On 24/07/2020 16:32, Phil Hobbs wrote: >>> On 2020-07-23 22:16, Chris Jones wrote: >>>> On 24/07/2020 09:14, Phil Hobbs wrote: >>>>> On 2020-07-23 16:25, Ricketty C wrote: >>>>>> On Thursday, July 23, 2020 at 7:39:36 AM UTC-4, pcdh...@gmail.com >>>>>> wrote: >>>>>>>> I guess I'm not following what this gains?&Acirc;&nbsp; You get lots more >>>>>>>> steps, but >you don't know any more about where they are.&Acirc;&nbsp; What >>>>>>>> am I missing? >>>>>>> >>>>>>> What it gets me is a combination of settability and bandwidth that >>>>>>> I can't get in any single part at any price. I'll dump in a bit of >>>>>>> pseudorandom DNL and see if that causes any issues. I don't expect >>>>>>> it to--it'll get homogenized much like the regular steps. >>>>>>> >>>>>>> For an iterative adjustment (think LC filter tuning) I can get >>>>>>> close by setting the fine pot to 0, adjusting the coarse pot, then >>>>>>> walking the two together to find the optimum. Near full scale the >>>>>>> possible search range is wider, but I don't think it'll take too >>>>>>> long provided that I'm willing to settle for "good enough" rather >>>>>>> than a global optimum. >>>>>>> >>>>>>> There are two mildly-interacting adjustments, but the response >>>>>>> surface is simple--no local minima. >>>>>>> >>>>>>> It's for taking out the effect of extrinsic emitter resistance in >>>>>>> the BJT diff pair that's the heart of the circuit. It works by >>>>>>> applying a signal to one of the bases that cancels out the >>>>>>> effective emitter degeneration and so restores the desired >>>>>>> Ebers-Moll behaviour at higher photocurrents. >>>>>>> >>>>>>> If I feel like getting fancy, once I've found the right >>>>>>> neighbourhood I can give the turd a final polish to get a bit >>>>>>> closer. ;) >>>>>>> >>>>>>> There are non-negligible effects such as the tempco of wiper >>>>>>> resistance that limit how good this approach can be. Sure beats a >>>>>>> trimpot on a motor though! >>>>> >>>>>> >>>>>> Ok, so this circuit is not a real time control loop as much as it is >>>>>> like a successive approximation.&Acirc;&nbsp; You don't mind the value varying >>>>>> unpredictably as long as it eventually reaches a setting. >>>>>> >>>>> >>>>> Right.&Acirc;&nbsp; It'll be done only occasionally, since R_E of a BJT is a >>>>> weak function of everything.&Acirc;&nbsp; In a diff pair, R_E produces >>>>> degeneration, which tends to make the current split by the ratios >>>>> of the R_E values, whereas we want it to do the Ebers-Moll thing >>>>> and split strictly as exp(delta V_BE/kT):1.&Acirc;&nbsp; R_E degeneration >>>>> generally dominates the error budget at higher photocurrents. >>>>> >>>>> If transistors Q1 and Q2 have resistances R_E1 and R_E2, then the >>>>> required voltage is >>>>> >>>>> dV_BEcor = I_E2 * R_E2&Acirc;&nbsp; - I_E1 * R_E1. >>>>> >>>>> Since V_B1 is controlled by a feedback loop trying to make the >>>>> total DC current from two or more photodiodes cancel out, the >>>>> correction voltage gets applied to the base of Q2.&Acirc;&nbsp; Since the >>>>> resistances are nearly constant, rapid adjustment of the correction >>>>> law is not required. >>>>> >>>>> However, the unwanted degeneration applies at AC as well as DC, and >>>>> the attainable benefit is limited by phase shift and amplitude >>>>> error in the correction voltage.&Acirc;&nbsp; Thus&Acirc;&nbsp; I do need to form the >>>>> correction in analogue with as high a bandwidth as reasonably >>>>> possible. >>>>> >>>>> There'll be a cal table in flash, which will probably rely on the >>>>> AD5273's decent DNL to interpolate relatively coarsely between >>>>> steps--aiming for, say, 8-bit accuracy.&Acirc;&nbsp; That's potentially enough >>>>> to reduce the degeneration effect by 50 dB in the spherical cow >>>>> universe. >>>>> >>>>> Then there'll be a magic pushbutton on the box so that users in lab >>>>> settings can tell it that their experiment is all set up, so please >>>>> optimize the noise cancellation for these exact conditions.&Acirc;&nbsp; Nobody >>>>> will mind if that takes 5 seconds or so.&Acirc;&nbsp; (The command will be >>>>> available electronically as well--USB serial or maybe a dedicated >>>>> control line.) >>>>> >>>>> The magic BFP640, with its ~1-kV Early voltage, excellent >>>>> saturation behaviour, and ultralow C_CB, allows all sorts of cute >>>>> tricks. I'm dorking V_CE on one side of the diff pair in real time >>>>> to force the power dissipation of Q1 and Q2 to be equal >>>>> irrespective of the splitting ratio (within limits). >>>>> >>>>> With some routed slots to control local temperature gradients, it >>>>> looks like I can get performance equivalent to a 40-GHz monolithic >>>>> dual. (We'll see how spherical the cows are in real life--I've seen >>>>> some pretty fat ones.) ;) >>>>> >>>>> Fun. >>>>> >>>>> Cheers >>>>> >>>>> Phil Hobbs >>>>> >>>>> (Who's stocking up on BFP640s as he did on BF862s) >>>>> >>>> >>>> By the way, I don't know if you've seen this, but google and >>>> skywater fab are open-sourcing the PDK for a 0.13um CMOS process, >>>> and google is paying for some free shuttle runs, subject to certain >>>> rules including that all designs be open-sourced too. >>>> >>>> If you can think of anything that you really wish you could get on a >>>> chip, and that can be made in CMOS, and that is useful to you but >>>> not secret enough to not want to publish the design, now is the >>>> time! Shuttle run in November and you get hundreds of parts back, so >>>> maybe enough for a low-volume product. Lots of people are playing >>>> with the tools so quite likely someone else would lay it out for fun >>>> if you didn't have time. >>>> >>>> There is a link to the slack workspace here, with many more details: >>>> >>>> https://groups.google.com/forum/#!topic/skywater-pdk-announce/zijPNEsqsx4 >>>> >>>> >>>> >>> >>> Sounds like a good opportunity for sure.&Acirc;&nbsp; If somebody would do that >>> with a decent low-noise analogue bipolar process (say 0.5 um) I'd be >>> all over it. >>> >>> Cheers >>> >>> Phil Hobbs >>> >> >> >> I'd welcome ideas anyway. Even if there is only a small chance of it >> working in CMOS, it might be worth trying, maybe even just on the >> corner of someone else's chip. >> >> I'm mostly interested in trying something that takes little effort but >> that could not be replicated easily using off-the-shelf parts. Maybe a >> trigger comparator + adjustable delay + sampler with several channels, >> for a sampling scope frontend. Even just a beefy inverter in 0.13um >> might make a nice fast edge generator for testing scope risetime, >> especially as it'll be a flipchip chipscale package thing so no >> bondwires. If there is a usable bipolar then I might do a bandgap >> reference / LDO and see what noise performance I can get from burning >> a few mA rather than the few uA that they usually allow themselves. I >> quite likely won't have time though. >> > > Well, a high-bandwidth dpot/attenuator comes to mind.&Acirc;&nbsp; Something around > 1-2k and 8 bits, maybe an R-2R thing.&Acirc;&nbsp; If the switches were as good as > the TMUX1511's, it could be pretty swoopy.
The best bandwidth would come from using 0.13um (behaving more like 0.18um I hear) NMOS-only switches but they are nominally 1.8V devices so might not be useful for some of your purposes (and driving the gates would get tricky if you need to use much of that range). How much swing will your DPOT see, and do you really want a true pot or just a variable resisor (rheostat)? What is the DC bias on it? If you wanted a variable resistor with one side grounded then the NMOS might work very well. If you could show me where it will be used, I might have other ideas that are more optimal than a general purpose DPOT. I'm pretty sure I won't come up with a general purpose DPOT better than the two remaining semiconductor companies can do, but I might be able to think of something that solves your circuit problem better. There are also 5V tolerant devices on the process but the capacitance for a given Ron will be worse of course. It seems like the PMOS devices are in wells but the NMOS are right in the substrate, (no NMOS in P well in N well). My very first chip used an R-2R attenuator with extra taps along the resistors to give 1dB steps, about 63dB range iirc. Using 0.35um and NMOS devices only, each switch a sort of T configuration but with 8 left arms connected to the resistor network, a shunt switch to ground, and all of the right arms of 8 T switches commoned as the output of the circuit. That worked fairly nicely at the 390MHz IF, though near maximum attenuation I had some subtrate coupling around the switches. I got to do a second revision which fixed it by moving some FETs around. I did subsequent dB attenuators with R-75R (or R/5-15R or R/15-5R maybe) instead of R-2R - easier than tapping the resistors of R-2R.
