On Friday, October 20, 2017 at 11:18:00 PM UTC-4, John Larkin wrote:> On Fri, 20 Oct 2017 18:36:05 -0700 (PDT), George Herold > <gherold@teachspin.com> wrote: > > >On Friday, October 20, 2017 at 9:03:52 PM UTC-4, John Larkin wrote: > >> On Fri, 20 Oct 2017 17:42:04 -0700 (PDT), George Herold > >> <gherold@teachspin.com> wrote: > >> > >> >On Friday, October 20, 2017 at 5:21:23 PM UTC-4, John Larkin wrote: > >> >> On Thu, 19 Oct 2017 18:00:12 -0700 (PDT), George Herold > >> >> <gherold@teachspin.com> wrote: > >> >> > >> >> >On Thursday, October 19, 2017 at 5:30:45 PM UTC-4, dagmarg...@yahoo.com wrote: > >> >> >> On Thursday, October 19, 2017 at 4:42:09 PM UTC-4, George Herold wrote: > >> >> >> > Hi all, months ago I bought this screaming fet opamp (THS463) to gin up an old photodiode circuit. Put it in today, and the step response doesn't look right. Here's 1 k ohm of gain w/ 10 pF across it. > >> >> >> > > >> >> >> > https://www.dropbox.com/s/ncswyuokm9hktb5/step-10pF.BMP?dl=0 > >> >> >> > A lot of wiggles, but the amplitude isn't big enough. > >> >> >> > > >> >> >> > Here's the uncompensated step. > >> >> >> > https://www.dropbox.com/s/rilf9gvmlwldajl/step.BMP?dl=0 > >> >> >> > (note time base change) > >> >> >> > > >> >> >> > It's only a 40 MHz 'scope so I'll try something faster. > >> >> >> > Maybe it's just the LED I'm blinking isn't fast enough... > >> >> >> > Chan. 1 (upper) is the voltage into the LED... Or my > >> >> >> > voltage step isn't fast enough. > >> >> >> > > >> >> >> > I could try a step with a 74HC14... hmm there was some flavor that > >> >> >> > was faster than the 'HC14? > >> >> >> > > >> >> >> > George H. > >> >> >> > >> >> >> Georrggggeeee, have you been using the white solderless breadboard again? > >> >> >> > >> >> >> Grins, > >> >> >> James Arthur > >> >> > > >> >> >Grin.. no but its a PD circuit I did ~15 years ago, big switch capacitance. > >> >> >I'm tuning out that and the PD capacitance. > >> >> > > >> >> >George H. > >> >> > >> >> You may be stuck with an old pcb layout, > >> > > >> >Once I terminated the coax, I didn't need any FB caps!(?) > >> >gain from 0.33k to 10M ~15MHz BW w/1k, but the gain > >> >dropped off 'too fast' above 33k ohm > >> >I'm reproducing this data from memory... data book's at work, > >> >(I'm 1/2 making up the numbers above 100k where it dropped > >> >off as a single pole.. the 'stray' feed back capacitance > >> >I assume.) > >> > > >> >Gain R 3dB > >> >1k 15M > >> >3.3k 9M > >> >10k 7M > >> >33k 4M > >> >100k 1.5M > >> >333k 400k > >> >1M 130k? > >> >3.3M 40k > >> >10M 15k? > >> > > >> >Putting in some numbers it looks like maybe > >> >1 pF a bit more, (I might have made a small math > >> >mistake.) ..but that's about what I would guess. > >> > > >> >I should do a new layout... I've been sending > >> >off a bunch of proto's to advanced circuits. > >> > I could stick a little pcb in the corner or on the end. > >> > > >> >'thinking about my pcb layout.' > >> >I might have some traces to the feed back R's lying over > >> >each other... a razor blade might make it better too! > >> > > >> >> but a simple cascode > >> >> transistor can really isolate the pd capacitance from the TIA > >> >> amplifier input, which can radically increase bandwidth and reduce the > >> >> feedback cap value. > >> >> > >> >> That is half of Phil's bootstrap cascode. > >> >Right, I figure I'd try the bootstrap 1/2 first. > >> >(I've got 100 of Phil's fav. jfet that's eol.) > >> >Does the cascode do anything weird at low currents, > >> >(besides no work as well.) > >> > >> One normally adds a resistor to donate a bit of extra current into the > >> cascode emitter to keep the emitter impedance down at low pd currents. > >> That makes a DC error at the TIA output, but that's easy to subtract > >> out. > >But what happens if you don't? It's a big R in series? Is that right? > >'scribble' > > Yes. The input impedance Re at the emitter of the cascode is 25/Ie, > where Ie is transistor current in mA. So at 1 uA, the impedance is > 25K. What will happen is that diode conduction and maybe leakage > current will pull the emitter positive in the dark. > > https://www.dropbox.com/s/983wckcv1o2e58l/Cascode_TIA.JPG?raw=1 > > So when the light hits, the emitter current is low, the Cpd*Re time > constant is long, and the current builds up slowly as the emitter > current creeps up. Adding Rb pulls the emitter down and keeps Re low. > > You can add Rx, or something downstream of the TIA amp, to null out > the current in Rb. > > Here's an over-the-top version: > > https://www.dropbox.com/s/xczb3j6td9vmo69/J712B.pdf?dl=0 > > > >Let's say 10 nA of current (100mV @10M ohm) > >so it's V_thermal/I = 40 M ohm... > >I guess I could live with some offset at the highest gain, > >most of the time there's uA's of current. > >Why not a bootstrap first? > >well second, after the FB capacitance... > >maybe I need to boot strap that first! > > Booting is great for slowish (MHz range) high capacitance photodiodes. > It's usually not a good idea for fast stuff.I guess I'm talking/thinking about adding some driven shield plane around the inverting input, try to block some of the ~1.5pf of FB capacitance. 1MHz is fine, I could use a dual opamp. George H .> John Larkin Highland Technology, Inc > > lunatic fringe electronics
