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' 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! George H. (sorry late friday night..:^)> > > > -- > > John Larkin Highland Technology, Inc > picosecond timing precision measurement > > jlarkin att highlandtechnology dott com > http://www.highlandtechnology.com
TIA-PD compensation, by eye
Started by ●October 19, 2017
Reply by ●October 20, 20172017-10-20
Reply by ●October 21, 20172017-10-21
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. -- John Larkin Highland Technology, Inc lunatic fringe electronics
Reply by ●October 21, 20172017-10-21
>LEDs are ok. They can be pulsed blazingly fast.In some alternate universe where 40 ns is blazingly fast. ;) UV ones are faster than that, and if you really bomb a high-current IR one, you can get the recombination to enhance enough to get them down to 100-ns territory. LEDs are slow compared with even cleaved-cavity lasers, which will do 200 ps pretty easily. VCSELs are much faster than that. Cheers Phil Hobbs
Reply by ●October 21, 20172017-10-21
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. [...] -- Regards, Joerg http://www.analogconsultants.com/
Reply by ●October 21, 20172017-10-21
On 2017-10-20 17:45, 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. > Some visible ones are not so not so fast. > The NIR led I used says 14 ns rise and fall times, > (870 nm?) I'm sure there are faster ones. >They can be pulsed faster, it's mostly a matter of the drive circuit: http://users.ox.ac.uk/~atdgroup/technicalnotes/Fast%20optical%20LED%20pulse%20generator.pdf You'll also have to keep all the leads very short and close to each other, ideally use coax for everything. [...] -- Regards, Joerg http://www.analogconsultants.com/
Reply by ●October 21, 20172017-10-21
On 2017-10-21 04:49, pcdhobbs@gmail.com wrote:>> LEDs are ok. They can be pulsed blazingly fast. > > In some alternate universe where 40 ns is blazingly fast. ;) UV ones > are faster than that, and if you really bomb a high-current IR one, > you can get the recombination to enhance enough to get them down to > 100-ns territory. > > LEDs are slow compared with even cleaved-cavity lasers, which will do > 200 ps pretty easily. VCSELs are much faster than that. >The onset can be quite fast, just not the trailing edge: https://www.mpi-hd.mpg.de/gerda/public/2008/t08_vlvnt08_led_bl.pdf When I pulsed LEDs I never tried it with the really old ones I still have from my teenage days (when I still had hair on top). Maybe I should. Those need at least 10mA to be reasonably visible during the day. -- Regards, Joerg http://www.analogconsultants.com/
Reply by ●October 21, 20172017-10-21
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. -- Regards, Joerg http://www.analogconsultants.com/
Reply by ●October 21, 20172017-10-21
On Sat, 21 Oct 2017 04:49:45 -0700 (PDT), pcdhobbs@gmail.com wrote:>>LEDs are ok. They can be pulsed blazingly fast. > >In some alternate universe where 40 ns is blazingly fast. ;) UV ones are faster than that, and if you really bomb a high-current IR one, you can get the recombination to enhance enough to get them down to 100-ns territory. > >LEDs are slow compared with even cleaved-cavity lasers, which will do 200 ps pretty easily. VCSELs are much faster than that. > >Cheers > >Phil HobbsThis is the best we've done with a butterfly-packaged fiber-coupled laser: https://www.dropbox.com/s/d5zwp8yvg5j683v/95_ps_optical_pulse.jpg?dl=0 To get that sort of pulse, you have to tease the pulse width and amplitude to get a sort of optical avalanche effect. Lots of lasers have so much inductance that they can't go fast. Or they were intended for telecom, and the impedances are wrong for pulsing. Here's the driver, which has a built-on one-shot to make the pulse width: https://www.dropbox.com/s/ku5onzl5fiao7z8/T165_3.jpg?raw=1 -- John Larkin Highland Technology, Inc lunatic fringe electronics
Reply by ●October 21, 20172017-10-21
On 2017-10-20 17:42, George Herold 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 ohmLeave 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. [...] -- Regards, Joerg http://www.analogconsultants.com/
Reply by ●October 21, 20172017-10-21