On 2020-07-24 10:22, Chris Jones wrote:
> On 24/07/2020 23:10, Phil Hobbs wrote: >> On 2020-07-24 07:43, Chris Jones wrote: >>> On 24/07/2020 16:32, Phil Hobbs wrote: >>>> On 2020-07-23 22:16, Chris Jones wrote: >>>>> On 24/07/2020 09:14, Phil Hobbs wrote: >>>>>> On 2020-07-23 16:25, Ricketty C wrote: >>>>>>> On Thursday, July 23, 2020 at 7:39:36 AM UTC-4, >>>>>>> pcdh...@gmail.com wrote: >>>>>>>>> I guess I'm not following what this gains? You get >>>>>>>>> lots more steps, but >you don't know any more about >>>>>>>>> where they are. What am I missing? >>>>>>>> >>>>>>>> What it gets me is a combination of settability and >>>>>>>> bandwidth that I can't get in any single part at any >>>>>>>> price. I'll dump in a bit of pseudorandom DNL and see >>>>>>>> if that causes any issues. I don't expect it to--it'll >>>>>>>> get homogenized much like the regular steps. >>>>>>>> >>>>>>>> For an iterative adjustment (think LC filter tuning) I >>>>>>>> can get close by setting the fine pot to 0, adjusting >>>>>>>> the coarse pot, then walking the two together to find >>>>>>>> the optimum. Near full scale the possible search range >>>>>>>> is wider, but I don't think it'll take too long >>>>>>>> provided that I'm willing to settle for "good enough" >>>>>>>> rather than a global optimum. >>>>>>>> >>>>>>>> There are two mildly-interacting adjustments, but the >>>>>>>> response surface is simple--no local minima. >>>>>>>> >>>>>>>> It's for taking out the effect of extrinsic emitter >>>>>>>> resistance in the BJT diff pair that's the heart of the >>>>>>>> circuit. It works by applying a signal to one of the >>>>>>>> bases that cancels out the effective emitter >>>>>>>> degeneration and so restores the desired Ebers-Moll >>>>>>>> behaviour at higher photocurrents. >>>>>>>> >>>>>>>> If I feel like getting fancy, once I've found the >>>>>>>> right neighbourhood I can give the turd a final polish >>>>>>>> to get a bit closer. ;) >>>>>>>> >>>>>>>> There are non-negligible effects such as the tempco of >>>>>>>> wiper resistance that limit how good this approach can >>>>>>>> be. Sure beats a trimpot on a motor though! >>>>>> >>>>>>> >>>>>>> Ok, so this circuit is not a real time control loop as >>>>>>> much as it is like a successive approximation. You don't >>>>>>> mind the value varying unpredictably as long as it >>>>>>> eventually reaches a setting. >>>>>>> >>>>>> >>>>>> Right. It'll be done only occasionally, since R_E of a BJT >>>>>> is a weak function of everything. In a diff pair, R_E >>>>>> produces degeneration, which tends to make the current >>>>>> split by the ratios of the R_E values, whereas we want it >>>>>> to do the Ebers-Moll thing and split strictly as exp(delta >>>>>> V_BE/kT):1. R_E degeneration generally dominates the >>>>>> error budget at higher photocurrents. >>>>>> >>>>>> If transistors Q1 and Q2 have resistances R_E1 and R_E2, >>>>>> then the required voltage is >>>>>> >>>>>> dV_BEcor = I_E2 * R_E2 - I_E1 * R_E1. >>>>>> >>>>>> Since V_B1 is controlled by a feedback loop trying to make >>>>>> the total DC current from two or more photodiodes cancel >>>>>> out, the correction voltage gets applied to the base of Q2. >>>>>> Since the resistances are nearly constant, rapid adjustment >>>>>> of the correction law is not required. >>>>>> >>>>>> However, the unwanted degeneration applies at AC as well as >>>>>> DC, and the attainable benefit is limited by phase shift >>>>>> and amplitude error in the correction voltage. Thus I do >>>>>> need to form the correction in analogue with as high a >>>>>> bandwidth as reasonably possible. >>>>>> >>>>>> There'll be a cal table in flash, which will probably rely >>>>>> on the AD5273's decent DNL to interpolate relatively >>>>>> coarsely between steps--aiming for, say, 8-bit accuracy. >>>>>> That's potentially enough to reduce the degeneration effect >>>>>> by 50 dB in the spherical cow universe. >>>>>> >>>>>> Then there'll be a magic pushbutton on the box so that >>>>>> users in lab settings can tell it that their experiment is >>>>>> all set up, so please optimize the noise cancellation for >>>>>> these exact conditions. Nobody will mind if that takes 5 >>>>>> seconds or so. (The command will be available >>>>>> electronically as well--USB serial or maybe a dedicated >>>>>> control line.) >>>>>> >>>>>> The magic BFP640, with its ~1-kV Early voltage, excellent >>>>>> saturation behaviour, and ultralow C_CB, allows all sorts >>>>>> of cute tricks. I'm dorking V_CE on one side of the diff >>>>>> pair in real time to force the power dissipation of Q1 and >>>>>> Q2 to be equal irrespective of the splitting ratio (within >>>>>> limits). >>>>>> >>>>>> With some routed slots to control local temperature >>>>>> gradients, it looks like I can get performance equivalent >>>>>> to a 40-GHz monolithic dual. (We'll see how spherical the >>>>>> cows are in real life--I've seen some pretty fat ones.) ;) >>>>>> >>>>>> Fun. >>>>>> >>>>>> Cheers >>>>>> >>>>>> Phil Hobbs >>>>>> >>>>>> (Who's stocking up on BFP640s as he did on BF862s) >>>>>> >>>>> >>>>> By the way, I don't know if you've seen this, but google and >>>>> skywater fab are open-sourcing the PDK for a 0.13um CMOS >>>>> process, and google is paying for some free shuttle runs, >>>>> subject to certain rules including that all designs be >>>>> open-sourced too. >>>>> >>>>> If you can think of anything that you really wish you could >>>>> get on a chip, and that can be made in CMOS, and that is >>>>> useful to you but not secret enough to not want to publish >>>>> the design, now is the time! Shuttle run in November and you >>>>> get hundreds of parts back, so maybe enough for a low-volume >>>>> product. Lots of people are playing with the tools so quite >>>>> likely someone else would lay it out for fun if you didn't >>>>> have time. >>>>> >>>>> There is a link to the slack workspace here, with many more >>>>> details: >>>>> >>>>> https://groups.google.com/forum/#!topic/skywater-pdk-announce/zijPNEsqsx4 >>>>> >>>>> >>>>> >>>>> >>>>> >>>>> >>>> >>>> Sounds like a good opportunity for sure. If somebody would do >>>> that with a decent low-noise analogue bipolar process (say 0.5 >>>> um) I'd be all over it. >>>> >>>> Cheers >>>> >>>> Phil Hobbs >>>> >>> >>> >>> I'd welcome ideas anyway. Even if there is only a small chance of >>> it working in CMOS, it might be worth trying, maybe even just on >>> the corner of someone else's chip. >>> >>> I'm mostly interested in trying something that takes little >>> effort but that could not be replicated easily using >>> off-the-shelf parts. Maybe a trigger comparator + adjustable >>> delay + sampler with several channels, for a sampling scope >>> frontend. Even just a beefy inverter in 0.13um might make a nice >>> fast edge generator for testing scope risetime, especially as >>> it'll be a flipchip chipscale package thing so no bondwires. If >>> there is a usable bipolar then I might do a bandgap reference / >>> LDO and see what noise performance I can get from burning a few >>> mA rather than the few uA that they usually allow themselves. I >>> quite likely won't have time though. >>> >> >> Well, a high-bandwidth dpot/attenuator comes to mind. Something >> around 1-2k and 8 bits, maybe an R-2R thing. If the switches were >> as good as the TMUX1511's, it could be pretty swoopy. > > The best bandwidth would come from using 0.13um (behaving more like > 0.18um I hear) NMOS-only switches but they are nominally 1.8V devices > so might not be useful for some of your purposes (and driving the > gates would get tricky if you need to use much of that range). How > much swing will your DPOT see, and do you really want a true pot or > just a variable resisor (rheostat)? What is the DC bias on it?
I'd be willing to be very flexible in exchange for, say, 100 MHz BW. The present application needs to put about +-20 mV across an ohm or two or five, to compensate the R_E degeneration in the diff pair of a laser noise canceller. (Less is better since the BFP640 has only about 3 ohms R_B, and it's a shame to waste that sort of noise performance.)
> > If you wanted a variable resistor with one side grounded then the > NMOS might work very well. If you could show me where it will be > used, I might have other ideas that are more optimal than a general > purpose DPOT. I'm pretty sure I won't come up with a general purpose > DPOT better than the two remaining semiconductor companies can do, > but I might be able to think of something that solves your circuit > problem better.