TIA-PD compensation, by eye
Started by ●October 19, 2017
Reply by ●October 21, 20172017-10-21
Reply by ●October 21, 20172017-10-21
On Saturday, October 21, 2017 at 10:50:49 AM UTC-4, Joerg wrote:> On 2017-10-20 18:09, George Herold wrote: > > On Friday, October 20, 2017 at 7:36:18 PM UTC-4, Joerg wrote: > >> On 2017-10-20 14:32, Joerg wrote: > >>> On 2017-10-19 18:17, George Herold wrote: > >>>> On Thursday, October 19, 2017 at 7:33:58 PM UTC-4, John Larkin wrote: > >>>>> On Thu, 19 Oct 2017 18:57:20 -0400, Martin Riddle > >>>>> <martin_ridd@verizon.net> wrote: > >>>>> > >>>>>> On Thu, 19 Oct 2017 14:30:41 -0700 (PDT), dagmargoodboat@yahoo.com > >>>>>> wrote: > >>>>>> > >>>>>>> On Thursday, October 19, 2017 at 4:42:09 PM UTC-4, George Herold > >>>>>>> wrote: > >>>>>>>> Hi all, months ago I bought this screaming fet opamp (THS463) to > >>>>>>>> gin up an old photodiode circuit. Put it in today, and the step > >>>>>>>> response doesn't look right. Here's 1 k ohm of gain w/ 10 pF > >>>>>>>> across it. > >>>>>>>> > >>>>>>>> https://www.dropbox.com/s/ncswyuokm9hktb5/step-10pF.BMP?dl=0 > >>>>>>>> A lot of wiggles, but the amplitude isn't big enough. > >>>>>>>> > >>>>>>>> Here's the uncompensated step. > >>>>>>>> https://www.dropbox.com/s/rilf9gvmlwldajl/step.BMP?dl=0 > >>>>>>>> (note time base change) > >>>>>>>> > >>>>>>>> It's only a 40 MHz 'scope so I'll try something faster. > >>>>>>>> Maybe it's just the LED I'm blinking isn't fast enough... > >>>>>>>> Chan. 1 (upper) is the voltage into the LED... Or my > >>>>>>>> voltage step isn't fast enough. > >>>>>>>> > >>>>>>>> I could try a step with a 74HC14... hmm there was some flavor that > >>>>>>>> was faster than the 'HC14? > >>>>>>>> > >>>>>>>> George H. > >>>>>>> > >>>>>>> Georrggggeeee, have you been using the white solderless breadboard > >>>>>>> again? > >>>>>>> > >>>>>>> Grins, > >>>>>>> James Arthur > >>>>>> > >>>>>> I was going to say ground lead oscillations, but that is a low > >>>>>> frequency. > >>>>>> > >>>>>> Cheers > >>>>> > >>>>> If this is a classic vanilla TIA, it may be the photodiode capacitance > >>>>> at the summing point making it ring. A bigger feedback cap, or a > >>>>> cascode thing, can fix that. I don't know why the amplitude should > >>>>> change. > >>>> > >>>> Right, Sorry, classic vanilla TIA-PD. > >>>> I'm just picking the tuning cap by eye... shape > >>>> of the step response. But the shape doesn't look right. > >>>> I've tuned a few TIA's with slower opamps and maybe > >>>> more C on the inverting input. It has this > >>>> classic two pole shape. You can add more feeed back C and pick > >>>> your Q by eye. This doesn't, so it's my light pulse > >>>> or my electronics. I'm mostly stuck with the old pcb layout > >>>> so I'm just trying to make sure my light source is not the > >>>> problem, or fix it if it is. > >>>> > >>>> Instead of an led, I could wiggle a diode laser, that's > >>>> less convenient. > >>> > >>> > >>> LEDs are ok. They can be pulsed blazingly fast. > >>> > >> > >> Which leaves the question which photodiode you used and how much you > >> pre-biased it. That can have a major impact on overall bandwidth. > > osi-optoelectronics 44pin (there might be some letters in there. > > or pin44) > > IIRC ~100 pF at 10V, mines biased to 12, via 10 ohm and > > 100uF Al to ground... that's hacked in too... Well, you're going > > to have to wait till Monday for the pics. > > > > The biased side of the PD needs an RF-compliant small ceramic cap to > ground. 0.1uF in 0603 package or similar, very short path to the ground > plane. Short meaning a via right next to its ground pad. The RF > impedance of an electrolytic cap is like roulette.Hmm, I can try adding a ceramic to the bias supply. There's no ground plane in this circuit. Well the whole thing lives in a grounded aluminum box, ~1"x1"x2" maybe a bit bigger. Inside, capacitance is not your friend at 1 meg ohm. (I've been measuring this outside of the Al box, I wonder if it will change by much inside?) George H.> > [...] > > -- > Regards, Joerg > > http://www.analogconsultants.com/
Reply by ●October 21, 20172017-10-21
