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.
TIA-PD compensation, by eye
Started by ●October 19, 2017
Reply by ●October 19, 20172017-10-19
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
Reply by ●October 19, 20172017-10-19
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 ArthurI was going to say ground lead oscillations, but that is a low frequency. Cheers
Reply by ●October 19, 20172017-10-19
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. > >CheersIf 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. -- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
Reply by ●October 19, 20172017-10-19
On 2017-10-19 15:57, Martin Riddle 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. >>>They aren't exactly my definition of screaming fast. Screaming starts around 2GHz.>>> https://www.dropbox.com/s/ncswyuokm9hktb5/step-10pF.BMP?dl=0 A >>> lot of wiggles, but the amplitude isn't big enough. >>>Then something fundamental must be off. Wrong resistor?>>> 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? >>>Yes, 74AUC1G14, it's a single gate version with sports exhaust and all.>>> 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. >It could be something like that or maybe there is a tortilla-sized circuitry connected to IN-. George, can you post of photo of that whole lab bench setup? -- Regards, Joerg http://www.analogconsultants.com/
Reply by ●October 19, 20172017-10-19
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 ArthurGrin.. 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.
Reply by ●October 19, 20172017-10-19
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. Right now I'm driving the led from a 20 MHz Rigol func. gen. Maybe that's the slow spot? George H.> > > -- > > John Larkin Highland Technology, Inc > picosecond timing precision measurement > > jlarkin att highlandtechnology dott com > http://www.highlandtechnology.com
Reply by ●October 19, 20172017-10-19
On Thursday, October 19, 2017 at 7:40:57 PM UTC-4, Joerg wrote:> On 2017-10-19 15:57, Martin Riddle 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. > >>> > > They aren't exactly my definition of screaming fast. Screaming starts > around 2GHz.Well it's all relative, the previous opamp was a opa124.> > > >>> https://www.dropbox.com/s/ncswyuokm9hktb5/step-10pF.BMP?dl=0 A > >>> lot of wiggles, but the amplitude isn't big enough. > >>> > > Then something fundamental must be off. Wrong resistor? > > > >>> 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? > >>> > > Yes, 74AUC1G14, it's a single gate version with sports exhaust and all.I don't suppose that comes in a dip? There was a TDR paper Steve posted with a fast(ish) '14 in a dip. (I'll find it.)> > > >>> 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. > > > > It could be something like that or maybe there is a tortilla-sized > circuitry connected to IN-. George, can you post of photo of that > whole lab bench setup?Grin, well switch sized. At least I got the right side of the rotary switch connected to the inverting input. (sometimes it's enough to be lucky.) It's a hack/mod of a pcb I did years ago. I'll post a pic. I'm driving a long piece of coax too... (details details, It's sometimes hard to know what's important.) George H.> > -- > Regards, Joerg > > http://www.analogconsultants.com/
Reply by ●October 19, 20172017-10-19
George Herold <gherold@teachspin.com> wrote:>> Yes, 74AUC1G14, it's a single gate version with sports exhaust and all. > I don't suppose that comes in a dip? There was a TDR paper Steve > posted with a fast(ish) '14 in a dip. (I'll find it.)Save you some time: http://www.epanorama.net/circuits/tdr.html Youtube video: https://www.youtube.com/watch?v=9cP6w2odGUc A 40 MHz scope is useless for these frequencies. You need 1 GHz or more. A Rigol goes for 8k or so, but a good sampling scope on eBay can be much cheaper if you are lucky. Or maybe you could pick up a used PicoScope 6000 series. 2ns risetime is definitely 50 ohm coax territory. 6" scope probe grounds will ring like crazy. A 499 ohm resistor in series with 50 ohm coax, terminated at the scope with 50 ohms, will give a 20 dB attenuator which will have negligible effect on the signal. Ground the coax shield at the source. A ferrite toroid over the coax will also help reduce common mode ground bounce. This is also ground plane territory. It is a bit frightening to glue a $50 ic to copperclad, but do it live bug so you can salvage it later. Really good grounding and bypassing is necessary, along with zero length leads. Read the Application Information section of the datasheet: http://www.ti.com/lit/ds/symlink/ths4631.pdf
Reply by ●October 20, 20172017-10-20
On Thursday, October 19, 2017 at 10:25:26 PM UTC-4, Steve Wilson wrote:> George Herold <gherold@teachspin.com> wrote: > > >> Yes, 74AUC1G14, it's a single gate version with sports exhaust and all. > > I don't suppose that comes in a dip? There was a TDR paper Steve > > posted with a fast(ish) '14 in a dip. (I'll find it.) > > Save you some time: > > http://www.epanorama.net/circuits/tdr.html > > Youtube video: > > https://www.youtube.com/watch?v=9cP6w2odGUc > > A 40 MHz scope is useless for these frequencies. You need 1 GHz or more. A > Rigol goes for 8k or so, but a good sampling scope on eBay can be much > cheaper if you are lucky. Or maybe you could pick up a used PicoScope 6000 > series. > > 2ns risetime is definitely 50 ohm coax territory. 6" scope probe grounds > will ring like crazy. A 499 ohm resistor in series with 50 ohm coax, > terminated at the scope with 50 ohms, will give a 20 dB attenuator which > will have negligible effect on the signal. Ground the coax shield at the > source. A ferrite toroid over the coax will also help reduce common mode > ground bounce. > > This is also ground plane territory. It is a bit frightening to glue a $50 > ic to copperclad, but do it live bug so you can salvage it later. Really > good grounding and bypassing is necessary, along with zero length leads. > > Read the Application Information section of the datasheet: > > http://www.ti.com/lit/ds/symlink/ths4631.pdfThanks 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. George H.
