AES wrote:>>> Is there any hidden connection between the insistence of these LEDs on >>> continuing to emit light long after they've been turned off, and the >>> insistence of the participants in this thread in continuing to re-emit >>> screenfuls and screenfuls of previous posts, before finally adding a >>> one-sentence comment at the end?!?!?! > > >> Modern newsreaders often run on machines with LED backlights? ;) >> Cheers, Phil Hobbs > > This thread really has been off the charts in the unwillingness of > posters to trim out unnecessary cruft when they reply!Nah, that's situation normal on a sci.electronics.design. Even the flame wars are like that. :( Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal ElectroOptical Innovations 55 Orchard Rd Briarcliff Manor NY 10510 845-480-2058 email: hobbs (atsign) electrooptical (period) net http://electrooptical.net
LED on Photodiode step response
Started by ●April 11, 2011
Reply by ●April 12, 20112011-04-12
Reply by ●April 12, 20112011-04-12
George Herold wrote:> On Apr 12, 10:45 am, Phil Hobbs > <pcdhSpamMeSensel...@electrooptical.net> wrote: >> Phil Hobbs wrote: >>> George Herold wrote: >>>> On Apr 11, 10:59 pm, John Larkin >>>> <jjlar...@highNOTlandTHIStechnologyPART.com> wrote: >>>>> On Mon, 11 Apr 2011 16:35:20 -0700 (PDT), George Herold >> >>>>> <gher...@teachspin.com> wrote: >>>>>> On Apr 11, 7:04 pm, John Larkin >>>>>> <jjlar...@highNOTlandTHIStechnologyPART.com> wrote: >>>>>>> On Mon, 11 Apr 2011 12:46:16 -0700 (PDT), George Herold >> >>>>>>> <gher...@teachspin.com> wrote: >>>>>>>> On Apr 11, 3:32 pm, Phil Hobbs >>>>>>>> <pcdhSpamMeSensel...@electrooptical.net> wrote: >>>>>>>>> George Herold wrote: >>>>>>>>>> I looking at the step response of a photodiode (PD). I m stepping >>>>>>>>>> the current into a LED such that the step doubles the photo current >>>>>>>>>> from the PD. (I m not starting with the LED off, but on slightly.) >>>>>>>>>> Here s a scope shot. The bottom trace in the PD response. (The top >>>>>>>>>> is the voltage step to the LED (through 100 ohms)... AC coupled >>>>>>>>>> to get >>>>>>>>>> rid of a big DC offset.) >> >>>>>>>>>> http://img189.imageshack.us/i/tek0024.png/ >> >>>>>>>>>> There is this long tail on the step response. Here s the same >>>>>>>>>> picture >>>>>>>>>> with the timebase slowed down. >> >>>>>>>>>> http://img812.imageshack.us/i/tek0025.png/ >> >>>>>>>>>> Looks something like a 2-2.5 us tail on the step. >> >>>>>>>>>> What s it caused by? I first thought it was perhaps heating of the >>>>>>>>>> LED during the step. But I was reading Photodetectors by S. Donati, >>>>>>>>>> over the weekend. In Chapter 5 he talks about two sets of photo- >>>>>>>>>> generated carriers the majority are made in the depletion layer, >>>>>>>>>> and >>>>>>>>>> travel with the drift speed, (times in the nano-second range.) The >>>>>>>>>> second smaller set are those generated in the doped region. If they >>>>>>>>>> are within a diffusion length of the depletion region then these >>>>>>>>>> have >>>>>>>>>> a chance of adding to the photocurrent. Donati says that these >>>>>>>>>> have a >>>>>>>>>> time constant in the microsecond range! I wonder if this is what >>>>>>>>>> I am >>>>>>>>>> seeing? >> >>>>>>>>>> I looked at the step response with a different color LED. (you >>>>>>>>>> expect >>>>>>>>>> more absorption in the doped region with shorter wavelength light.) >>>>>>>>>> So here is the pulse response from a 635 nm red and 594 nm Amber >>>>>>>>>> LED. >> >>>>>>>>>> http://img52.imageshack.us/i/tek0026.png/ >>>>>>>>>> The red LED is the trace that is a bit higher. I m looking to >>>>>>>>>> see if >>>>>>>>>> I can dig up any blue or white LED s. >> >>>>>>>>>> Any other ideas how I might determine