On Saturday, October 21, 2017 at 11:08:12 AM UTC-4, Joerg wrote:> On 2017-10-20 17:56, George Herold wrote: > > On Friday, October 20, 2017 at 5:31:54 PM UTC-4, Joerg wrote: > >> On 2017-10-19 18:17, George Herold wrote: > >>> On Thursday, October 19, 2017 at 7:33:58 PM UTC-4, John Larkin wrote: > >>>> On Thu, 19 Oct 2017 18:57:20 -0400, Martin Riddle > >>>> <martin_ridd@verizon.net> wrote: > >>>> > >>>>> On Thu, 19 Oct 2017 14:30:41 -0700 (PDT), dagmargoodboat@yahoo.com > >>>>> wrote: > >>>>> > >>>>>> On Thursday, October 19, 2017 at 4:42:09 PM UTC-4, George Herold wrote: > >>>>>>> Hi all, months ago I bought this screaming fet opamp (THS463) to gin up an old photodiode circuit. Put it in today, and the step response doesn't look right. Here's 1 k ohm of gain w/ 10 pF across it. > >>>>>>> > >>>>>>> https://www.dropbox.com/s/ncswyuokm9hktb5/step-10pF.BMP?dl=0 > >>>>>>> A lot of wiggles, but the amplitude isn't big enough. > >>>>>>> > >>>>>>> Here's the uncompensated step. > >>>>>>> https://www.dropbox.com/s/rilf9gvmlwldajl/step.BMP?dl=0 > >>>>>>> (note time base change) > >>>>>>> > >>>>>>> It's only a 40 MHz 'scope so I'll try something faster. > >>>>>>> Maybe it's just the LED I'm blinking isn't fast enough... > >>>>>>> Chan. 1 (upper) is the voltage into the LED... Or my > >>>>>>> voltage step isn't fast enough. > >>>>>>> > >>>>>>> I could try a step with a 74HC14... hmm there was some flavor that > >>>>>>> was faster than the 'HC14? > >>>>>>> > >>>>>>> George H. > >>>>>> > >>>>>> Georrggggeeee, have you been using the white solderless breadboard again? > >>>>>> > >>>>>> Grins, > >>>>>> James Arthur > >>>>> > >>>>> I was going to say ground lead oscillations, but that is a low > >>>>> frequency. > >>>>> > >>>>> Cheers > >>>> > >>>> If this is a classic vanilla TIA, it may be the photodiode capacitance > >>>> at the summing point making it ring. A bigger feedback cap, or a > >>>> cascode thing, can fix that. I don't know why the amplitude should > >>>> change. > >>> > >>> Right, Sorry, classic vanilla TIA-PD. > >>> I'm just picking the tuning cap by eye... shape > >>> of the step response. But the shape doesn't look right. > >>> I've tuned a few TIA's with slower opamps and maybe > >>> more C on the inverting input. It has this > >>> classic two pole shape. You can add more feeed back C and pick > >>> your Q by eye. This doesn't, so it's my light pulse > >>> or my electronics. I'm mostly stuck with the old pcb layout > >>> so I'm just trying to make sure my light source is not the > >>> problem, or fix it if it is. > >>> > >>> Instead of an led, I could wiggle a diode laser, that's > >>> less convenient. > >> > >> > >> LEDs are ok. They can be pulsed blazingly fast. > >> > >> > >>> ... Right now I'm driving the led from a > >>> 20 MHz Rigol func. gen. Maybe that's the slow spot? > >>> > >> > >> That would be slow. Just run it through a fast gate driver or the > >> Schmitt single inverter I posted about, that'll speed it up. RF > >> transistors work as well by slamming the current from the generator into > >> the base. The gain that this provides can results in more than an order > >> of magnitude of speed-up but the turn-off will be sluggish unless you > >> use a gold-doped part like a 2N2369. Since fast transistors are usually > >> available in a parts rack or can be pilfered from scrapped gear you > >> could save the wait time for an order. > >> > >> However, you already found the ringing issue in your unterminated coax > >> canle. Later you might work on that rotary switch connected ti IN- which > >> probably adds a lot of capacitance. > >> > >> -- > >> Regards, Joerg > >> > >> http://www.analogconsultants.com/ > > > > Thanks Joerg, I took pic's of my setup/pcb, but didn't post them. > > I did make a fast edge with a 74HC14, dip, all six in parallel. > > (now I've got two led /PD test jigs. :^) > > I'd mostly like more BW at high gain 1M ohm*, 100k is ok. > > seems like stray feedback capacitance is the current problem. > > > > Phil is the expert on that stuff. He might know tricks to milk more BW > at such high current/voltage ratios. > > IMHO you should not force all that gain into one stage. It may be better > to reduce to 1-5k in the TIA and then follow with a regular amplifier > stage to do the rest.Right, that's what I'm doing at the moment, more gain down stream. In theory you have the least noise with the most gain at the front end. In practice you don't always need the least noise. George H.> > -- > Regards, Joerg > > http://www.analogconsultants.com/
Reply by ●October 22, 20172017-10-22
On Friday, October 20, 2017 at 11:17:42 AM UTC-4, George Herold wrote:> Well I built a fast(er) edge with a 74HC14. > No change. But the problem was operator error. > My ~7 feet of coax was un-terminated. > 'Duh' Dope slap!Your 'scope pics show a ringing period of about 65nS, so, a 7-foot coax's lack of termination isn't the whole explanation. Are you driving the coax with your TIA? It might not like that. Cheers, James Arthur
Reply by ●October 22, 20172017-10-22