The circuit is basically Figure 3 in <https://electrooptical.net/static/media/uploads/Projects/LaserNoiseCanceller/noisecan.pdf> with the modifications of Figures 12 (R_E cancellation) and 19 (differential/high power version that reduces the current in the diff pair). (There's also some mildly complicated bootstrapping and cascoding going on, because I can't get fast PNPs anymore, but that's for another thread.) Sticking with Figure 3, overall feedback forces the current through D1 to be the same as the collector current of Q2. (In principle, this is true at all frequencies regardless of the loop bandwidth--that's why the circuit works.) In Figure 12, the correction voltage comes directly from Q1's collector current and a mirrored version of the tail current of the pair, scaled by a couple of low-value trimpots. That's pretty simple and works pretty well, as long as the user is a circuits guy with some vague clue as to how it all works. In the current circuit, I'm forcing the dissipation of Q1 to equal that of Q2 dorking its V_CE with a DAC controlling another BFP640 cascode transistor. That's where the effectively-infinite Early voltage comes in handy. With a C-shaped cutout in the board and a nice symmetric layout, that ought to keep the transistors within 0.1 C of each other. If so, that will basically eliminate temperature differences as a limiting factor, just as if I had a 40-GHz monolithic dual NPN. Since I need to control VC(Q1), I can't just dump IC(Q1) into the compensation network. (I also can't get an electronically controllable equivalent to a 10-ohm trimpot.) Anyway, D1 needs some reverse bias. Soooo, instead I'm using floating ADA4817 TIAs: one in the collector of the cascode to measure IC(Q1)) and one in the cathode of D1 to measure IC(Q2). I'm forming the correction term by a 499-ohm resistor in one TIA and one of these compound dpot gizmos in the other. An LM6171 and four 0.025% resistors are doing the diff amp honours to do the subtraction and shift the difference signal to near ground. (If I could get a 100-MHz instrumentation amp that handled 30-V supplies, I'd use that instead.) After the diff amp, there's a 499/100 ohm voltage divider, followed by another of these dpot gizmos running off +-2.5V to divide that voltage down to the +-20 mV level. The divider protects the dpots from overvoltage, and besides there's a bias current in D1's bootstrap circuit that I need to cancel out--the current goes into the 499-ohm TIA so I need a 499 ohm load to cancel it.) A rheostat with one end near ground but not actually connected there would do fine in both places, and I could live with 2V supplies if I had to. If I really had to, I could run the dpot's ground pin into the TIA and fix up the quiescent current afterwards. I'd do that in a heartbeat if it would get me >=100 MHz bandwidth.
> > There are also 5V tolerant devices on the process but the capacitance > for a given Ron will be worse of course. > > It seems like the PMOS devices are in wells but the NMOS are right in > the substrate, (no NMOS in P well in N well). > > My very first chip used an R-2R attenuator with extra taps along the > resistors to give 1dB steps, about 63dB range iirc. Using 0.35um and > NMOS devices only, each switch a sort of T configuration but with 8 > left arms connected to the resistor network, a shunt switch to > ground, and all of the right arms of 8 T switches commoned as the > output of the circuit. That worked fairly nicely at the 390MHz IF, > though near maximum attenuation I had some subtrate coupling around > the switches. I got to do a second revision which fixed it by moving > some FETs around. I did subsequent dB attenuators with R-75R (or > R/5-15R or R/15-5R maybe) instead of R-2R - easier than tapping the > resistors of R-2R.
Nice! Cheers Phil Hobbs (who never gets tired of talking about his gizmos, despite occasional glazed looks from his interlocutors) -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics Briarcliff Manor NY 10510 http://electrooptical.net http://hobbs-eo.com
I like the 0.35um process

Most fabs using this support voltages up to 50V, so you can really cover many circuits in a design. And, you can do tiny custom 500MHz opamps also, for a fraction of the commercial price 

Anyone here tried OpenCircuit, free Asic tools?

http://opencircuitdesign.com/magic/

Cheers 

Klaus 
On 25/07/2020 04:38, Phil Hobbs wrote:
> On 2020-07-24 10:22, Chris Jones wrote: >> On 24/07/2020 23:10, Phil Hobbs wrote: >>> On 2020-07-24 07:43, Chris Jones wrote: >>>> On 24/07/2020 16:32, Phil Hobbs wrote: >>>>> On 2020-07-23 22:16, Chris Jones wrote: >>>>>> On 24/07/2020 09:14, Phil Hobbs wrote: >>>>>>> On 2020-07-23 16:25, Ricketty C wrote: >>>>>>>> On Thursday, July 23, 2020 at 7:39:36 AM UTC-4, >>>>>>>> pcdh...@gmail.com wrote: >>>>>>>>>> I guess I'm not following what this gains?&Acirc;&nbsp; You get lots more >>>>>>>>>> steps, but >you don't know any more about where they are. >>>>>>>>>> What am I missing? >>>>>>>>> >>>>>>>>> What it gets me is a combination of settability and bandwidth >>>>>>>>> that I can't get in any single part at any price. I'll dump in >>>>>>>>> a bit of pseudorandom DNL and see if that causes any issues. I >>>>>>>>> don't expect it to--it'll >>>>>>>>> &Acirc;&nbsp;get homogenized much like the regular steps. >>>>>>>>> >>>>>>>>> For an iterative adjustment (think LC filter tuning) I >>>>>>>>> &Acirc;&nbsp;can get close by setting the fine pot to 0, adjusting >>>>>>>>> &Acirc;&nbsp;the coarse pot, then walking the two together to find >>>>>>>>> &Acirc;&nbsp;the optimum. Near full scale the possible search range >>>>>>>>> &Acirc;&nbsp;is wider, but I don't think it'll take too long provided that >>>>>>>>> I'm willing to settle for "good enough" rather than a global >>>>>>>>> optimum. >>>>>>>>> >>>>>>>>> There are two mildly-interacting adjustments, but the response >>>>>>>>> surface is simple--no local minima. >>>>>>>>> >>>>>>>>> It's for taking out the effect of extrinsic emitter resistance >>>>>>>>> in the BJT diff pair that's the heart of the >>>>>>>>> circuit. It works by applying a signal to one of the >>>>>>>>> bases that cancels out the effective emitter degeneration and >>>>>>>>> so restores the desired Ebers-Moll behaviour at higher >>>>>>>>> photocurrents. >>>>>>>>> >>>>>>>>> If I feel like getting fancy, once I've found the >>>>>>>>> right neighbourhood I can give the turd a final polish >>>>>>>>> to get a bit closer. ;) >>>>>>>>> >>>>>>>>> There are non-negligible effects such as the tempco of >>>>>>>>> &Acirc;&nbsp;wiper resistance that limit how good this approach can >>>>>>>>> &Acirc;&nbsp;be. Sure beats a trimpot on a motor though! >>>>>>> >>>>>>>> >>>>>>>> Ok, so this circuit is not a real time control loop as much as >>>>>>>> it is like a successive approximation.&Acirc;&nbsp; You don't >>>>>>>> mind the value varying unpredictably as long as it eventually >>>>>>>> reaches a setting. >>>>>>>> >>>>>>> >>>>>>> Right.&Acirc;&nbsp; It'll be done only occasionally, since R_E of a BJT >>>>>>> is a weak function of everything.&Acirc;&nbsp; In a diff pair, R_E produces >>>>>>> degeneration, which tends to make the current split by the ratios >>>>>>> of the R_E values, whereas we want it to do the Ebers-Moll thing >>>>>>> and split strictly as exp(delta >>>>>>> &Acirc;&nbsp;V_BE/kT):1.&Acirc;&nbsp; R_E degeneration generally dominates the error >>>>>>> budget at higher photocurrents. >>>>>>> >>>>>>> If transistors Q1 and Q2 have resistances R_E1 and R_E2, then the >>>>>>> required voltage is >>>>>>> >>>>>>> dV_BEcor = I_E2 * R_E2&Acirc;&nbsp; - I_E1 * R_E1. >>>>>>> >>>>>>> Since V_B1 is controlled by a feedback loop trying to make >>>>>>> &Acirc;&nbsp;the total DC current from two or more photodiodes cancel out, >>>>>>> the correction voltage gets applied to the base of Q2. >>>>>>> Since the resistances are nearly constant, rapid adjustment >>>>>>> of the correction law is not required. >>>>>>> >>>>>>> However, the unwanted degeneration applies at AC as well as >>>>>>> DC, and the attainable benefit is limited by phase shift >>>>>>> and amplitude error in the correction voltage.&Acirc;&nbsp; Thus I do >>>>>>> need to form the correction in analogue with as high a >>>>>>> bandwidth as reasonably possible. >>>>>>> >>>>>>> There'll be a cal table in flash, which will probably rely >>>>>>> &Acirc;&nbsp;on the AD5273's decent DNL to interpolate relatively coarsely >>>>>>> between steps--aiming for, say, 8-bit accuracy. That's >>>>>>> potentially enough to reduce the degeneration effect >>>>>>> by 50 dB in the spherical cow universe. >>>>>>> >>>>>>> Then there'll be a magic pushbutton on the box so that users in >>>>>>> lab settings can tell it that their experiment is >>>>>>> &Acirc;&nbsp;all set up, so please optimize the noise cancellation for >>>>>>> &Acirc;&nbsp;these exact conditions.&Acirc;&nbsp; Nobody will mind if that takes 5 >>>>>>> &Acirc;&nbsp;seconds or so.