This is a multi-part message in MIME format. --------------FF67D67D34458B072894D837 Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 7bit So I'm doing a spin of my swoopy nanoamp photoreceiver board, to make it smaller and more versatile. The original QL01 has two pots, one for offset and one for HF boost. The HF boost is a lead-lag network that attenuates the voltage the FB resistor sees, reducing the effect of its parallel capacitance. The first one can easily be a dpot, but the second one is a bit more of a challenge. It's not too hard to level-shift unidirectional I2c up to the op amp's output--a PNP CB stage with the base grounded, followed by an NPN CB stage with the base about 3V above the negative supply rail, as in the LTspice files below (i2cls.lib and I2Cshifter.asc. The dpot can be an AD5273BRJZ1, whose 6 MHz bandwidth will work fine. Problem is, at high frequency most of the signal swing appears across the dpot, so if I do it this way I'll have to reduce the supplies. I'm considering two things: first, leaving the resistors fixed and switching the caps in and out with a high-voltage mux, or else (more interestingly) transformer-coupling the dpot. The first It'll need a fair amount of inductance to look like 1k ohms at 200 kHz and above, probably a couple of mH, and a turns ratio of 2:1 or so to reduce the voltage swing. Anybody here done something like that? 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 --------------FF67D67D34458B072894D837 Content-Type: text/plain; charset=UTF-8; name="12CshifterOpAmp.asc" Content-Transfer-Encoding: base64 Content-Disposition: attachment; filename="12CshifterOpAmp.asc" VmVyc2lvbiA0ClNIRUVUIDEgMTQyOCA3MDAKV0lSRSAxNjAgLTQ0OCAxNDQgLTQ0OApXSVJF IDI4OCAtNDQ4IDI3MiAtNDQ4CldJUkUgNDAwIC00NDggMzg0IC00NDgKV0lSRSAtNTQ0IC00 MzIgLTcyMCAtNDMyCldJUkUgLTQzMiAtNDMyIC01NDQgLTQzMgpXSVJFIC0zNTIgLTQzMiAt NDMyIC00MzIKV0lSRSAtMjI0IC00MzIgLTM1MiAtNDMyCldJUkUgLTY0IC00MzIgLTIyNCAt NDMyCldJUkUgMTYwIC00MTYgMTYwIC00NDgKV0lSRSAyODggLTQxNiAyODggLTQ0OApXSVJF IDQwMCAtNDE2IDQwMCAtNDQ4CldJUkUgLTY0IC00MDAgLTY0IC00MzIKV0lSRSAtNTQ0IC0z NjggLTU0NCAtNDMyCldJUkUgLTQzMiAtMzY4IC00MzIgLTQzMgpXSVJFIC0xMTIgLTM2OCAt 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Transformer-coupled dpot
Started by ●August 17, 2020
Reply by ●August 18, 20202020-08-18
On Mon, 17 Aug 2020 17:30:47 -0400, Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:>So I'm doing a spin of my swoopy nanoamp photoreceiver board, to make it >smaller and more versatile. > >The original QL01 has two pots, one for offset and one for HF boost. >The HF boost is a lead-lag network that attenuates the voltage the FB >resistor sees, reducing the effect of its parallel capacitance. > >The first one can easily be a dpot, but the second one is a bit more of >a challenge. It's not too hard to level-shift unidirectional I2c up to >the op amp's output--a PNP CB stage with the base grounded, followed by >an NPN CB stage with the base about 3V above the negative supply rail, >as in the LTspice files below (i2cls.lib and I2Cshifter.asc. The dpot >can be an AD5273BRJZ1, whose 6 MHz bandwidth will work fine. > >Problem is, at high frequency most of the signal swing appears across >the dpot, so if I do it this way I'll have to reduce the supplies. > >I'm considering two things: first, leaving the resistors fixed and >switching the caps in and out with a high-voltage mux, or else (more >interestingly) transformer-coupling the dpot. The first > >It'll need a fair amount of inductance to look like 1k ohms at 200 kHz >and above, probably a couple of mH, and a turns ratio of 2:1 or so to >reduce the voltage swing. > >Anybody here done something like that? > >Cheers > >Phil HobbsI guess the object of the level shifter is to interface with the floating dpot. The sim can't find the TLV431 model, which I guess is just a sort of floating power supply. A transformer is an interesting way to isolate a variable part, like a pot or a varicap or something. We use COILCRAFT PWB1010LB which is a 780 uH 1:1 transformer spec'd for 3 KHz to 125 MHz. but we use it for signal coupling, not "impedance isolation." Why not a varicap to ground, inside the feedback loop? Sorta in place of C3. -- John Larkin Highland Technology, Inc Science teaches us to doubt. Claude Bernard