if that's what I'm seeing... >> >>>>>>>>>> Oh, the PD is a OSI-optoelectroncs PIN-3CD (3.2mm^2 area) reversed >>>>>>>>>> biased at ~12V. >> >>>>>>>>>> George H. >> >>>>>>>>> Crank up the bias till the diode is fully depleted, and see. >> >>>>>>>>> Cheers >> >>>>>>>>> Phil Hobbs >>>>>>>>>> Thanks Phil, >> >>>>>>>> I can try more bias. Is there any way to know it's fully depleted? >>>>>>>> (Or do I just take it to the maximum as specified by the maker. (30 >>>>>>>> volts in this case.) >> >>>>>>>> The white LED showed more of the long tail... but less of a sharp >>>>>>>> turn- >>>>>>>> on, I wonder if it's just the LED time response that I'm seeing. >> >>>>>>> A white LED has a phosphor that is likely slow. >> >>>>>> Yeah, I noticed that.. Thanks to Grant too. >> >>>>>> I didn't save the screeen shot on my flash. (I should hit the save >>>>>> button more often.) >>>>>> IR led's are slow too. >> >>>>>>> Try a laser diode. They are (usually) fast. >> >>>>>>> I did encounter some laser diodes that had a weird slow step response >>>>>>> when used at low pulse rates. They behaved, electrically and >>>>>>> optically, as if there were an inductor in series with the diode. At >>>>>>> high rep-rates, 1 MHz maybe, the effect disappeared. >> >>>>>> Bring out the big guns ehh? I've never looked at the step response >>>>> >from our laser diode, but we've modulated it up to ~150 MHz. There's >>>>>> a reported change in the modulation sensitivity, at the microsecond >>>>>> range and longer its due to heating, at shorter times it drops by an >>>>>> order of magnitude and is due to (index of refraction?) carrier >>>>>> density effects. At very high frequency you can get an enhancement >>>>>> due to relaxation oscialltion in the LD... about 6 GHz, in the LD's I >>>>>> know. >> >>>>>>> Try looking at the voltage across the LED, and see if it's a clean >>>>>>> step. >> >>>>>>> What's the signal path from the photodiode to the scope? Since all the >>>>>>> LEDs seem to have similar waveforms, maybe the detector is the >>>>>>> problem. >>>>>> Yeah sorry I didn't save all the crummy 'scope shots. >>>>>> But I'm certainly open to some silly measurement error. There's a TIA >>>>>> with OPA134 (GBW~8MHz, 1-3pF Cin) and 10kohm feedback resistor, with >>>>>> 3.3pF parallel capacitance (plus some extra stray... call it ~2 pF, >>>>>> perhaps a bit less). >> >>>>>> I think I'm seeing a LED temperature effect. Does the light output go >>>>>> up as an LED gets hotter? If it's linear with delta T, I can't change >>>>>> drive level and make it go away. >> >>>>> LEDs are less efficient when they're hot. But the forward drop goes >>>>> down. So there is usually some source resistance where the effects >>>>> cancel. >> >>>> Thanks John, If it's due to more current because of a drop in forward >>>> voltage, then I should see that in the current. (Which I don't... >>>> sigh) >> >>>> OK let me try a totally different idea. Can the stray capacitance of >>>> 'stuff' have a long tail like that. Charges moving around on pieces >>>> of plastic/fiber glass or something? >> >>>> I could air wire the circuit and see what that does... >> >>>> George H. >> >>>>> John- Hide quoted text - >> >>>>> - Show quoted text - >> >>> How about posting the schematic? If the edges are symmetrical, it's >>> probably the TIA--the LED would be cooling down when it wasn't emitting >>> light, so you wouldn't see the thermal tail. >> >>> Op amps have really terrible supply rejection when it comes to fast >>> transients--depending on the feedback, you can easily get _gain_ from >>> the supply pin to the output. (PSR is specified with respect to the >>> amplifier _inputs_.) >> >>> Another possibility is phase funnies in the open