&Acirc;&nbsp; (The command will be available electronically as >>>>>>> well--USB serial or maybe a dedicated control line.) >>>>>>> >>>>>>> The magic BFP640, with its ~1-kV Early voltage, excellent >>>>>>> saturation behaviour, and ultralow C_CB, allows all sorts of cute >>>>>>> tricks. I'm dorking V_CE on one side of the diff pair in real >>>>>>> time to force the power dissipation of Q1 and >>>>>>> &Acirc;&nbsp;Q2 to be equal irrespective of the splitting ratio (within >>>>>>> &Acirc;&nbsp;limits). >>>>>>> >>>>>>> With some routed slots to control local temperature gradients, it >>>>>>> looks like I can get performance equivalent to a 40-GHz >>>>>>> monolithic dual. (We'll see how spherical the cows are in real >>>>>>> life--I've seen some pretty fat ones.) ;) >>>>>>> >>>>>>> Fun. >>>>>>> >>>>>>> Cheers >>>>>>> >>>>>>> Phil Hobbs >>>>>>> >>>>>>> (Who's stocking up on BFP640s as he did on BF862s) >>>>>>> >>>>>> >>>>>> By the way, I don't know if you've seen this, but google and >>>>>> &Acirc;&nbsp;skywater fab are open-sourcing the PDK for a 0.13um CMOS process, >>>>>> and google is paying for some free shuttle runs, subject to >>>>>> certain rules including that all designs be open-sourced too. >>>>>> >>>>>> If you can think of anything that you really wish you could get on >>>>>> a chip, and that can be made in CMOS, and that is useful to you >>>>>> but not secret enough to not want to publish the design, now is >>>>>> the time! Shuttle run in November and you >>>>>> &Acirc;&nbsp;get hundreds of parts back, so maybe enough for a low-volume >>>>>> &Acirc;&nbsp;product. Lots of people are playing with the tools so quite >>>>>> &Acirc;&nbsp;likely someone else would lay it out for fun if you didn't have >>>>>> time. >>>>>> >>>>>> There is a link to the slack workspace here, with many more details: >>>>>> >>>>>> https://groups.google.com/forum/#!topic/skywater-pdk-announce/zijPNEsqsx4 >>>>>> >>>>>> >>>>>> >>>>>> >>>>>> >>>>>> >>>>>> >>>>> >>>>> Sounds like a good opportunity for sure.&Acirc;&nbsp; If somebody would do >>>>> &Acirc;&nbsp;that with a decent low-noise analogue bipolar process (say 0.5 >>>>> &Acirc;&nbsp;um) I'd be all over it. >>>>> >>>>> Cheers >>>>> >>>>> Phil Hobbs >>>>> >>>> >>>> >>>> I'd welcome ideas anyway. Even if there is only a small chance of >>>> it working in CMOS, it might be worth trying, maybe even just on >>>> the corner of someone else's chip. >>>> >>>> I'm mostly interested in trying something that takes little effort >>>> but that could not be replicated easily using off-the-shelf parts. >>>> Maybe a trigger comparator + adjustable delay + sampler with several >>>> channels, for a sampling scope frontend. Even just a beefy inverter >>>> in 0.13um might make a nice >>>> &Acirc;&nbsp;fast edge generator for testing scope risetime, especially as it'll >>>> be a flipchip chipscale package thing so no bondwires. If there is a >>>> usable bipolar then I might do a bandgap reference / LDO and see >>>> what noise performance I can get from burning a few mA rather than >>>> the few uA that they usually allow themselves. I quite likely won't >>>> have time though. >>>> >>> >>> Well, a high-bandwidth dpot/attenuator comes to mind.&Acirc;&nbsp; Something >>> around 1-2k and 8 bits, maybe an R-2R thing.&Acirc;&nbsp; If the switches were >>> &Acirc;&nbsp;as good as the TMUX1511's, it could be pretty swoopy. >> >> The best bandwidth would come from using 0.13um (behaving more like >> 0.18um I hear) NMOS-only switches but they are nominally 1.8V devices >> so might not be useful for some of your purposes (and driving the >> gates would get tricky if you need to use much of that range). How >> much swing will your DPOT see, and do you really want a true pot or >> just a variable resisor (rheostat)? What is the DC bias on it? > > I'd be willing to be very flexible in exchange for, say, 100 MHz BW. > The present application needs to put about +-20 mV across an ohm or two > or five, to compensate the R_E degeneration in the diff pair of a laser > noise canceller.&Acirc;&nbsp; (Less is better since the BFP640 has only about 3 ohms > R_B, and it's a shame to waste that sort of noise performance.) > >> >> If you wanted a variable resistor with one side grounded then the NMOS >> might work very well. If you could show me where it will be used, I >> might have other ideas that are more optimal than a general purpose >> DPOT. I'm pretty sure I won't come up with a general purpose >> &Acirc;&nbsp;DPOT better than the two remaining semiconductor companies can do, >> but I might be able to think of something that solves your circuit >> problem better. > > The circuit is basically Figure 3 in > <https://electrooptical.net/static/media/uploads/Projects/LaserNoiseCanceller/noisecan.pdf> > > with the modifications of Figures 12 (R_E cancellation) and 19 > (differential/high power version that reduces the current in the diff > pair).&Acirc;&nbsp; (There's also some mildly complicated bootstrapping and > cascoding going on, because I can't get fast PNPs anymore, but that's > for another thread.) > > Sticking with Figure 3, overall feedback forces the current through D1 > to be the same as the collector current of Q2. (In principle, this is > true at all frequencies regardless of the loop bandwidth--that's why the > circuit works.)&Acirc;&nbsp; In Figure 12, the correction voltage comes directly > from Q1's collector current and a mirrored version of the tail current > of the pair, scaled by a couple of low-value trimpots.&Acirc;&nbsp; That's pretty > simple and works pretty well, as long as the user is a circuits guy with > some vague clue as to how it all works. > > In the current circuit, I'm forcing the dissipation of Q1 to equal that > of Q2 dorking its V_CE with a DAC controlling another BFP640 cascode > transistor.&Acirc;&nbsp; That's where the effectively-infinite Early voltage comes > in handy.&Acirc;&nbsp; With a C-shaped cutout in the board and a nice symmetric > layout, that ought to keep the transistors within 0.1 C of each other. > If so, that will basically eliminate temperature differences as a > limiting factor, just as if I had a 40-GHz monolithic dual NPN. > > Since I need to control VC(Q1), I can't just dump IC(Q1) into the > compensation network.&Acirc;&nbsp; (I also can't get an electronically controllable > equivalent to a 10-ohm trimpot.)&Acirc;&nbsp; Anyway, D1 needs some reverse bias. > > Soooo, instead I'm using floating ADA4817 TIAs: one in the collector of > the cascode to measure IC(Q1)) and one in the cathode of D1 to measure > IC(Q2).&Acirc;&nbsp; I'm forming the correction term by a 499-ohm resistor in one > TIA and one of these compound dpot gizmos in the other.&Acirc;&nbsp; An LM6171 and > four 0.025% resistors are doing the diff amp honours to do the > subtraction and shift the difference signal to near ground.&Acirc;&nbsp; (If I could > get a 100-MHz instrumentation amp that handled 30-V supplies, I'd use > that instead.) > > After the diff amp, there's a 499/100 ohm voltage divider, followed by > another of these dpot gizmos running off +-2.5V to divide that voltage > down to the +-20 mV level.&Acirc;&nbsp; The divider protects the dpots from > overvoltage,
So far so good, I think I understand up to here...
> and besides there's a bias current in D1's bootstrap > circuit that I need to cancel out--the current goes into the 499-ohm TIA > so I need a 499 ohm load to cancel it.)
Ok now I am confused about this bit.
> A rheostat with one end near ground but not actually connected there > would do fine in both places
In the place where the 20 Ohm pot is shown in the circuit of fig. 12 I guess one end really would be grounded, unless I misunderstand. In the other DPOT location, I understand this to be the feedback resistor of the TIA that senses the current in Q1. I it seems to me that this one is biased up at whatever voltage you need for achieving the right amount of heating in Q1, so it would not be possible for the second DPOT to reside on the same die as the first DPOT that feeds the base of Q1, which is sad. So, maybe there isn't a dpot on the base of Q1 but a fixed resistive divider, and do the scaling elsewhere... Do both floating ADA4817 TIAs opamps share the same voltage on their non-inverting inputs? Otherwise I don't understand how the LM6171 and four 0.025% resistor diff amp would manage the subtraction without further complication. If the TIAs are biased up at the same voltage, then I guess both TIAs could use the new wideband DPOTs/rheostats and could float up there together on the same die. So the gains of both TIAs would be adjusted separately and the the results of that would get subtracted and shifted down by the diff amp and scaled by a fixed resistive divider to feed the base of Q1. I am guessing that in this case you would want a ~500 Ohm value rheostat for each TIA, but might have to go for a lower value in order to limit the voltage at the end of the rheostat. > and I could live with 2V supplies if I had to. If I really had to, I > could run the dpot's ground pin into the TIA and fix up the quiescent > current afterwards. Do you mean the wiper, or the VSS? VSS of the dpot thing could be driven by whatever sets the non-inverting input of the TIA opamps, I think. One part I don't understand is the correction of bootstrap bias current that you mentioned. If only usenet had a whiteboard. Also this whole circuit would be great in a fast complementary bipolar process like XFCB3. Oh well. Stuck with CMOS, it is tempting to also try some sort of multiplying current DAC to scale the comparison photodiode current until it is equal to the signal photodiode current. Getting enough resolution for good cancellation, and over a wide bandwidth sounds very hard though. Also it would only ever work for cases where the ratio of the photocurrents is pretty constant over a measurement run.