Reply by ●August 18, 20202020-08-18
On Mon, 17 Aug 2020 17:30:47 -0400, Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:>So I'm doing a spin of my swoopy nanoamp photoreceiver board, to make it >smaller and more versatile. > >The original QL01 has two pots, one for offset and one for HF boost. >The HF boost is a lead-lag network that attenuates the voltage the FB >resistor sees, reducing the effect of its parallel capacitance. > >The first one can easily be a dpot, but the second one is a bit more of >a challenge. It's not too hard to level-shift unidirectional I2c up to >the op amp's output--a PNP CB stage with the base grounded, followed by >an NPN CB stage with the base about 3V above the negative supply rail, >as in the LTspice files below (i2cls.lib and I2Cshifter.asc. The dpot >can be an AD5273BRJZ1, whose 6 MHz bandwidth will work fine. > >Problem is, at high frequency most of the signal swing appears across >the dpot, so if I do it this way I'll have to reduce the supplies. > >I'm considering two things: first, leaving the resistors fixed and >switching the caps in and out with a high-voltage mux, or else (more >interestingly) transformer-coupling the dpot. The first > >It'll need a fair amount of inductance to look like 1k ohms at 200 kHz >and above, probably a couple of mH, and a turns ratio of 2:1 or so to >reduce the voltage swing. > >Anybody here done something like that? > >Cheers > >Phil HobbsYou might use a digital capacitor to tweak HF response. https://www.mouser.com/Passive-Components/Capacitors/Trimmer-Variable-Capacitors/_/N-5g7r?keyword=digital -- John Larkin Highland Technology, Inc Science teaches us to doubt. Claude Bernard
Reply by ●August 18, 20202020-08-18
On 2020-08-18 10:19, jlarkin@highlandsniptechnology.com wrote:> On Mon, 17 Aug 2020 17:30:47 -0400, Phil Hobbs > <pcdhSpamMeSenseless@electrooptical.net> wrote: > >> So I'm doing a spin of my swoopy nanoamp photoreceiver board, to >> make it smaller and more versatile. >> >> The original QL01 has two pots, one for offset and one for HF >> boost. The HF boost is a lead-lag network that attenuates the >> voltage the FB resistor sees, reducing the effect of its parallel >> capacitance. >> >> The first one can easily be a dpot, but the second one is a bit >> more of a challenge. It's not too hard to level-shift >> unidirectional I2c up to the op amp's output--a PNP CB stage with >> the base grounded, followed by an NPN CB stage with the base about >> 3V above the negative supply rail, as in the LTspice files below >> (i2cls.lib and I2Cshifter.asc. The dpot can be an AD5273BRJZ1, >> whose 6 MHz bandwidth will work fine. >> >> Problem is, at high frequency most of the signal swing appears >> across the dpot, so if I do it this way I'll have to reduce the >> supplies. >> >> I'm considering two things: first, leaving the resistors fixed and >> switching the caps in and out with a high-voltage mux, or else >> (more interestingly) transformer-coupling the dpot. The first >> >> It'll need a fair amount of inductance to look like 1k ohms at 200 >> kHz and above, probably a couple of mH, and a turns ratio of 2:1 or >> so to reduce the voltage swing. >> >> Anybody here done something like that? >> >> Cheers >> >> Phil Hobbs > > You might use a digital capacitor to tweak HF response. > > https://www.mouser.com/Passive-Components/Capacitors/Trimmer-Variable-Capacitors/_/N-5g7r?keyword=digitalThose are very pretty. I sort of need at least a 10V swing, though. 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