On 2017-10-21 18:44, George Herold wrote:> On Saturday, October 21, 2017 at 10:50:49 AM UTC-4, Joerg wrote: >> On 2017-10-20 18:09, George Herold wrote: >>> On Friday, October 20, 2017 at 7:36:18 PM UTC-4, Joerg wrote: >>>> On 2017-10-20 14:32, Joerg wrote: >>>>> On 2017-10-19 18:17, George Herold wrote: >>>>>> On Thursday, October 19, 2017 at 7:33:58 PM UTC-4, John Larkin wrote: >>>>>>> On Thu, 19 Oct 2017 18:57:20 -0400, Martin Riddle >>>>>>> <martin_ridd@verizon.net> wrote: >>>>>>> >>>>>>>> On Thu, 19 Oct 2017 14:30:41 -0700 (PDT), dagmargoodboat@yahoo.com >>>>>>>> wrote: >>>>>>>> >>>>>>>>> On Thursday, October 19, 2017 at 4:42:09 PM UTC-4, George Herold >>>>>>>>> wrote: >>>>>>>>>> Hi all, months ago I bought this screaming fet opamp (THS463) to >>>>>>>>>> gin up an old photodiode circuit. Put it in today, and the step >>>>>>>>>> response doesn't look right. Here's 1 k ohm of gain w/ 10 pF >>>>>>>>>> across it. >>>>>>>>>> >>>>>>>>>> https://www.dropbox.com/s/ncswyuokm9hktb5/step-10pF.BMP?dl=0 >>>>>>>>>> A lot of wiggles, but the amplitude isn't big enough. >>>>>>>>>> >>>>>>>>>> Here's the uncompensated step. >>>>>>>>>> https://www.dropbox.com/s/rilf9gvmlwldajl/step.BMP?dl=0 >>>>>>>>>> (note time base change) >>>>>>>>>> >>>>>>>>>> It's only a 40 MHz 'scope so I'll try something faster. >>>>>>>>>> Maybe it's just the LED I'm blinking isn't fast enough... >>>>>>>>>> Chan. 1 (upper) is the voltage into the LED... Or my >>>>>>>>>> voltage step isn't fast enough. >>>>>>>>>> >>>>>>>>>> I could try a step with a 74HC14... hmm there was some flavor that >>>>>>>>>> was faster than the 'HC14? >>>>>>>>>> >>>>>>>>>> George H. >>>>>>>>> >>>>>>>>> Georrggggeeee, have you been using the white solderless breadboard >>>>>>>>> again? >>>>>>>>> >>>>>>>>> Grins, >>>>>>>>> James Arthur >>>>>>>> >>>>>>>> I was going to say ground lead oscillations, but that is a low >>>>>>>> frequency. >>>>>>>> >>>>>>>> Cheers >>>>>>> >>>>>>> If this is a classic vanilla TIA, it may be the photodiode capacitance >>>>>>> at the summing point making it ring. A bigger feedback cap, or a >>>>>>> cascode thing, can fix that. I don't know why the amplitude should >>>>>>> change. >>>>>> >>>>>> Right, Sorry, classic vanilla TIA-PD. >>>>>> I'm just picking the tuning cap by eye... shape >>>>>> of the step response. But the shape doesn't look right. >>>>>> I've tuned a few TIA's with slower opamps and maybe >>>>>> more C on the inverting input. It has this >>>>>> classic two pole shape. You can add more feeed back C and pick >>>>>> your Q by eye. This doesn't, so it's my light pulse >>>>>> or my electronics. I'm mostly stuck with the old pcb layout >>>>>> so I'm just trying to make sure my light source is not the >>>>>> problem, or fix it if it is. >>>>>> >>>>>> Instead of an led, I could wiggle a diode laser, that's >>>>>> less convenient. >>>>> >>>>> >>>>> LEDs are ok. They can be pulsed blazingly fast. >>>>> >>>> >>>> Which leaves the question which photodiode you used and how much you >>>> pre-biased it. That can have a major impact on overall bandwidth. >>> osi-optoelectronics 44pin (there might be some letters in there. >>> or pin44) >>> IIRC ~100 pF at 10V, mines biased to 12, via 10 ohm and >>> 100uF Al to ground... that's hacked in too... Well, you're going >>> to have to wait till Monday for the pics. >>> >> >> The biased side of the PD needs an RF-compliant small ceramic cap to >> ground. 0.1uF in 0603 package or similar, very short path to the ground >> plane. Short meaning a via right next to its ground pad. The RF >> impedance of an electrolytic cap is like roulette. > Hmm, I can try adding a ceramic to the bias supply. > There's no ground plane in this circuit.Yikes! Then it's almost a wonder that a 200MHZ amp runs in there. Although a 40MHz scope won't necessarily show you if it does tarantella dances.> ... Well the whole thing > lives in a grounded aluminum box, ~1"x1"x2" maybe a bit bigger. > Inside, capacitance is not your friend at 1 meg ohm. > (I've been measuring this outside of the Al box, > I wonder if it will change by much inside?) >The RF behavior can't be ascertained well from outside. Or even inside, without a ground plane. Trying nanosecond stuff that way doesn't make much sense, I'd rebuild that with a ground plane. For prototyping a piece of copperclad will do. -- Regards, Joerg http://www.analogconsultants.com/
Reply by ●October 22, 20172017-10-22
On 2017-10-21 19:09, George Herold wrote:> On Saturday, October 21, 2017 at 11:08:12 AM UTC-4, Joerg wrote: >> On 2017-10-20 17:56, George Herold wrote: >>> On Friday, October 20, 2017 at 5:31:54 PM UTC-4, Joerg wrote: >>>> On 2017-10-19 18:17, George Herold wrote: >>>>> On Thursday, October 19, 2017 at 7:33:58 PM UTC-4, John Larkin wrote: >>>>>> On Thu, 19 Oct 2017 18:57:20 -0400, Martin Riddle >>>>>> <martin_ridd@verizon.net> wrote: >>>>>> >>>>>>> On Thu, 19 Oct 2017 14:30:41 -0700 (PDT), dagmargoodboat@yahoo.com >>>>>>> wrote: >>>>>>> >>>>>>>> On Thursday, October 19, 2017 at 4:42:09 PM UTC-4, George Herold wrote: >>>>>>>>> Hi all, months ago I bought this screaming fet opamp (THS463) to gin up an old photodiode circuit. Put it in today, and the step response doesn't look right. Here's 1 k ohm of gain w/ 10 pF across it. >>>>>>>>> >>>>>>>>> https://www.dropbox.com/s/ncswyuokm9hktb5/step-10pF.BMP?dl=0 >>>>>>>>> A lot of wiggles, but the amplitude isn't big enough. >>>>>>>>> >>>>>>>>> Here's the uncompensated step. >>>>>>>>> https://www.dropbox.com/s/rilf9gvmlwldajl/step.BMP?dl=0 >>>>>>>>> (note time base change) >>>>>>>>> >>>>>>>>> It's only a 40 MHz 'scope so I'll try something faster. >>>>>>>>> Maybe it's just the LED I'm blinking isn't fast enough... >>>>>>>>> Chan. 