loop gain. Composite >>> amps are notorious for that sort of thing. The long-time settling >>> behaviour is dominated by the lowest-frequency pole or zero in the open >>> loop transfer function--lead-lag networks can cause this sort of thing >>> even though the closed-loop response looks nice on a spectrum analyzer. >> >>> Try touching the summing junction with your finger and see what happens >>> to the step response. >> >>> Cheers >> >>> Phil Hobbs >> >> Rereading, it looks like you have something like a 20 pF summing >> junction capacitance and a 2 pF feedback cap. That means you have an >> open-loop pole/zero pair separated by a factor of 10 in frequency, up >> near the GBW of the op amp. That will lead to some entertaining >> settling behaviour, which at this point would be my best guess as to >> what you're seeing. > > Yeah perhaps more like 3.3 pF across the feedback. > > The pulse response through a 'big' resistor looks just fine, (no PD > or LED). I went back and added the 'missing' 12 pF to ground, just in > case...(That's the removed PD capacitance, the strays and opamp C are > still there.) > > Hey this might be some 'real' physics and not just a circuit screw- > up. > > George H.Well, it does look like the photodiode, which suggests that the aforementioned diffusion tail mechanism is doing it. I'm surprised that jacking up the bias didn't fix it. It isn't usually as bad as that. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal ElectroOptical Innovations 55 Orchard Rd Briarcliff Manor NY 10510 845-480-2058 email: hobbs (atsign) electrooptical (period) net http://electrooptical.net
Reply by ●April 12, 20112011-04-12
George Herold <gherold@teachspin.com> writes:> On Apr 12, 11:12 am, John Devereux <j...@devereux.me.uk> wrote: >> George Herold <gher...@teachspin.com> writes: >> > I’ looking at the step response of a photodiode (PD). I’m stepping >> > the current into a LED such that the step doubles the photo current >> > from the PD. (I’m not starting with the LED off, but on slightly.) >> > Here’s a ‘scope shot. The bottom trace in the PD response. (The top >> > is the voltage step to the LED (through 100 ohms)... AC coupled to get >> > rid of a big DC offset.) >> >> >http://img189.imageshack.us/i/tek0024.png/ >> >> > There is this long tail on the step response. Here’s the same picture >> > with the timebase slowed down. >> >> >http://img812.imageshack.us/i/tek0025.png/ >> >> > Looks something like a 2-2.5 us tail on the step.[...]>> >> >http://img52.imageshack.us/i/tek0026.png/ >> > The red LED is the trace that is a bit higher. I’m looking to see if >> > I can dig up any blue or white LED’s. >> >> > Any other ideas how I might determine if that's what I'm seeing... >> >> > Oh, the PD is a OSI-optoelectroncs PIN-3CD (3.2mm^2 area) reversed >> > biased at ~12V. >> >> Hi George, >> >> You may find VCSELs easier to use than laster diodes. They can usually >> be safely driven at constant current and have ~circular beams, some of >> them have built-in collimator lenses too, They are noisy compared to >> LEDs if that is an issue, but are designed to be modulated very quickly >> (GHz), > > Thanks John, I've got lotsa laser diodes. VCSELs sound expensive. > Who sells them?They're quite affordable, comparable to laser diodes really depending on type of course. I bought some from Roithner <http://www.roithner-laser.com/> (European company, they have a huge range of laser diodes and leds). Looks like cheapest is $9.25 for PM85-D1POU (2mW 850nm with dome lens, 2 degree beam). Just remembered there are also "resonant cavity" leds, similar pricing, these are also designed for fast modulation (3ns rise/fall times on the one I am looking at now). Don't know really what their noise is like but being "leds" I would guess they are better than laser diodes. -- John Devereux