Reply by ●August 18, 20202020-08-18
On Tue, 18 Aug 2020 10:58:30 -0400, Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:>On 2020-08-18 10:19, jlarkin@highlandsniptechnology.com wrote: >> On Mon, 17 Aug 2020 17:30:47 -0400, Phil Hobbs >> <pcdhSpamMeSenseless@electrooptical.net> wrote: >> >>> So I'm doing a spin of my swoopy nanoamp photoreceiver board, to >>> make it smaller and more versatile. >>> >>> The original QL01 has two pots, one for offset and one for HF >>> boost. The HF boost is a lead-lag network that attenuates the >>> voltage the FB resistor sees, reducing the effect of its parallel >>> capacitance. >>> >>> The first one can easily be a dpot, but the second one is a bit >>> more of a challenge. It's not too hard to level-shift >>> unidirectional I2c up to the op amp's output--a PNP CB stage with >>> the base grounded, followed by an NPN CB stage with the base about >>> 3V above the negative supply rail, as in the LTspice files below >>> (i2cls.lib and I2Cshifter.asc. The dpot can be an AD5273BRJZ1, >>> whose 6 MHz bandwidth will work fine. >>> >>> Problem is, at high frequency most of the signal swing appears >>> across the dpot, so if I do it this way I'll have to reduce the >>> supplies. >>> >>> I'm considering two things: first, leaving the resistors fixed and >>> switching the caps in and out with a high-voltage mux, or else >>> (more interestingly) transformer-coupling the dpot. The first >>> >>> It'll need a fair amount of inductance to look like 1k ohms at 200 >>> kHz and above, probably a couple of mH, and a turns ratio of 2:1 or >>> so to reduce the voltage swing. >>> >>> Anybody here done something like that? >>> >>> Cheers >>> >>> Phil Hobbs >> >> You might use a digital capacitor to tweak HF response. >> >> https://www.mouser.com/Passive-Components/Capacitors/Trimmer-Variable-Capacitors/_/N-5g7r?keyword=digital > >Those are very pretty. I sort of need at least a 10V swing, though. > >Cheers > >Phil HobbsSlide it a little closer to the summing point maybe? -- John Larkin Highland Technology, Inc Science teaches us to doubt. Claude Bernard
Reply by ●August 18, 20202020-08-18
On 2020-08-18 11:05, jlarkin@highlandsniptechnology.com wrote:> On Tue, 18 Aug 2020 10:58:30 -0400, Phil Hobbs > <pcdhSpamMeSenseless@electrooptical.net> wrote: > >> On 2020-08-18 10:19, jlarkin@highlandsniptechnology.com wrote: >>> On Mon, 17 Aug 2020 17:30:47 -0400, Phil Hobbs >>> <pcdhSpamMeSenseless@electrooptical.net> wrote: >>> >>>> So I'm doing a spin of my swoopy nanoamp photoreceiver board, to >>>> make it smaller and more versatile. >>>> >>>> The original QL01 has two pots, one for offset and one for HF >>>> boost. The HF boost is a lead-lag network that attenuates the >>>> voltage the FB resistor sees, reducing the effect of its parallel >>>> capacitance. >>>> >>>> The first one can easily be a dpot, but the second one is a bit >>>> more of a challenge. It's not too hard to level-shift >>>> unidirectional I2c up to the op amp's output--a PNP CB stage with >>>> the base grounded, followed by an NPN CB stage with the base about >>>> 3V above the negative supply rail, as in the LTspice files below >>>> (i2cls.lib and I2Cshifter.asc. The dpot can be an AD5273BRJZ1, >>>> whose 6 MHz bandwidth will work fine. >>>> >>>> Problem is, at high frequency most of the signal swing appears >>>> across the dpot, so if I do it this way I'll have to reduce the >>>> supplies. >>>> >>>> I'm considering two things: first, leaving the resistors fixed and >>>> switching the caps in and out with a high-voltage mux, or else >>>> (more interestingly) transformer-coupling the dpot. The first >>>> >>>> It'll need a fair amount of inductance to look like 1k ohms at 200 >>>> kHz and above, probably a couple of mH, and a turns ratio of 2:1 or >>>> so to