1 (upper) is the voltage into the LED... Or my >>>>>>>>> voltage step isn't fast enough. >>>>>>>>> >>>>>>>>> I could try a step with a 74HC14... hmm there was some flavor that >>>>>>>>> was faster than the 'HC14? >>>>>>>>> >>>>>>>>> George H. >>>>>>>> >>>>>>>> Georrggggeeee, have you been using the white solderless breadboard again? >>>>>>>> >>>>>>>> Grins, >>>>>>>> James Arthur >>>>>>> >>>>>>> I was going to say ground lead oscillations, but that is a low >>>>>>> frequency. >>>>>>> >>>>>>> Cheers >>>>>> >>>>>> If this is a classic vanilla TIA, it may be the photodiode capacitance >>>>>> at the summing point making it ring. A bigger feedback cap, or a >>>>>> cascode thing, can fix that. I don't know why the amplitude should >>>>>> change. >>>>> >>>>> Right, Sorry, classic vanilla TIA-PD. >>>>> I'm just picking the tuning cap by eye... shape >>>>> of the step response. But the shape doesn't look right. >>>>> I've tuned a few TIA's with slower opamps and maybe >>>>> more C on the inverting input. It has this >>>>> classic two pole shape. You can add more feeed back C and pick >>>>> your Q by eye. This doesn't, so it's my light pulse >>>>> or my electronics. I'm mostly stuck with the old pcb layout >>>>> so I'm just trying to make sure my light source is not the >>>>> problem, or fix it if it is. >>>>> >>>>> Instead of an led, I could wiggle a diode laser, that's >>>>> less convenient. >>>> >>>> >>>> LEDs are ok. They can be pulsed blazingly fast. >>>> >>>> >>>>> ... Right now I'm driving the led from a >>>>> 20 MHz Rigol func. gen. Maybe that's the slow spot? >>>>> >>>> >>>> That would be slow. Just run it through a fast gate driver or the >>>> Schmitt single inverter I posted about, that'll speed it up. RF >>>> transistors work as well by slamming the current from the generator into >>>> the base. The gain that this provides can results in more than an order >>>> of magnitude of speed-up but the turn-off will be sluggish unless you >>>> use a gold-doped part like a 2N2369. Since fast transistors are usually >>>> available in a parts rack or can be pilfered from scrapped gear you >>>> could save the wait time for an order. >>>> >>>> However, you already found the ringing issue in your unterminated coax >>>> canle. Later you might work on that rotary switch connected ti IN- which >>>> probably adds a lot of capacitance. >>>> >>>> -- >>>> Regards, Joerg >>>> >>>> http://www.analogconsultants.com/ >>> >>> Thanks Joerg, I took pic's of my setup/pcb, but didn't post them. >>> I did make a fast edge with a 74HC14, dip, all six in parallel. >>> (now I've got two led /PD test jigs. :^) >>> I'd mostly like more BW at high gain 1M ohm*, 100k is ok. >>> seems like stray feedback capacitance is the current problem. >>> >> >> Phil is the expert on that stuff. He might know tricks to milk more BW >> at such high current/voltage ratios. >> >> IMHO you should not force all that gain into one stage. It may be better >> to reduce to 1-5k in the TIA and then follow with a regular amplifier >> stage to do the rest. > Right, that's what I'm doing at the moment, more gain down stream. > > In theory you have the least noise with the most gain at the front end. > In practice you don't always need the least noise. >With 1k in there the difference is so miniscule you couldn't even measure it. -- Regards, Joerg http://www.analogconsultants.com/
Reply by ●October 22, 20172017-10-22
On 2017-10-21 13:02, pcdhobbs@gmail.com wrote:>> Leave it at 1k and place an amp behind the TIA if more gain is required. >> Using high resistance in a fast TIA makes no sense. > > True, _provided_ that the minimum photocurrent you care about is at least 100-200 uA. >Depends on the desired dynamic range, and use a low noise amp in the first stage. Most of all, build it with proper RF guidelines in mind and that's where this project could use some upgrading. Otherwise anyone with a GSM phone walking around could disturb the signal especially if the amp has BJT structures in its input area. -- Regards, Joerg http://www.analogconsultants.com/
Reply by ●October 23, 20172017-10-23
On Sunday, October 22, 2017 at 10:40:39 AM UTC-4, Joerg wrote:> On 2017-10-21 18:44, George Herold wrote: > > On Saturday, October 21, 2017 at 10:50:49 AM UTC-4, Joerg wrote: > >> On 2017-10-20 18:09, George Herold wrote: > >>> On Friday, October 20, 2017 at 7:36:18 PM UTC-4, Joerg wrote: > >>>> On 2017-10-20 14:32, Joerg wrote: > >>>>> On 2017-10-19 18:17, George Herold wrote: > >>>>>> On Thursday, October 19, 2017 at 7:33:58 PM UTC-4, John Larkin wrote: > >>>>>>> On Thu, 19 Oct 2017 18:57:20 -0400, Martin Riddle > >>>>>>> <martin_ridd@verizon.net> wrote: > >>>>>>> > >>>>>>>> On Thu, 19 Oct 2017 14:30:41 -0700 (PDT), dagmargoodboat@yahoo.com > >>>>>>>> wrote: > >>>>>>>> > >>>>>>>>> On Thursday, October 19, 2017 at 4:42:09 PM UTC-4, George Herold > >>>>>>>>> wrote: > >>>>>>>>>> Hi all, months ago I bought this screaming fet opamp (THS463) to > >>>>>>>>>> gin up an old photodiode circuit. Put it in today, and the step > >>>>>>>>>> response doesn't look right. Here's 1 k ohm of gain w/ 10 pF > >>>>>>>>>> across it. > >>>>>>>>>> > >>>>>>>>>> https://www.dropbox.com/s/ncswyuokm9hktb5/step-10pF.BMP?dl=0 > >>>>>>>>>> A lot of wiggles, but the amplitude isn't big enough. > >>>>>>>>>> > >>>>>>>>>> Here's the uncompensated step. > >>>>>>>>>> https://www.dropbox.com/s/rilf9gvmlwldajl/step.BMP?dl=0 > >>>>>>>>>> (note time base change) > >>>>>>>>>> > >>>>>>>>>> It's only a 40 MHz 'scope so I'll try something faster. > >>>>>>>>>> Maybe it's just the LED I'm blinking isn't fast enough... > >>>>>>>>>> Chan. 1 (upper) is the voltage into the LED... Or my > >>>>>>>>>> voltage step isn't fast enough. > >>>>>>>>>> > >>>>>>>>>> I could try a step with a 74HC14... hmm there was some flavor that > >>>>>>>>>> was faster than the 'HC14? > >>>>>>>>>> > >>>>>>>>>> George H. > >>>>>>>>> > >>>>>>>>> Georrggggeeee, have you been using the white solderless breadboard > >>>>>>>>> again? > >>>>>>>>> > >>>>>>>>> Grins, > >>>>>>>>> James Arthur > >>>>>>>> > >>>>>>>> I was going to say ground lead oscillations, but that is a low > >>>>>>>> frequency. > >>>>>>>> > >>>>>>>> Cheers > >>>>>>> > >>>>>>> If this is a classic vanilla TIA, it may be the photodiode capacitance > >>>>>>> at the summing point making it ring. A bigger feedback cap, or a > >>>>>>> cascode thing, can fix that. I don't know why the amplitude should > >>>>>>> change. > >>>>>> > >>>>>> Right, Sorry, classic vanilla TIA-PD. > >>>>>> I'm just picking the tuning cap by eye... shape > >>>>>> of the step response. But the shape doesn't look right. > >>>>>> I've tuned a few TIA's with slower opamps and maybe > >>>>>> more C on the inverting input. It has this > >>>>>> classic two pole shape. You can add more feeed back C and pick > >>>>>> your Q by eye. This doesn't, so it's my light pulse > >>>>>> or my electronics. I'm mostly stuck with the old pcb layout > >>>>>> so I'm just trying to make sure my light source is not the > >>>>>> problem, or fix it if it is. > >>>>>> > >>>>>> Instead of an led, I could wiggle a diode laser, that's > >>>>>> less convenient. > >>>>> > >>>>> > >>>>> LEDs are ok. They can be pulsed blazingly fast. > >>>>> > >>>> > >>>> Which leaves the question which photodiode you used and how much you > >>>> pre-biased it. That can have a major impact on overall bandwidth. > >>> osi-optoelectronics 44pin (there might be some letters in there. > >>> or pin44) > >>> IIRC ~100 pF at 10V, mines biased to 12, via 10 ohm and > >>> 100uF Al to ground... that's hacked in too... Well, you're going > >>> to have to wait till Monday for the pics. > >>> > >> > >> The biased side of the PD needs an RF-compliant small ceramic cap to > >> ground. 0.1uF in 0603 package or similar, very short path to the ground > >> plane. Short meaning a via right next to its ground pad. The RF > >> impedance of an electrolytic cap is like roulette. > > Hmm, I can try adding a ceramic to the bias supply. > > There's no ground plane in this circuit. > > > Yikes! Then it's almost a wonder that a 200MHZ amp runs in there. > Although a 40MHz scope won't necessarily show you if it does tarantella > dances. > > > > ... Well the whole thing > > lives in a grounded aluminum box, ~1"x1"x2" maybe a bit bigger. > > Inside, capacitance is not your friend at 1 meg ohm. > > (I've been measuring this outside of the Al box, > > I wonder if it will change by much inside?) > > > > The RF behavior can't be ascertained well from outside. Or even inside, > without a ground plane. Trying nanosecond stuff that way doesn't make > much sense, I'd rebuild that with a ground plane. For prototyping a > piece of copperclad will do. > > -- > Regards, Joerg > > http://www.analogconsultants.com/Hi Joerg, (others) Here's a couple of pics. https://www.dropbox.com/s/2ivoas0759qz03i/PD-front.JPG?dl=0 https://www.dropbox.com/s/9ls9kkm7n6i0l6v/PD-back.JPG?dl=0 https://www.dropbox.com/s/0jxk4vzf66g6xrv/PD-case1.JPG?dl=0 https://www.dropbox.com/s/p3ox17ngwhu4ruo/PD-case2.JPG?dl=0 James, yeah the opamp TIA drives the coax too. It was oscillating happily when I had only 330 ohms of TIA R. :^) But it seems OK when terminated. So as you say, the ringing is due somewhat to the cable capacitance.. I guess, I'm not really sure how to think of 7' of coax.. transmission line or capacitance? Ground planes and such. Joerg I'm not sure, what I'd like is a big ass photodiode at 1 M ohm gain and 1 MHz BW. (typically ~1 uA of current.) 1 MHz is sorta border line RF, and with 1 Meg ohm a pF of capacitance can ruin your whole day. (I've got hacked pcb's where I learned this... ground plane sanded off around PD and TIA... ugly.) I bought a couple of THS4361's so maybe I'll build up a pcb w/o the switch and see what that look like. Oh the numbers I post upstream are not right. I'm a little bothered by the high gain end, cause I think I was getting more (HF) gain there with a slower opamp, so I need to redo this more carefully. Gain R 3dB 330 20M(Hz) 1k 15M 3.3k 9M 10k 6M 33k 3M 100k 400k 330k 90k 1Meg 30k George H.