On 2020-07-25 11:40, Chris Jones wrote:
> On 25/07/2020 04:38, Phil Hobbs wrote: >> On 2020-07-24 10:22, Chris Jones wrote: >>> On 24/07/2020 23:10, Phil Hobbs wrote: >>>> On 2020-07-24 07:43, Chris Jones wrote: >>>>> On 24/07/2020 16:32, Phil Hobbs wrote: >>>>>> On 2020-07-23 22:16, Chris Jones wrote: >>>>>>> On 24/07/2020 09:14, Phil Hobbs wrote: >>>>>>>> On 2020-07-23 16:25, Ricketty C wrote: >>>>>>>>> On Thursday, July 23, 2020 at 7:39:36 AM UTC-4, >>>>>>>>> pcdh...@gmail.com wrote: >>>>>>>>>>> I guess I'm not following what this gains? You >>>>>>>>>>> get lots more steps, but >you don't know any more >>>>>>>>>>> about where they are. What am I missing? >>>>>>>>>> >>>>>>>>>> What it gets me is a combination of settability and >>>>>>>>>> bandwidth that I can't get in any single part at >>>>>>>>>> any price. I'll dump in a bit of pseudorandom DNL >>>>>>>>>> and see if that causes any issues. I don't expect >>>>>>>>>> it to--it'll get homogenized much like the regular >>>>>>>>>> steps. >>>>>>>>>> >>>>>>>>>> For an iterative adjustment (think LC filter >>>>>>>>>> tuning) I can get close by setting the fine pot to >>>>>>>>>> 0, adjusting the coarse pot, then walking the two >>>>>>>>>> together to find the optimum. Near full scale the >>>>>>>>>> possible search range is wider, but I don't think >>>>>>>>>> it'll take too long provided that I'm willing to >>>>>>>>>> settle for "good enough" rather than a global >>>>>>>>>> optimum. >>>>>>>>>> >>>>>>>>>> There are two mildly-interacting adjustments, but >>>>>>>>>> the response surface is simple--no local minima. >>>>>>>>>> >>>>>>>>>> It's for taking out the effect of extrinsic emitter >>>>>>>>>> resistance in the BJT diff pair that's the heart of >>>>>>>>>> the circuit. It works by applying a signal to one >>>>>>>>>> of the bases that cancels out the effective emitter >>>>>>>>>> degeneration and so restores the desired Ebers-Moll >>>>>>>>>> behaviour at higher photocurrents. >>>>>>>>>> >>>>>>>>>> If I feel like getting fancy, once I've found the >>>>>>>>>> right neighbourhood I can give the turd a final >>>>>>>>>> polish to get a bit closer. ;) >>>>>>>>>> >>>>>>>>>> There are non-negligible effects such as the tempco >>>>>>>>>> of wiper resistance that limit how good this >>>>>>>>>> approach can be. Sure beats a trimpot on a motor >>>>>>>>>> though! >>>>>>>> >>>>>>>>> >>>>>>>>> Ok, so this circuit is not a real time control loop >>>>>>>>> as much as it is like a successive approximation. >>>>>>>>> You don't mind the value varying unpredictably as >>>>>>>>> long as it eventually reaches a setting. >>>>>>>>> >>>>>>>> >>>>>>>> Right. It'll be done only occasionally, since R_E of a >>>>>>>> BJT is a weak function of everything. In a diff pair, >>>>>>>> R_E produces degeneration, which tends to make the >>>>>>>> current split by the ratios of the R_E values, whereas >>>>>>>> we want it to do the Ebers-Moll thing and split >>>>>>>> strictly as exp(delta V_BE/kT):1. R_E degeneration >>>>>>>> generally dominates the error budget at higher >>>>>>>> photocurrents. >>>>>>>> >>>>>>>> If transistors Q1 and Q2 have resistances R_E1 and >>>>>>>> R_E2, then the required voltage is >>>>>>>> >>>>>>>> dV_BEcor = I_E2 * R_E2 - I_E1 * R_E1. >>>>>>>> >>>>>>>> Since V_B1 is controlled by a feedback loop trying to >>>>>>>> make the total DC current from two or more photodiodes >>>>>>>> cancel out, the correction voltage gets applied to the >>>>>>>> base of Q2. Since the resistances are nearly constant, >>>>>>>> rapid adjustment of the correction law is not >>>>>>>> required. >>>>>>>> >>>>>>>> However, the unwanted degeneration applies at AC as >>>>>>>> well as DC, and the attainable benefit is limited by >>>>>>>> phase shift and amplitude error in the correction >>>>>>>> voltage. Thus I do need to form the correction in >>>>>>>> analogue with as high a bandwidth as reasonably >>>>>>>> possible. >>>>>>>> >>>>>>>> There'll be a cal table in flash, which will probably >>>>>>>> rely on the AD5273's decent DNL to interpolate >>>>>>>> relatively coarsely between steps--aiming for, say, >>>>>>>> 8-bit accuracy. That's potentially enough to reduce the >>>>>>>> degeneration effect by 50 dB in the spherical cow >>>>>>>> universe. >>>>>>>> >>>>>>>> Then there'll be a magic pushbutton on the box so that >>>>>>>> users in lab settings can tell it that their experiment >>>>>>>> is all set up, so please optimize the noise >>>>>>>> cancellation for these exact conditions. Nobody will >>>>>>>> mind if that takes 5 seconds or so. (The command will >>>>>>>> be available electronically as well--USB serial or >>>>>>>> maybe a dedicated control line.) >>>>>>>> >>>>>>>> The magic BFP640, with its ~1-kV Early voltage, >>>>>>>> excellent saturation behaviour, and ultralow C_CB, >>>>>>>> allows all sorts of cute tricks. I'm dorking V_CE on >>>>>>>> one side of the diff pair in real time to force the >>>>>>>> power dissipation of Q1 and Q2 to be equal irrespective >>>>>>>> of the splitting ratio (within limits). >>>>>>>> >>>>>>>> With some routed slots to control local temperature >>>>>>>> gradients, it looks like I can get performance >>>>>>>> equivalent to a 40-GHz monolithic dual. (We'll see how >>>>>>>> spherical the cows are in real life--I've seen some >>>>>>>> pretty fat ones.) ;) >>>>>>>> >>>>>>>> Fun. >>>>>>>> >>>>>>>> Cheers >>>>>>>> >>>>>>>> Phil Hobbs >>>>>>>> >>>>>>>> (Who's stocking up on BFP640s as he did on BF862s) >>>>>>>> >>>>>>> >>>>>>> By the way, I don't know if you've seen this, but google >>>>>>> and skywater fab are open-sourcing the PDK for a 0.13um >>>>>>> CMOS process, and google is paying for some free shuttle >>>>>>> runs, subject to certain rules including that all designs >>>>>>> be open-sourced too. >>>>>>> >>>>>>> If you can think of anything that you really wish you >>>>>>> could get on a chip, and that can be made in CMOS, and >>>>>>> that is useful to you but not secret enough to not want >>>>>>> to publish the design, now is the time! Shuttle run in >>>>>>> November and you get hundreds of parts back, so maybe >>>>>>> enough for a low-volume product. Lots of people are >>>>>>> playing with the tools so quite likely someone else would >>>>>>> lay it out for fun if you didn't have time. >>>>>>> >>>>>>> There is a link to the slack workspace here, with many >>>>>>> more details: >>>>>>> >>>>>>> https://groups.google.com/forum/#!topic/skywater-pdk-announce/zijPNEsqsx4 >>>>>>> >>>>>>> >>>>>>> >>>>>>> >>>>>>> >>>>>>> >>>>>>> >>>>>>> >>>>>> >>>>>> Sounds like a good opportunity for sure. If somebody would >>>>>> do that with a decent low-noise analogue bipolar process >>>>>> (say 0.5 um) I'd be all over it. >>>>>> >>>>>> Cheers >>>>>> >>>>>> Phil Hobbs >>>>>> >>>>> >>>>> >>>>> I'd welcome ideas anyway. Even if there is only a small >>>>> chance of it working in CMOS, it might be worth trying, maybe >>>>> even just on the corner of someone else's chip. >>>>> >>>>> I'm mostly interested in trying something that takes little >>>>> effort but that could not be replicated easily using >>>>> off-the-shelf parts. Maybe a trigger comparator + adjustable >>>>> delay + sampler with several channels, for a sampling scope >>>>> frontend. Even just a beefy inverter in 0.13um might make a >>>>> nice fast edge generator for testing scope risetime, >>>>> especially as it'll be a flipchip chipscale package thing so >>>>> no bondwires. If there is a usable bipolar then I might do a >>>>> bandgap reference / LDO and see what noise performance I can >>>>> get from burning a few mA rather than the few uA that they >>>>> usually allow themselves. I quite likely won't have time >>>>> though. >>>>> >>>> >>>> Well, a high-bandwidth dpot/attenuator comes to mind. >>>> Something around 1-2k and 8 bits, maybe an R-2R thing. If the >>>> switches were as good as the TMUX1511's, it could be pretty >>>> swoopy. >>> >>> The best bandwidth would come from using 0.13um (behaving more >>> like 0.18um I hear) NMOS-only switches but they are nominally >>> 1.8V devices so might not be useful for some of your purposes >>> (and driving the gates would get tricky if you need to use much >>> of that range). How much swing will your DPOT see, and do you >>> really want a true pot or just a variable resisor (rheostat)? >>> What is the DC bias on it? >> >> I'd be willing to be very flexible in exchange for, say, 100 MHz >> BW. The present application needs to put about +-20 mV across an >> ohm or two or five, to compensate the R_E degeneration in the diff >> pair of a laser noise canceller. (Less is better since the BFP640 >> has only about 3 ohms R_B, and it's a shame to waste that sort of >> noise performance.) >> >>> >>> If you wanted a variable resistor with one side grounded then the >>> NMOS might work very well. If you could show me where it will be >>> used, I might have other ideas that are more optimal than a >>> general purpose DPOT. I'm pretty sure I won't come up with a >>> general purpose DPOT better than the two remaining semiconductor >>> companies can do, but I might be able to think of something that >>> solves your circuit problem better. >> >> The circuit is basically Figure 3 in >> <https://electrooptical.net/static/media/uploads/Projects/LaserNoiseCanceller/noisecan.pdf> >> >> >> with the modifications of Figures 12 (R_E cancellation) and 19 >> (differential/high power version that reduces the current in the >> diff pair). (There's also some mildly complicated bootstrapping >> and cascoding going on, because I can't get fast PNPs anymore, but >> that's for another thread.) >> >> Sticking with Figure 3, overall feedback forces the current through >> D1 to be the same as the collector current of Q2. (In principle, >> this is true at all frequencies regardless of the loop >> bandwidth--that's why the circuit works.) In Figure 12, the >> correction voltage comes directly from Q1's collector current and a >> mirrored version of the tail current of the pair, scaled by a >> couple of low-value trimpots. That's pretty simple and works >> pretty well, as long as the user is a circuits guy with some vague >> clue as to how it all works. >> >> In the current circuit, I'm forcing the dissipation of Q1 to equal >> that of Q2 dorking its V_CE with a DAC controlling another BFP640 >> cascode transistor. That's where the effectively-infinite Early >> voltage comes in handy. With a C-shaped cutout in the board and a >> nice symmetric layout, that ought to keep the transistors within >> 0.1 C of each other. If so, that will basically eliminate >> temperature differences as a limiting factor, just as if I had a >> 40-GHz monolithic dual NPN. >> >> Since I need to control VC(Q1), I can't just dump IC(Q1) into the >> compensation network. (I also can't get an electronically >> controllable equivalent to a 10-ohm trimpot.) Anyway, D1 needs >> some reverse bias. >> >> Soooo, instead I'm using floating ADA4817 TIAs: one in the >> collector of the cascode to measure IC(Q1)) and one in the cathode >> of D1 to measure IC(Q2). I'm forming the correction term by a >> 499-ohm resistor in one TIA and one of these compound dpot gizmos >> in the other. An LM6171 and four 0.025% resistors are doing the >> diff amp honours to do the subtraction and shift the difference >> signal to near ground. (If I could get a 100-MHz instrumentation >> amp that handled 30-V supplies, I'd use that instead.) >> >> After the diff amp, there's a 499/100 ohm voltage divider, followed >> by another of these dpot gizmos running off +-2.5V to divide that >> voltage down to the +-20 mV level. The divider protects the dpots >> from overvoltage, > So far so good, I think I understand up to here... > >> and besides there's a bias current in D1's bootstrap circuit that I >> need to cancel out--the current goes into the 499-ohm TIA so I need >> a 499 ohm load to cancel it.) > Ok now I am confused about this bit.