reduce the voltage swing. >>>> >>>> Anybody here done something like that? >>>> >>>> Cheers >>>> >>>> Phil Hobbs >>> >>> You might use a digital capacitor to tweak HF response. >>> >>> https://www.mouser.com/Passive-Components/Capacitors/Trimmer-Variable-Capacitors/_/N-5g7r?keyword=digital >> >> Those are very pretty. I sort of need at least a 10V swing, though. >> >> Cheers >> >> Phil Hobbs > > Slide it a little closer to the summing point maybe? > > >Hard to do without destroying the SNR, because the larger Johnson noise of the small segment of the feedback resistor can flow through the lead-lag network. The same thing happens if you try splitting the resistor and bootstrapping the midpoint via a lower-resistance string--the SNR is the same as if you just used the first resistor by itself. If transformer coupling a dpot doesn't work, I'll probably wind up switching capacitors to ground with a DG409 or something like that. The caps would be in the hundreds of pF, in series with 120 ohms or so, which wouldn't tax the HV mux's capabilities too much. I'd really like to use a dpot though, because that can be programmed at test time whereas using a mux would mean I'd need a micro on a hitherto all-analogue board. Mini-Circuits sells transformers that work down to 4 kHz at 50 ohms, so ISTM they should work at 40 kHz at 500 ohms. We'll see. 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
Reply by ●August 18, 20202020-08-18
This is a multi-part message in MIME format. --------------EBC507EA42E5FA4349FF5C52 Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 8bit On 2020-08-18 11:27, Phil Hobbs wrote:> On 2020-08-18 11:05, jlarkin@highlandsniptechnology.com wrote: >> On Tue, 18 Aug 2020 10:58:30 -0400, Phil Hobbs >> <pcdhSpamMeSenseless@electrooptical.net> wrote: >> >>> On 2020-08-18 10:19, jlarkin@highlandsniptechnology.com wrote: >>>> On Mon, 17 Aug 2020 17:30:47 -0400, Phil Hobbs >>>> <pcdhSpamMeSenseless@electrooptical.net> wrote: >>>> >>>>> So I'm doing a spin of my swoopy nanoamp photoreceiver board, to >>>>> make it smaller and more versatile. >>>>> >>>>> The original QL01 has two pots, one for offset and one for HF >>>>> boost. The HF boost is a lead-lag network that attenuates the >>>>> voltage the FB resistor sees, reducing the effect of its parallel >>>>> capacitance. >>>>> >>>>> The first one can easily be a dpot, but the second one is a bit >>>>> more of a challenge. It's not too hard to level-shift >>>>> unidirectional I2c up to the op amp's output--a PNP CB stage with >>>>> the base grounded, followed by an NPN CB stage with the base about >>>>> 3V above the negative supply rail, as in the LTspice files below >>>>> (i2cls.lib and I2Cshifter.asc. The dpot can be an AD5273BRJZ1, >>>>> whose 6 MHz bandwidth will work fine. >>>>> >>>>> Problem is, at high frequency most of the signal swing appears >>>>> across the dpot, so if I do it this way I'll have to reduce the >>>>> supplies. >>>>> >>>>> I'm considering two things: first, leaving the resistors fixed and >>>>> switching the caps in and out with a high-voltage mux, or else >>>>> (more interestingly) transformer-coupling the dpot. The first >>>>> >>>>> It'll need a fair amount of inductance to look like 1k ohms at 200 >>>>> kHz and above, probably a couple of mH, and a turns ratio of 2:1 or >>>>> so to reduce the voltage swing. >>>>> >>>>> Anybody here done something like that? >>>>> >>>>> Cheers >>>>> >>>>> Phil Hobbs >>>> >>>> You might use a digital capacitor to tweak HF response. >>>> >>>> https://www.mouser.com/Passive-Components/Capacitors/Trimmer-Variable-Capacitors/_/N-5g7r?keyword=digital >>>> >>> >>> Those are very pretty. I sort of need at least a 10V swing, though. >>> >>> Cheers >>> >>> Phil Hobbs >> >> Slide it a little closer to the summing point maybe? >> >> >> > Hard to do without destroying the SNR, because the larger Johnson noise > of the small segment of the feedback resistor can flow through the > lead-lag network. The same thing happens if you try splitting the > resistor and bootstrapping the midpoint via a lower-resistance > string--the SNR is the same as if you just used the first resistor by > itself. > > If transformer coupling a dpot doesn't work, I'll probably wind up > switching capacitors to ground with a DG409 or something like that. The > caps would be in the hundreds of pF, in series with 120 ohms or so, > which wouldn't tax the HV mux's capabilities too much. > > I'd really like to use a dpot though, because that can be programmed at > test time whereas using a mux would mean I'd need a micro on a hitherto > all-analogue board. > > Mini-Circuits sells transformers that work down to 4 kHz at 50 ohms, so > ISTM they should work at 40 kHz at 500 ohms. We'll see. >Looks like it should work if the coupling is high enough (see attached)--hopefully these will work better since there's no custom symbol). 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 --------------EBC507EA42E5FA4349FF5C52 Content-Type: text/plain; charset=UTF-8; name="TransformerDPOT.asc" Content-Transfer-Encoding: base64 Content-Disposition: attachment; filename="TransformerDPOT.asc" VmVyc2lvbiA0ClNIRUVUIDEgMTQyOCA3MDAKV0lSRSAtMTEzNiAtNDQ4IC0xMTg0IC00NDgK V0lSRSAtMTAyNCAtNDQ4IC0xMDcyIC00NDgKV0lSRSAtMzY4IC00MzIgLTM4NCAtNDMyCldJ UkUgLTI1NiAtNDMyIC0yNzIgLTQzMgpXSVJFIC0zNjggLTQwMCAtMzY4IC00MzIKV0lSRSAt MjU2IC00MDAgLTI1NiAtNDMyCldJUkUgLTExODQgLTM4NCAtMTE4NCAtNDQ4CldJUkUgLTEx ODQgLTM4NCAtMTIzMiAtMzg0CldJUkUgLTExMzYgLTM4NCAtMTE4NCAtMzg0CldJUkUgLTEw MjQgLTM4NCAtMTAyNCAtNDQ4CldJUkUgLTEwMjQgLTM4NCAtMTA1NiAtMzg0CldJUkUgLTk3 NiAtMzg0IC0xMDI0IC0zODQKV0lSRSAtODgwIC0zODQgLTk3NiAtMzg0CldJUkUgLTc2OCAt 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Reply by ●August 18, 20202020-08-18
On Tue, 18 Aug 2020 14:03:11 -0400, Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:>On 2020-08-18 11:27, Phil Hobbs wrote: >> On 2020-08-18 11:05, jlarkin@highlandsniptechnology.com wrote: >>> On Tue, 18 Aug 2020 10:58:30 -0400, Phil Hobbs >>> <pcdhSpamMeSenseless@electrooptical.net> wrote: >>> >>>> On 2020-08-18 10:19, jlarkin@highlandsniptechnology.com wrote: >>>>> On Mon, 17 Aug 2020 17:30:47 -0400, Phil Hobbs >>>>> <pcdhSpamMeSenseless@electrooptical.net> wrote: >>>>> >>>>>> So I'm doing a spin of my swoopy nanoamp photoreceiver board, to >>>>>> make it smaller and more versatile. >>>>>> >>>>>> The original QL01 has two pots, one for offset and one for HF >>>>>> boost. The HF boost is a lead-lag network that attenuates the >>>>>> voltage the FB resistor sees, reducing the effect of its parallel >>>>>> capacitance. >>>>>> >>>>>> The first one can easily be a dpot, but the second one is a bit >>>>>> more of a challenge.� It's not too hard to level-shift >>>>>> unidirectional I2c up to the op amp's output--a PNP CB stage with >>>>>> the base grounded, followed by an NPN CB stage with the base about >>>>>> 3V above the negative supply rail, as in the LTspice files below >>>>>> (i2cls.lib and I2Cshifter.asc.� The dpot can be an AD5273BRJZ1, >>>>>> whose 6 MHz bandwidth will work fine. >>>>>> >>>>>> Problem is, at high frequency most of the signal swing appears >>>>>> across the dpot, so if I do it this way I'll have to reduce the >>>>>> supplies. >>>>>> >>>>>> I'm considering two things: first, leaving the resistors fixed and >>>>>> switching the caps in and out with a high-voltage mux, or else >>>>>> (more interestingly) transformer-coupling the dpot.� The first >>>>>> >>>>>> It'll need a fair amount of inductance to look like 1k ohms at 200 >>>>>> kHz and above, probably a couple of mH, and a turns ratio of 2:1 or >>>>>> so to reduce the voltage swing. >>>>>> >>>>>> Anybody here done something like that? >>>>>> >>>>>> Cheers >>>>>> >>>>>> Phil Hobbs >>>>> >>>>> You might use a digital capacitor to tweak HF response. >>>>> >>>>> https://www.mouser.com/Passive-Components/Capacitors/Trimmer-Variable-Capacitors/_/N-5g7r?keyword=digital >>>>> >>>> >>>> Those are very pretty.