Reply by ●October 23, 20172017-10-23
George Herold <gherold@teachspin.com> wrote:> James, yeah the opamp TIA drives the coax too. It was oscillating > happily when I had only 330 ohms of TIA R. :^) But it seems OK when > terminated. So as you say, the ringing is due somewhat to the cable > capacitance.. I guess, I'm not really sure how to think of 7' of coax.. > transmission line or capacitance? > > Ground planes and such. Joerg I'm not sure, what I'd like is a big ass > photodiode at 1 M ohm gain and 1 MHz BW. (typically ~1 uA of current.) > 1 MHz is sorta border line RF, and with 1 Meg ohm a pF of capacitance > can ruin your whole day.Build on solid ground plane with good RF bypassing. Use Manhattan for non-critical components. Air wire the sensitive node. Do you really need to drive the coax directly? Can you add a series resistor to reduce the loading? Any ground bounce from poor supply bypassing can give feedback to the inputs. The datasheet Application Information section has a lot of useful information on feedback resistor selection, layout, bypassing, alternate configurations, etc. http://datasheet.octopart.com/THS4631DDAEVM-Texas-Instruments-datasheet- 32750733.pdf They definitely do not recommend a two-sided pcb.
Reply by ●October 23, 20172017-10-23
On 2017-10-23 06:45, George Herold wrote:> On Sunday, October 22, 2017 at 10:40:39 AM UTC-4, Joerg wrote: >> On 2017-10-21 18:44, George Herold wrote: >>> On Saturday, October 21, 2017 at 10:50:49 AM UTC-4, Joerg wrote: >>>> On 2017-10-20 18:09, George Herold wrote: >>>>> On Friday, October 20, 2017 at 7:36:18 PM UTC-4, Joerg wrote: >>>>>> On 2017-10-20 14:32, Joerg wrote: >>>>>>> On 2017-10-19 18:17, George Herold wrote: >>>>>>>> On Thursday, October 19, 2017 at 7:33:58 PM UTC-4, John Larkin wrote: >>>>>>>>> On Thu, 19 Oct 2017 18:57:20 -0400, Martin Riddle >>>>>>>>> <martin_ridd@verizon.net> wrote: >>>>>>>>> >>>>>>>>>> On Thu, 19 Oct 2017 14:30:41 -0700 (PDT), dagmargoodboat@yahoo.com >>>>>>>>>> wrote: >>>>>>>>>> >>>>>>>>>>> On Thursday, October 19, 2017 at 4:42:09 PM UTC-4, George Herold >>>>>>>>>>> wrote: >>>>>>>>>>>> Hi all, months ago I bought this screaming fet opamp (THS463) to >>>>>>>>>>>> gin up an old photodiode circuit. Put it in today, and the step >>>>>>>>>>>> response doesn't look right. Here's 1 k ohm of gain w/ 10 pF >>>>>>>>>>>> across it. >>>>>>>>>>>> >>>>>>>>>>>> https://www.dropbox.com/s/ncswyuokm9hktb5/step-10pF.BMP?dl=0 >>>>>>>>>>>> A lot of wiggles, but the amplitude isn't big enough. >>>>>>>>>>>> >>>>>>>>>>>> Here's the uncompensated step. >>>>>>>>>>>> https://www.dropbox.com/s/rilf9gvmlwldajl/step.BMP?dl=0 >>>>>>>>>>>> (note time base change) >>>>>>>>>>>> >>>>>>>>>>>> It's only a 40 MHz 'scope so I'll try something faster. >>>>>>>>>>>> Maybe it's just the LED I'm blinking isn't fast enough... >>>>>>>>>>>> Chan. 1 (upper) is the voltage into the LED... Or my >>>>>>>>>>>> voltage step isn't fast enough. >>>>>>>>>>>> >>>>>>>>>>>> I could try a step with a 74HC14... hmm there was some flavor that >>>>>>>>>>>> was faster than the 'HC14? >>>>>>>>>>>> >>>>>>>>>>>> George H. >>>>>>>>>>> >>>>>>>>>>> Georrggggeeee, have you been using the white solderless breadboard >>>>>>>>>>> again? >>>>>>>>>>> >>>>>>>>>>> Grins, >>>>>>>>>>> James Arthur >>>>>>>>>> >>>>>>>>>> I was going to say ground lead oscillations, but that is a low >>>>>>>>>> frequency. >>>>>>>>>> >>>>>>>>>> Cheers >>>>>>>>> >>>>>>>>> If this is a classic vanilla TIA, it may be the photodiode capacitance >>>>>>>>> at the summing point making it ring. A bigger feedback cap, or a >>>>>>>>> cascode thing, can fix that. I don't know why the amplitude should >>>>>>>>> change. >>>>>>>> >>>>>>>> Right, Sorry, classic vanilla TIA-PD. >>>>>>>> I'm just picking the tuning cap by eye... shape >>>>>>>> of the step response. But the shape doesn't look right. >>>>>>>> I've tuned a few TIA's with slower opamps and maybe >>>>>>>> more C on the inverting input. It has this >>>>>>>> classic two pole shape. You can add more feeed back C and pick >>>>>>>> your Q by eye. This