The bootstraps/cascodes are Darlington-connected BFP640s, each with a 5-ohm bead in series with the base. The beads turn them from twitchy magic Ferraris into boring magic Hondas--they keep their 500 beta, 1 kV Early voltage, 0.3 nV 1-Hz flatband noise, and 0.6 ohm R_E, but become nice and stable although probably 20x slower. However, the combo's rolloff is roughly quadratic rather than linear below about 500 MHz, so from DC to VHF they're nearly indistinguishable from the untouched magic microwave transistor. In the present application, I'm aiming for accuracies of around 1 part in 10**4 over a 100:1 range of photocurrent, 50 uA to 5 mA. (The complete instrument won't be quite that good--this is the budget per photodiode.) So the bootstraps and cascodes are Darlingtons with additional external bias for the driver transistor to keep the bandwidth from going into the tank at low photocurrent.(*) For a normal photodiode bootstrap, where we measure the end that goes to the TIA and jiggle the end that goes to the bias supply, there's no issue--we don't care about mixing the photocurrent and bias current on that end of the device. However, since there are no fast PNPs available anymore, we have to bootstrap D1 with NPNs cascode-style: measure the anode and jiggle the cathode. That means that we have to get our photocurrent measurement from the collectors of the Darlington. Since both collectors connect to the TIA input, the external bias current for the driver transistor corrupts the TIA's output. That bias comes via a 10k resistor connected someplace near ground, so since I chose that TIA to be the one with the fixed 499-ohm resistor, the output is in error by about 499 ohms * Vbias/10k ohms. Conveniently, that TIA connects to the - input of the diff amp, so the bias current makes the diff amp's output go too low. Thus by hanging a 499 ohm resistor in series with the diff amp output, I can null out the offset by simply connecting the other end of the 10k resistor to the far end of the 499 ohms. As the diff amp output changes, the bias current will change a little, but Kirchhoff says it'll stay nulled. ;) This is true regardless of the 499 / 100 ohm divider--it works at any ratio.
>> A rheostat with one end near ground but not actually connected >> there would do fine in both places > In the place where the 20 Ohm pot is shown in the circuit of fig. 12 > I guess one end really would be grounded, unless I misunderstand. > > In the other DPOT location, I understand this to be the feedback > resistor of the TIA that senses the current in Q1. I it seems to me > that this one is biased up at whatever voltage you need for achieving > the right amount of heating in Q1, so it would not be possible for > the second DPOT to reside on the same die as the first DPOT that > feeds the base of Q1, which is sad.
Yeah, their supplies are offset by like 18V from each other, but I'm totally happy using two chips. If I wind up wiring VSS of the dpot to the TIA input it'd have to be a single anyway. > So, maybe there isn't a dpot on the base of Q1
> but a fixed resistive divider, and do the scaling elsewhere... > > Do both floating ADA4817 TIAs opamps share the same voltage on their > non-inverting inputs?
Yes. They're both connected to a 'floating ground' hung 5V below the main VCC rail. Their VEE pins are a couple of volts below that. Both rails courtesy of that TCA0372 I was whining about on another thread. ;)
> Otherwise I don't understand how the LM6171 and four 0.025% resistor > diff amp would manage the subtraction without further complication. > If the TIAs are biased up at the same voltage, then I guess both TIAs > could use the new wideband DPOTs/rheostats and could float up there > together on the same die.
I think that would probably work fine, as long as the bottom end of the rheostats were separate rather than connecting directly to VSS. Since the input transistor of the D1 Darlington is above the floating ground and the voltage drop doesn't vary rapidly, I could probably get rid of the bias current effect adequately using another dpot and software to distribute it between the two TIAs correctly.
> So the gains of both TIAs would be adjusted separately and the the > results of that would get subtracted and shifted down by the diff amp > and scaled by a fixed resistive divider to feed the base of Q1. I am > guessing that in this case you would want a ~500 Ohm value rheostat > for each TIA, but might have to go for a lower value in order to > limit the voltage at the end of the rheostat.
I'm totally happy bootstrapping the supplies in any way necessary, so as long as the cold ends of the rheostats aren't both grounded, we're good.
> >> and I could live with 2V supplies if I had to. If I really had to, >> I could run the dpot's ground pin into the TIA and fix up the >> quiescent current afterwards. > Do you mean the wiper, or the VSS? VSS of the dpot thing could be > driven by whatever sets the non-inverting input of the TIA opamps, I > think. > > One part I don't understand is the correction of bootstrap bias > current that you mentioned. If only usenet had a whiteboard.
I hope I've explained it adequately. I'll happily post a schematic fragment if needed.
> Also this whole circuit would be great in a fast complementary > bipolar process like XFCB3. Oh well. > > Stuck with CMOS, it is tempting to also try some sort of multiplying > current DAC to scale the comparison photodiode current until it is > equal to the signal photodiode current. Getting enough resolution for > good cancellation, and over a wide bandwidth sounds very hard though. > Also it would only ever work for cases where the ratio of the > photocurrents is pretty constant over a measurement run.