� I sort of need at least a 10V swing, though. >>>> >>>> Cheers >>>> >>>> Phil Hobbs >>> >>> Slide it a little closer to the summing point maybe? >>> >>> >>> >> Hard to do without destroying the SNR, because the larger Johnson noise >> of the small segment of the feedback resistor can flow through the >> lead-lag network.� The same thing happens if you try splitting the >> resistor and bootstrapping the midpoint via a lower-resistance >> string--the SNR is the same as if you just used the first resistor by >> itself. >> >> If transformer coupling a dpot doesn't work, I'll probably wind up >> switching capacitors to ground with a DG409 or something like that. The >> caps would be in the hundreds of pF, in series with 120 ohms or so, >> which wouldn't tax the HV mux's capabilities too much. >> >> I'd really like to use a dpot though, because that can be programmed at >> test time whereas using a mux would mean I'd need a micro on a hitherto >> all-analogue board. >> >> Mini-Circuits sells transformers that work down to 4 kHz at 50 ohms, so >> ISTM they should work at 40 kHz at 500 ohms.� We'll see. >> > >Looks like it should work if the coupling is high enough (see >attached)--hopefully these will work better since there's no custom symbol). > >Cheers > >Phil HobbsSeems to me that the transformer doesn't do much. Couldn't you use the dpot as a pot, with one end grounded, one end driven from the amp (with a resistor maybe) and the wiper driving the compensation?
Reply by ●August 18, 20202020-08-18
On 2020-08-18 14:52, John Larkin wrote:> On Tue, 18 Aug 2020 14:03:11 -0400, Phil Hobbs > <pcdhSpamMeSenseless@electrooptical.net> wrote: > >> On 2020-08-18 11:27, Phil Hobbs wrote: >>> On 2020-08-18 11:05, jlarkin@highlandsniptechnology.com wrote: >>>> On Tue, 18 Aug 2020 10:58:30 -0400, Phil Hobbs >>>> <pcdhSpamMeSenseless@electrooptical.net> wrote: >>>> >>>>> On 2020-08-18 10:19, jlarkin@highlandsniptechnology.com >>>>> wrote: >>>>>> On Mon, 17 Aug 2020 17:30:47 -0400, Phil Hobbs >>>>>> <pcdhSpamMeSenseless@electrooptical.net> wrote: >>>>>> >>>>>>> So I'm doing a spin of my swoopy nanoamp photoreceiver >>>>>>> board, to make it smaller and more versatile. >>>>>>> >>>>>>> The original QL01 has two pots, one for offset and one >>>>>>> for HF boost. The HF boost is a lead-lag network that >>>>>>> attenuates the voltage the FB resistor sees, reducing >>>>>>> the effect of its parallel capacitance. >>>>>>> >>>>>>> The first one can easily be a dpot, but the second one >>>>>>> is a bit more of a challenge. It's not too hard to >>>>>>> level-shift unidirectional I2c up to the op amp's >>>>>>> output--a PNP CB stage with the base grounded, followed >>>>>>> by an NPN CB stage with the base about 3V above the >>>>>>> negative supply rail, as in the LTspice files below >>>>>>> (i2cls.lib and I2Cshifter.asc. The dpot can be an >>>>>>> AD5273BRJZ1, whose 6 MHz bandwidth will work fine. >>>>>>> >>>>>>> Problem is, at high frequency most of the signal swing >>>>>>> appears across the dpot, so if I do it this way I'll >>>>>>> have to reduce the supplies. >>>>>>> >>>>>>> I'm considering two things: first, leaving the resistors >>>>>>> fixed and switching the caps in and out with a >>>>>>> high-voltage mux, or else (more interestingly) >>>>>>> transformer-coupling the dpot. The first >>>>>>> >>>>>>> It'll need a fair amount of inductance to look like 1k >>>>>>> ohms at 200 kHz and above, probably a couple of mH, and >>>>>>> a turns ratio of 2:1 or so to reduce the voltage swing. >>>>>>> >>>>>>> Anybody here done something like that? >>>>>>> >>>>>>> Cheers >>>>>>> >>>>>>> Phil Hobbs >>>>>> >>>>>> You might use a digital capacitor to tweak HF response. >>>>>> >>>>>> https://www.mouser.com/Passive-Components/Capacitors/Trimmer-Variable-Capacitors/_/N-5g7r?keyword=digital >>>>>> >>>>> >>>>> >>>>>> >>>>>>Those are very pretty. I sort of need at least a 10V swing, though.