Reply by ●April 12, 20112011-04-12
On Tue, 12 Apr 2011 21:17:38 +0100, John Devereux <john@devereux.me.uk> wrote:>George Herold <gherold@teachspin.com> writes: > >> On Apr 12, 11:12�am, John Devereux <j...@devereux.me.uk> wrote: >>> George Herold <gher...@teachspin.com> writes: >>> > I� looking at the step response of a photodiode (PD). �I�m stepping >>> > the current into a LED such that the step doubles the photo current >>> > from the PD. �(I�m not starting with the LED off, but on slightly.) >>> > Here�s a �scope shot. �The bottom trace in the PD response. �(The top >>> > is the voltage step to the LED (through 100 ohms)... AC coupled to get >>> > rid of a big DC offset.) >>> >>> >http://img189.imageshack.us/i/tek0024.png/ >>> >>> > There is this long tail on the step response. �Here�s the same picture >>> > with the timebase slowed down. >>> >>> >http://img812.imageshack.us/i/tek0025.png/ >>> >>> > Looks something like a 2-2.5 us tail on the step. > >[...] > >>> >>> >http://img52.imageshack.us/i/tek0026.png/ >>> > The red LED is the trace that is a bit higher. �I�m looking to see if >>> > I can dig up any blue or white LED�s. >>> >>> > Any other ideas how I might determine if that's what I'm seeing... >>> >>> > Oh, the PD is a OSI-optoelectroncs PIN-3CD (3.2mm^2 area) reversed >>> > biased at ~12V. >>> >>> Hi George, >>> >>> You may find VCSELs easier to use than laster diodes. They can usually >>> be safely driven at constant current and have ~circular beams, some of >>> them have built-in collimator lenses too, They are noisy compared to >>> LEDs if that is an issue, but are designed to be modulated very quickly >>> (GHz), >> >> Thanks John, I've got lotsa laser diodes. VCSELs sound expensive. >> Who sells them? > >They're quite affordable, comparable to laser diodes really depending on >type of course. > >I bought some from Roithner <http://www.roithner-laser.com/> (European >company, they have a huge range of laser diodes and leds). Looks like >cheapest is $9.25 for PM85-D1POU (2mW 850nm with dome lens, 2 degree >beam).Nice. Any hassles with end-use certificates or do they just ship whatever you order asap?
Reply by ●April 12, 20112011-04-12
On Tue, 12 Apr 2011 12:34:54 -0700, AES <siegman@stanford.edu> wrote:>> > Is there any hidden connection between the insistence of these LEDs on >> > continuing to emit light long after they've been turned off, and the >> > insistence of the participants in this thread in continuing to re-emit >> > screenfuls and screenfuls of previous posts, before finally adding a >> > one-sentence comment at the end?!?!?! > > >> Modern newsreaders often run on machines with LED backlights? ;) >> Cheers, Phil Hobbs > >This thread really has been off the charts in the unwillingness of >posters to trim out unnecessary cruft when they reply!Get some scroll bars. They're not expensive. John
Reply by ●April 12, 20112011-04-12
On Tue, 12 Apr 2011 13:45:20 -0500, wiljan@nospam.pobox.com (Will Janoschka) wrote:>On Tue, 12 Apr 2011 16:21:14, George Herold <gherold@teachspin.com> >wrote: > >> On Apr 12, 10:19�am, John Larkin >> <jjlar...