It's generally pretty well that way. Thanks! Phil Hobbs (*) In an unbiased Darlington with beta = 500 and I_C = 50 uA, the driver's collector current is only 100 nA. Even a BFP640 isn't very fast at that level. -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics Briarcliff Manor NY 10510 http://electrooptical.net http://hobbs-eo.com
On 26/07/2020 04:10, Phil Hobbs wrote:
> On 2020-07-25 11:40, Chris Jones wrote: >> On 25/07/2020 04:38, Phil Hobbs wrote: >>> On 2020-07-24 10:22, Chris Jones wrote: >>>> On 24/07/2020 23:10, Phil Hobbs wrote: >>>>> On 2020-07-24 07:43, Chris Jones wrote: >>>>>> On 24/07/2020 16:32, Phil Hobbs wrote: >>>>>>> On 2020-07-23 22:16, Chris Jones wrote: >>>>>>>> On 24/07/2020 09:14, Phil Hobbs wrote: >>>>>>>>> On 2020-07-23 16:25, Ricketty C wrote: >>>>>>>>>> On Thursday, July 23, 2020 at 7:39:36 AM UTC-4, >>>>>>>>>> pcdh...@gmail.com wrote: >>>>>>>>>>>> I guess I'm not following what this gains?&Acirc;&nbsp; You >>>>>>>>>>>> get lots more steps, but >you don't know any more >>>>>>>>>>>> about where they are. What am I missing? >>>>>>>>>>> >>>>>>>>>>> What it gets me is a combination of settability and >>>>>>>>>>> bandwidth that I can't get in any single part at >>>>>>>>>>> any price. I'll dump in a bit of pseudorandom DNL >>>>>>>>>>> and see if that causes any issues. I don't expect >>>>>>>>>>> it to--it'll get homogenized much like the regular >>>>>>>>>>> steps. >>>>>>>>>>> >>>>>>>>>>> For an iterative adjustment (think LC filter >>>>>>>>>>> tuning) I can get close by setting the fine pot to >>>>>>>>>>> 0, adjusting the coarse pot, then walking the two >>>>>>>>>>> together to find the optimum. Near full scale the >>>>>>>>>>> possible search range is wider, but I don't think >>>>>>>>>>> it'll take too long provided that I'm willing to >>>>>>>>>>> settle for "good enough" rather than a global optimum. >>>>>>>>>>> >>>>>>>>>>> There are two mildly-interacting adjustments, but >>>>>>>>>>> the response surface is simple--no local minima. >>>>>>>>>>> >>>>>>>>>>> It's for taking out the effect of extrinsic emitter >>>>>>>>>>> resistance in the BJT diff pair that's the heart of >>>>>>>>>>> the circuit. It works by applying a signal to one >>>>>>>>>>> of the bases that cancels out the effective emitter >>>>>>>>>>> degeneration and so restores the desired Ebers-Moll >>>>>>>>>>> behaviour at higher photocurrents. >>>>>>>>>>> >>>>>>>>>>> If I feel like getting fancy, once I've found the right >>>>>>>>>>> neighbourhood I can give the turd a final >>>>>>>>>>> polish to get a bit closer. ;) >>>>>>>>>>> >>>>>>>>>>> There are non-negligible effects such as the tempco >>>>>>>>>>> of wiper resistance that limit how good this >>>>>>>>>>> approach can be. Sure beats a trimpot on a motor >>>>>>>>>>> though! >>>>>>>>> >>>>>>>>>> >>>>>>>>>> Ok, so this circuit is not a real time control loop >>>>>>>>>> as much as it is like a successive approximation. >>>>>>>>>> You don't mind the value varying unpredictably as >>>>>>>>>> long as it eventually reaches a setting. >>>>>>>>>> >>>>>>>>> >>>>>>>>> Right.&Acirc;&nbsp; It'll be done only occasionally, since R_E of a >>>>>>>>> BJT is a weak function of everything.&Acirc;&nbsp; In a diff pair, >>>>>>>>> R_E produces degeneration, which tends to make the >>>>>>>>> current split by the ratios of the R_E values, whereas >>>>>>>>> we want it to do the Ebers-Moll thing and split >>>>>>>>> strictly as exp(delta V_BE/kT):1.&Acirc;&nbsp; R_E degeneration >>>>>>>>> generally dominates the error budget at higher >>>>>>>>> photocurrents. >>>>>>>>> >>>>>>>>> If transistors Q1 and Q2 have resistances R_E1 and >>>>>>>>> R_E2, then the required voltage is >>>>>>>>> >>>>>>>>> dV_BEcor = I_E2 * R_E2&Acirc;&nbsp; - I_E1 * R_E1. >>>>>>>>> >>>>>>>>> Since V_B1 is controlled by a feedback loop trying to >>>>>>>>> make the total DC current from two or more photodiodes >>>>>>>>> cancel out, the correction voltage gets applied to the >>>>>>>>> base of Q2. Since the resistances are nearly constant, >>>>>>>>> rapid adjustment of the correction law is not >>>>>>>>> required. >>>>>>>>> >>>>>>>>> However, the unwanted degeneration applies at AC as >>>>>>>>> well as DC, and the attainable benefit is limited by >>>>>>>>> phase shift and amplitude error in the correction >>>>>>>>> voltage.&Acirc;&nbsp; Thus I do need to form the correction in >>>>>>>>> analogue with as high a bandwidth as reasonably >>>>>>>>> possible. >>>>>>>>> >>>>>>>>> There'll be a cal table in flash, which will probably >>>>>>>>> rely on the AD5273's decent DNL to interpolate >>>>>>>>> relatively coarsely between steps--aiming for, say, >>>>>>>>> 8-bit accuracy. That's potentially enough to reduce the >>>>>>>>> degeneration effect by 50 dB in the spherical cow >>>>>>>>> universe. >>>>>>>>> >>>>>>>>> Then there'll be a magic pushbutton on the box so that >>>>>>>>> users in lab settings can tell it that their experiment >>>>>>>>> is all set up, so please optimize the noise >>>>>>>>> cancellation for these exact conditions.&Acirc;&nbsp; Nobody will >>>>>>>>> mind if that takes 5 seconds or so.&Acirc;&nbsp; (The command will >>>>>>>>> be available electronically as well--USB serial or >>>>>>>>> maybe a dedicated control line.) >>>>>>>>> >>>>>>>>> The magic BFP640, with its ~1-kV Early voltage, >>>>>>>>> excellent saturation behaviour, and ultralow C_CB, >>>>>>>>> allows all sorts of cute tricks. I'm dorking V_CE on >>>>>>>>> one side of the diff pair in real time to force the >>>>>>>>> power dissipation of Q1 and Q2 to be equal irrespective >>>>>>>>> of the splitting ratio (within limits). >>>>>>>>> >>>>>>>>> With some routed slots to control local temperature >>>>>>>>> gradients, it looks like I can get performance >>>>>>>>> equivalent to a 40-GHz monolithic dual. (We'll see how >>>>>>>>> spherical the cows are in real life--I've seen some >>>>>>>>> pretty fat ones.) ;) >>>>>>>>> >>>>>>>>> Fun. >>>>>>>>> >>>>>>>>> Cheers >>>>>>>>> >>>>>>>>> Phil Hobbs >>>>>>>>> >>>>>>>>> (Who's stocking up on BFP640s as he did on BF862s) >>>>>>>>> >>>>>>>> >>>>>>>> By the way, I don't know if you've seen this, but google >>>>>>>> and skywater fab are open-sourcing the PDK for a 0.13um >>>>>>>> CMOS process, and google is paying for some free shuttle >>>>>>>> runs, subject to certain rules including that all designs >>>>>>>> be open-sourced too. >>>>>>>> >>>>>>>> If you can think of anything that you really wish you >>>>>>>> could get on a chip, and that can be made in CMOS, and >>>>>>>> that is useful to you but not secret enough to not want >>>>>>>> to publish the design, now is the time! Shuttle run in >>>>>>>> November and you get hundreds of parts back, so maybe >>>>>>>> enough for a low-volume product. Lots of people are >>>>>>>> playing with the tools so quite likely someone else would >>>>>>>> lay it out for fun if you didn't have time. >>>>>>>> >>>>>>>> There is a link to the slack workspace here, with many >>>>>>>> more details: >>>>>>>> >>>>>>>> https://groups.google.com/forum/#!topic/skywater-pdk-announce/zijPNEsqsx4 >>>>>>>> >>>>>>>> >>>>>>>> >>>>>>>> >>>>>>>> >>>>>>>> >>>>>>>> >>>>>>>> >>>>>>>> >>>>>>> >>>>>>> Sounds like a good opportunity for sure.&Acirc;&nbsp; If somebody would >>>>>>> do that with a decent low-noise analogue bipolar process >>>>>>> (say 0.5 um) I'd be all over it. >>>>>>> >>>>>>> Cheers >>>>>>> >>>>>>> Phil Hobbs >>>>>>> >>>>>> >>>>>> >>>>>> I'd welcome ideas anyway. Even if there is only a small >>>>>> chance of it working in CMOS, it might be worth trying, maybe >>>>>> even just on the corner of someone else's chip. >>>>>> >>>>>> I'm mostly interested in trying something that takes little >>>>>> effort but that could not be replicated easily using >>>>>> off-the-shelf parts. Maybe a trigger comparator + adjustable >>>>>> delay + sampler with several channels, for a sampling scope >>>>>> frontend. Even just a beefy inverter in 0.13um might make a >>>>>> nice fast edge generator for testing scope risetime, >>>>>> especially as it'll be a flipchip chipscale package thing so >>>>>> no bondwires. If there is a usable bipolar then I might do a >>>>>> bandgap reference / LDO and see what noise performance I can >>>>>> get from burning a few mA rather than the few uA that they >>>>>> usually allow themselves. I quite likely won't have time >>>>>> though. >>>>>> >>>>> >>>>> Well, a high-bandwidth dpot/attenuator comes to mind. >>>>> Something around 1-2k and 8 bits, maybe an R-2R thing.&Acirc;&nbsp; If the >>>>> switches were as good as the TMUX1511's, it could be pretty >>>>> swoopy. >>>> >>>> The best bandwidth would come from using 0.13um (behaving more >>>> like 0.18um I hear) NMOS-only switches but they are nominally >>>> 1.8V devices so might not be useful for some of your purposes >>>> (and driving the gates would get tricky if you need to use much >>>> of that range). How much swing will your DPOT see, and do you >>>> really want a true pot or just a variable resisor (rheostat)? >>>> What is the DC bias on it? >>> >>> I'd be willing to be very flexible in exchange for, say, 100 MHz >>> BW. The present application needs to put about +-20 mV across an >>> ohm or two or five, to compensate the R_E degeneration in the diff >>> pair of a laser noise canceller.&Acirc;&nbsp; (Less is better since the BFP640 >>> has only about 3 ohms R_B, and it's a shame to waste that sort of >>> noise performance.) >>> >>>> >>>> If you wanted a variable resistor with one side grounded then the >>>> &Acirc;&nbsp;NMOS might work very well. If you could show me where it will be >>>> &Acirc;&nbsp;used, I might have other ideas that are more optimal than a >>>> general purpose DPOT. I'm pretty sure I won't come up with a >>>> general purpose DPOT better than the two remaining semiconductor >>>> companies can do, but I might be able to think of something that >>>> solves your circuit problem better. >>> >>> The circuit is basically Figure 3 in >>> <https://electrooptical.net/static/media/uploads/Projects/LaserNoiseCanceller/noisecan.pdf> >>> >>> >>> >>> with the modifications of Figures 12 (R_E cancellation) and 19 >>> (differential/high power version that reduces the current in the >>> diff pair).&Acirc;&nbsp; (There's also some mildly complicated bootstrapping >>> and cascoding going on, because I can't get fast PNPs anymore, but >>> that's for another thread.) >>> >>> Sticking with Figure 3, overall feedback forces the current through >>> D1 to be the same as the collector current of Q2. (In principle, >>> this is true at all frequencies regardless of the loop >>> bandwidth--that's why the circuit works.)