doesn't, so it's my light pulse >>>>>>>> or my electronics. I'm mostly stuck with the old pcb layout >>>>>>>> so I'm just trying to make sure my light source is not the >>>>>>>> problem, or fix it if it is. >>>>>>>> >>>>>>>> Instead of an led, I could wiggle a diode laser, that's >>>>>>>> less convenient. >>>>>>> >>>>>>> >>>>>>> LEDs are ok. They can be pulsed blazingly fast. >>>>>>> >>>>>> >>>>>> Which leaves the question which photodiode you used and how much you >>>>>> pre-biased it. That can have a major impact on overall bandwidth. >>>>> osi-optoelectronics 44pin (there might be some letters in there. >>>>> or pin44) >>>>> IIRC ~100 pF at 10V, mines biased to 12, via 10 ohm and >>>>> 100uF Al to ground... that's hacked in too... Well, you're going >>>>> to have to wait till Monday for the pics. >>>>> >>>> >>>> The biased side of the PD needs an RF-compliant small ceramic cap to >>>> ground. 0.1uF in 0603 package or similar, very short path to the ground >>>> plane. Short meaning a via right next to its ground pad. The RF >>>> impedance of an electrolytic cap is like roulette. >>> Hmm, I can try adding a ceramic to the bias supply. >>> There's no ground plane in this circuit. >> >> >> Yikes! Then it's almost a wonder that a 200MHZ amp runs in there. >> Although a 40MHz scope won't necessarily show you if it does tarantella >> dances. >> >> >>> ... Well the whole thing >>> lives in a grounded aluminum box, ~1"x1"x2" maybe a bit bigger. >>> Inside, capacitance is not your friend at 1 meg ohm. >>> (I've been measuring this outside of the Al box, >>> I wonder if it will change by much inside?) >>> >> >> The RF behavior can't be ascertained well from outside. Or even inside, >> without a ground plane. Trying nanosecond stuff that way doesn't make >> much sense, I'd rebuild that with a ground plane. For prototyping a >> piece of copperclad will do. >> >> -- >> Regards, Joerg >> >> http://www.analogconsultants.com/ > > Hi Joerg, (others) Here's a couple of pics. > https://www.dropbox.com/s/2ivoas0759qz03i/PD-front.JPG?dl=0 > https://www.dropbox.com/s/9ls9kkm7n6i0l6v/PD-back.JPG?dl=0 > https://www.dropbox.com/s/0jxk4vzf66g6xrv/PD-case1.JPG?dl=0 > https://www.dropbox.com/s/p3ox17ngwhu4ruo/PD-case2.JPG?dl=0 >That is not likely going to work reliably. You can get away with double-sided PCB but one side has to be a solid or almost solid ground plane.> > James, yeah the opamp TIA drives the coax too. It was oscillating happily > when I had only 330 ohms of TIA R. :^)That is one of the telltale signs that the whole enchilada is teetering on the edge when it comes to stability.> ... But it seems OK when terminated. > So as you say, the ringing is due somewhat to the cable capacitance.. > I guess, I'm not really sure how to think of 7' of coax.. transmission > line or capacitance? >For coax you need a staunch driver. I believe the lowest load resistance the THS4631 is supposed to drive is 100ohms.> Ground planes and such. Joerg I'm not sure, what I'd like is a big ass > photodiode at 1 M ohm gain and 1 MHz BW. (typically ~1 uA of current.) > 1 MHz is sorta border line RF, and with 1 Meg ohm a pF of capacitance > can ruin your whole day. (I've got hacked pcb's where I learned this... > ground plane sanded off around PD and TIA... ugly.) >You can leave open a small island around IN- but this sort of requirement asks for Phil's medicine. Cascode and such.> I bought a couple of THS4361's so maybe I'll build up a pcb w/o the switch > and see what that look like. >Yes, and with a solid ground plane.> Oh the numbers I post upstream are not right. > I'm a little bothered by the high gain end, cause I think I was > getting more (HF) gain there with a slower opamp, so I need to redo this > more carefully. > > Gain R 3dB > 330 20M(Hz) > 1k 15M > 3.3k 9M > 10k 6M > 33k 3M > 100k 400k > 330k 90k > 1Meg 30k >At 1M it is bound to fall apart. You might also want to look for amps with lower input noise so you don't need such high feedbakc resistor values. -- Regards, Joerg http://www.analogconsultants.com/
