George Herold <gherold@teachspin.com> wrote:> Thanks Steve, I knew I had that page bookmarked at work. > 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!> George H.Don't kill yourself. You are a physicist. There is no way any of us could compare with you in your own field. Electronics has a bunch of gotchas that take years to learn. This is why we are here, so you've come to the right place. On high frequency oscilloscopes, there might be another method that may interest you. The Rigol DS1052E is a 50 MHz two channel scope that sells for around $349. A german hacker found that removing the input bandwidth filter in front of the ADC chip would raise the bandwidth to around 450 MHz. Unfortunately I did not save his page, and I'm having trouble locating it again. I happen to have two of these scopes along with a bunch of others, and it would be very interesting to do the mod. There is another hack that involves changing the firmware to convince the DS1052E that it is a different scope. This changes the input filter bandwidth to 100 MHz and sets the maximum timebase to 2 ns instead of 5 ns. A different version raises the bandwidth to 150 MHz. The hack is a bit dangerous since you can brick the scope if you make a mistake. And current revisions have to be downgraded to an earlier version before applying the hack, then upgraded back to the new version. More opportunity to brick the scope. So I probably won't try that one. But I'll let you know if I find the 450 MHz page.
TIA-PD compensation, by eye
Started by ●October 19, 2017
Reply by ●October 20, 20172017-10-20
Reply by ●October 20, 20172017-10-20
On Friday, October 20, 2017 at 1:08:44 PM UTC-4, Steve Wilson wrote:> George Herold <gherold@teachspin.com> wrote: > > > Thanks Steve, I knew I had that page bookmarked at work. > > 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! > > > George H. > > Don't kill yourself. You are a physicist. There is no way any of us could > compare with you in your own field. Electronics has a bunch of gotchas that > take years to learn. This is why we are here, so you've come to the right > place.Grin, I don't like compliments, mostly 'cause they're not true. Physics is no harder or easier than other fields.. it's mostly just a choice of what you like and are good at. I'm sorta a mid grade physicist, plenty are smarter and others less smart. I guess the one thing I pride myself on is trouble shooting, which I think I do well. Forgetting to terminate a fast signal, then seeing ringing, (a big hint) deserves a dope slap. It's fine though, my other talent is making mistakes. :^)> > On high frequency oscilloscopes, there might be another method that may > interest you. The Rigol DS1052E is a 50 MHz two channel scope that sells > for around $349. A german hacker found that removing the input bandwidth > filter in front of the ADC chip would raise the bandwidth to around 450 > MHz. Unfortunately I did not save his page, and I'm having trouble locating > it again. I happen to have two of these scopes along with a bunch of > others, and it would be very interesting to do the mod. > > There is another hack that involves changing the firmware to convince the > DS1052E that it is a different scope. This changes the input filter > bandwidth to 100 MHz and sets the maximum timebase to 2 ns instead of 5 ns. > > A different version raises the bandwidth to 150 MHz. > > The hack is a bit dangerous since you can brick the scope if you make a > mistake. And current revisions have to be downgraded to an earlier version > before applying the hack, then upgraded back to the new version. More > opportunity to brick the scope. So I probably won't try that one. > > But I'll let you know if I find the 450 MHz page.I've got a 50 MHz Rigol, and I've seen the hack that can make it 100 MHz, I haven't bothered. We have a 200 MHz TEK if I need it. If I ever get around to playing with the TDR stuff, I might need a faster 'scope. Speaking of 'scopes I noticed the new low end Rigol's now have separate knobs for gain and offset for each channel. (Good move) https://www.rigolna.com/products/digital-oscilloscopes/2000a/ And 0.5mV/div gain, up to 8,000 number of averages. Those would be nice. Maybe next year. George H.
Reply by ●October 20, 20172017-10-20
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, 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. I recently tried a simple NPN cascode between a photodiode and a fast opamp. Risetime was 450 ps, about 5x better than the vanilla circuit. I also tried a cascode into a mini-circuits MMIC. That didn't work. -- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
Reply by ●October 20, 20172017-10-20
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/
Reply by ●October 20, 20172017-10-20
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.> >> ... 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/
Reply by ●October 20, 20172017-10-20
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.) George H.> > I recently tried a simple NPN cascode between a photodiode and a fast > opamp. Risetime was 450 ps, about 5x better than the vanilla circuit. > > I also tried a cascode into a mini-circuits MMIC. That didn't work. > > > -- > > John Larkin Highland Technology, Inc > picosecond timing precision measurement > > jlarkin att highlandtechnology dott com > http://www.highlandtechnology.com
Reply by ●October 20, 20172017-10-20
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. George H.> > > > ... 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/
Reply by ●October 20, 20172017-10-20
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. George H.
Reply by ●October 20, 20172017-10-20
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. -- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
Reply by ●October 20, 20172017-10-20
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. George H.> > > > > >> ... 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/