>>>>> >>>>> Cheers >>>>> >>>>> Phil Hobbs >>>> >>>> Slide it a little closer to the summing point maybe? >>>> >>>> >>>> >>> Hard to do without destroying the SNR, because the larger >>> Johnson noise of the small segment of the feedback resistor can >>> flow through the lead-lag network. The same thing happens if you >>> try splitting the resistor and bootstrapping the midpoint via a >>> lower-resistance string--the SNR is the same as if you just used >>> the first resistor by itself. >>> >>> If transformer coupling a dpot doesn't work, I'll probably wind >>> up switching capacitors to ground with a DG409 or something like >>> that. The caps would be in the hundreds of pF, in series with >>> 120 ohms or so, which wouldn't tax the HV mux's capabilities too >>> much. >>> >>> I'd really like to use a dpot though, because that can be >>> programmed at test time whereas using a mux would mean I'd need >>> a micro on a hitherto all-analogue board. >>> >>> Mini-Circuits sells transformers that work down to 4 kHz at 50 >>> ohms, so ISTM they should work at 40 kHz at 500 ohms. We'll >>> see. >>> >> >> Looks like it should work if the coupling is high enough (see >> attached)--hopefully these will work better since there's no >> custom symbol).> > Seems to me that the transformer doesn't do much.From a compensation POV, that's a feature rather than a bug. ;) The transformer more than halves the voltage applied to the dpot, so I can use AD5273BRJZ1's with a 6V supply and hang one end on the +3.3V rail. It also eliminates the need to level-shift the I2C.> Couldn't you use the dpot as a pot, with one end grounded, one end > driven from the amp (with a resistor maybe) and the wiper driving > the compensation?I'd have to drop the power supplies way down to avoid frying the dpot, so the feedback resistor would have to go way down as well, which would hurt the SNR at low currents. The Johnson noise of the feedback resistor equals the shot noise when it's dropping 2kT/e (50 mV at room temperature). With 10M, that's 5 nA, but with 2M it's 25 nA, and I care about that quite a lot. I have some of these on order. Both have 10 kHz minimum frequency quoted. <https://www.minicircuits.com/WebStore/dashboard.html?model=T-626%2B> which is 1:1:1, , and <https://www.minicircuits.com/WebStore/dashboard.html?model=ADT4-6T%2B> which is 1:4 Using my trusty AADE, I measured a T622 1:1:1 (100 kHz-200 MHz). It was 35 uH per winding open, 35 nH with one winding shorted. Since L_sc/L_oc = 1-k**2, that works out to k = 0.9995. If the ratios are the same, the 10 kHz 4:1 will be about 350 uH on the primary and 1.4 mH on the secondary. That leads to a bit of tilt on the pulse tops unless I halve the impedance level of the lead-lag network. I can probably use a fixed resistor there to do most of the work, and run the dpot with the two ends shorted, which gives vernier action near half-scale. It might be easier to fix the tilt in the second stage by giving a bit of a LF gain cut. Fun. 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
Reply by ●August 18, 20202020-08-18
On 19/08/2020 01:27, Phil Hobbs wrote:> I'd really like to use a dpot though, because that can be programmed at > test time whereas using a mux would mean I'd need a micro on a hitherto > all-analogue board.If you're going to program it at test, could you put two or three DPOTs in series (so less voltage across each one), and power them either with photovoltaic isolators, or some fancy op-amp bootstrapping circuit. Yes the SPI level shifters could get interesting, but those could be a one-off monstrosity residing in your tester, behind the pogo pins or whatever.