&Acirc;&nbsp; In Figure 12, the >>> correction voltage comes directly from Q1's collector current and a >>> mirrored version of the tail current of the pair, scaled by a >>> couple of low-value trimpots.&Acirc;&nbsp; That's pretty simple and works >>> pretty well, as long as the user is a circuits guy with some vague >>> clue as to how it all works. >>> >>> In the current circuit, I'm forcing the dissipation of Q1 to equal >>> that of Q2 dorking its V_CE with a DAC controlling another BFP640 >>> cascode transistor.&Acirc;&nbsp; That's where the effectively-infinite Early >>> voltage comes in handy.&Acirc;&nbsp; With a C-shaped cutout in the board and a >>> nice symmetric layout, that ought to keep the transistors within >>> 0.1 C of each other. If so, that will basically eliminate >>> temperature differences as a limiting factor, just as if I had a >>> 40-GHz monolithic dual NPN. >>> >>> Since I need to control VC(Q1), I can't just dump IC(Q1) into the >>> compensation network.&Acirc;&nbsp; (I also can't get an electronically >>> controllable equivalent to a 10-ohm trimpot.)&Acirc;&nbsp; Anyway, D1 needs >>> some reverse bias. >>> >>> Soooo, instead I'm using floating ADA4817 TIAs: one in the >>> collector of the cascode to measure IC(Q1)) and one in the cathode >>> of D1 to measure IC(Q2).&Acirc;&nbsp; I'm forming the correction term by a >>> 499-ohm resistor in one TIA and one of these compound dpot gizmos >>> in the other.&Acirc;&nbsp; An LM6171 and four 0.025% resistors are doing the >>> diff amp honours to do the subtraction and shift the difference >>> signal to near ground.&Acirc;&nbsp; (If I could get a 100-MHz instrumentation >>> amp that handled 30-V supplies, I'd use that instead.) >>> >>> After the diff amp, there's a 499/100 ohm voltage divider, followed >>> by another of these dpot gizmos running off +-2.5V to divide that >>> voltage down to the +-20 mV level.&Acirc;&nbsp; The divider protects the dpots >>> from overvoltage, >> So far so good, I think I understand up to here... >> >>> and besides there's a bias current in D1's bootstrap circuit that I >>> need to cancel out--the current goes into the 499-ohm TIA so I need >>> a 499 ohm load to cancel it.) >> Ok now I am confused about this bit. > > The bootstraps/cascodes are Darlington-connected BFP640s, each with a > 5-ohm bead in series with the base.&Acirc;&nbsp; The beads turn them from twitchy > magic Ferraris into boring magic Hondas--they keep their 500 beta, 1 kV > Early voltage, 0.3 nV 1-Hz flatband noise, and 0.6 ohm R_E, but become > nice and stable although probably 20x slower.&Acirc;&nbsp; However, the combo's > rolloff is roughly quadratic rather than linear below about 500 MHz, so > from DC to VHF they're nearly indistinguishable from the untouched magic > microwave transistor. > > In the present application, I'm aiming for accuracies of around 1 part > in 10**4 over a 100:1 range of photocurrent, 50 uA to 5 mA.&Acirc;&nbsp; (The > complete instrument won't be quite that good--this is the budget per > photodiode.)&Acirc;&nbsp; So the bootstraps and cascodes are Darlingtons with > additional external bias for the driver transistor to keep the bandwidth > from going into the tank at low photocurrent.(*)&Acirc;&nbsp; For a normal > photodiode bootstrap, where we measure the end that goes to the TIA and > jiggle the end that goes to the bias supply, there's no issue--we don't > care about mixing the photocurrent and bias current on that end of the > device. > > However, since there are no fast PNPs available anymore, we have to > bootstrap D1 with NPNs cascode-style: measure the anode and jiggle the > cathode.&Acirc;&nbsp; That means that we have to get our photocurrent measurement > from the collectors of the Darlington.&Acirc;&nbsp; Since both collectors connect to > the TIA input, the external bias current for the driver transistor > corrupts the TIA's output. > > That bias comes via a 10k resistor connected someplace near ground, so > since I chose that TIA to be the one with the fixed 499-ohm resistor, > the output is in error by about 499 ohms * Vbias/10k ohms. > Conveniently, that TIA connects to the - input of the diff amp, so the > bias current makes the diff amp's output go too low. > > Thus by hanging a 499 ohm resistor in series with the diff amp output, I > can null out the offset by simply connecting the other end of the 10k > resistor to the far&Acirc;&nbsp; end of the 499 ohms.&Acirc;&nbsp; As the diff amp output > changes, the bias current will change a little, but Kirchhoff says it'll > stay nulled.&Acirc;&nbsp; ;) This is true regardless of the 499 / 100 ohm > divider--it works at any ratio. > >>> A rheostat with one end near ground but not actually connected >>> there would do fine in both places >> In the place where the 20 Ohm pot is shown in the circuit of fig. 12 >> I guess one end really would be grounded, unless I misunderstand. >> >> In the other DPOT location, I understand this to be the feedback >> resistor of the TIA that senses the current in Q1. I it seems to me >> that this one is biased up at whatever voltage you need for achieving >> the right amount of heating in Q1, so it would not be possible for >> the second DPOT to reside on the same die as the first DPOT that >> feeds the base of Q1, which is sad. > > Yeah, their supplies are offset by like 18V from each other, but I'm > totally happy using two chips.&Acirc;&nbsp; If I wind up wiring VSS of the dpot to > the TIA input it'd have to be a single anyway. > > &Acirc;&nbsp;> So, maybe there isn't a dpot on the base of Q1 >> but a fixed resistive divider, and do the scaling elsewhere... >> >> Do both floating ADA4817 TIAs opamps share the same voltage on their >> &Acirc;&nbsp;non-inverting inputs? > > Yes. They're both connected to a 'floating ground' hung 5V below the > main VCC rail.&Acirc;&nbsp; Their VEE pins are a couple of volts below that.&Acirc;&nbsp; Both > rails courtesy of that TCA0372 I was whining about on another thread. ;) > >> Otherwise I don't understand how the LM6171 and four 0.025% resistor >> diff amp would manage the subtraction without further complication. >> If the TIAs are biased up at the same voltage, then I guess both TIAs >> could use the new wideband DPOTs/rheostats and could float up there >> together on the same die. > > I think that would probably work fine, as long as the bottom end of the > rheostats were separate rather than connecting directly to VSS. > > Since the input transistor of the D1 Darlington is above the > floating ground and the voltage drop doesn't vary rapidly, I could > probably get rid of the bias current effect adequately using another > dpot and software to distribute it between the two TIAs correctly. > >> So the gains of both TIAs would be adjusted separately and the the >> results of that would get subtracted and shifted down by the diff amp >> and scaled by a fixed resistive divider to feed the base of Q1. I am >> guessing that in this case you would want a ~500 Ohm value rheostat >> for each TIA, but might have to go for a lower value in order to >> limit the voltage at the end of the rheostat. > I'm totally happy bootstrapping the supplies in any way necessary, so as > long as the cold ends of the rheostats aren't both grounded, we're good. > >> >>> and I could live with 2V supplies if I had to.&Acirc;&nbsp; If I really had to, >>> I could run the dpot's ground pin into the TIA and fix up the >>> quiescent current afterwards. >> Do you mean the wiper, or the VSS? VSS of the dpot thing could be >> driven by whatever sets the non-inverting input of the TIA opamps, I >> think. >> >> One part I don't understand is the correction of bootstrap bias >> current that you mentioned. If only usenet had a whiteboard. > > I hope I've explained it adequately.&Acirc;&nbsp; I'll happily post a schematic > fragment if needed.
I'm afraid I am still very confused, after trying quite hard to interpret your explanation in the context of Fig. 3 from your link. I appreciate that you have put a lot of work into the explanation, and I was going to point out each part that I didn't understand, but I think re-explaining all of that in text would perhaps not be an efficient use of your time. I think part of it is that avoiding the PNP probably makes the circuit fairly different from Fig. 3, and part of it is that I am imagining that there are three TIAs (the main one for the subtracted photocurrents that produces the final output, and one for monitoring the collector current of each transistor in the diff pair that does the current splitting), but it is not always clear to me in each case which TIA you are referring to above. If you could post a schematic, that would be very efficient, however I would understand if you prefer not to do that unless privately under formal or informal NDA, in which case I've sent you an e-mail by which we could arrange that if necessary. Curiousity aside, for implementing a fast DPOT the main thing I think I am missing is an understanding of how the bias current (of the Darlington) gets subtracted out, and how the diffamp is connected. Given that the tempco of the dpot resistors (and initial tolerance) is likely to be truly awful, there might be an advantage in implementing the two input resistors of the diffamp on-chip using the exact same awful tempco as the DPOT, to take out the gain drift. It may not matter that the voltage on one end of those unswitched resistors goes well outside the rails, as long as I am allowed to get creative with the ESD protection cells.
> >> Also this whole circuit would be great in a fast complementary >> bipolar process like XFCB3. Oh well. >> >> Stuck with CMOS, it is tempting to also try some sort of multiplying >> current DAC to scale the comparison photodiode current until it is >> equal to the signal photodiode current. Getting enough resolution for >> good cancellation, and over a wide bandwidth sounds very hard though. >> Also it would only ever work for cases where the ratio of the >> photocurrents is pretty constant over a measurement run. > > It's generally pretty well that way.
Most of the ideas that I have come up with involve a fast PNP somewhere. I wonder how good a 0.13um PMOS is, and if there is a way to use it that does not rely on decent "Early voltage" (or the fet equivalent), nor care about 1/f noise.