@highNOTlandTHIStechnologyPART.com> wrote: >> > On Tue, 12 Apr 2011 06:50:16 -0700 (PDT), George Herold >> > >> <Big snip> >> >> > >> > >Thanks John, �If it's due to more current because of a drop in forward >> > >voltage, then I should see that in the current. �(Which I don't... >> > >sigh) >> > >> > What's the LED driver circuit like? >> >> I've used a few different configurations... all give the same result. >> Coax from the signal generator, daisy chains to the 'scope and on to >> the the LED. There I had 50 ohms to ground and in parallel 500 ohms >> feeding the LED to ground. Originally there was a few inches of wire >> between the 500 ohm resistor and the LED, but I later tightened it up >> and put everything right at the LED. I later just feed the LED >> through a 50 ohm resistor. >> >> >> The PD circuit is standard TIA >> >> http://img651.imageshack.us/i/tia2.png/ >> >> I'm going to write to OSI (the PD maker.) I wanted to be sure I >> wasn't doing something stupid beofre contacting them... >> >> George H. >> > >> > >> > >> > >OK let me try a totally different idea. �Can the stray capacitance of >> > >'stuff' have a long tail like that. �Charges moving around on pieces >> > >of plastic/fiber glass or something? >> > >> > Probably not the tail you're seeing. PCB 'capacitors' can have 'hook' >> > behavior, but your waveforms don't look like that. >> > >> > Post (or email me) your LED driver and pd circuits, and I'll see if I >> > can spot anything. >> > >> > >> > >> > >I could air wire the circuit and see what that does... >> > >> > Time constants in the microseconds probably aren't strays. >> > >> > John- Hide quoted text - >> > >> > - Show quoted text - >> > >Do you have any kind of a chopper or shutter that you could >use to isolate the time constant(s) of the PD frome those of the >LED/ laser?It would be interesting to see how fast an edge you could create with a mechanical chopper. Nanoseconds would be tough. John
Reply by ●April 12, 20112011-04-12
On Apr 12, 2:45=A0pm, wil...@nospam.pobox.com (Will Janoschka) wrote:> On Tue, 12 Apr 2011 16:21:14, George Herold <gher...@teachspin.com> > wrote: > > > > > > > On Apr 12, 10:19 am, John Larkin > > <jjlar...@highNOTlandTHIStechnologyPART.com> wrote: > > > On Tue, 12 Apr 2011 06:50:16 -0700 (PDT), George Herold > > > <Big snip> > > > > >Thanks John, If it's due to more current because of a drop in forwar=d> > > >voltage, then I should see that in the current. (Which I don't... > > > >sigh) > > > > What's the LED driver circuit like? > > > I've used a few different configurations... all give the same result. > > Coax from the signal generator, daisy chains to the 'scope and on to > > the the LED. =A0There I had 50 ohms to ground and in parallel 500 ohms > > feeding the LED to ground. =A0Originally there was a few inches of wire > > between the 500 ohm resistor and the LED, but I later tightened it up > > and put everything right at the LED. =A0I later just feed the LED > > through a 50 ohm resistor. > > > The PD circuit is standard TIA > > >http://img651.imageshack.us/i/tia2.png/ > > > I'm going to write to OSI (the PD maker.) =A0I wanted to be sure I > > wasn't doing something stupid beofre contacting them... > > > George H. > > > > >OK let me try a totally different idea. Can the stray capacitance of > > > >'stuff' have a long tail like that. Charges moving around on pieces > > > >of plastic/fiber glass or something? > > > > Probably not the tail you're seeing. PCB 'capacitors' can have 'hook' > > > behavior, but your waveforms don't look like that. > > > > Post (or email me) your LED driver and pd circuits, and I'll see if I > > > can spot anything. > > > > >I could air wire the circuit and see what that does... > > > > Time constants in the microseconds probably aren't strays. > > > > John- Hide quoted text - > > > > - Show quoted text - > > Do you have any kind of a chopper or shutter that you could > use to isolate =A0the time constant(s) of the PD frome those of the > LED/ laser?- Hide quoted text - > > - Show quoted text -Yeah, I was thinking about sweeping a laser across the PD face, with a rotating mirror or something. But then it's real optics and not just sticking an LED against a PD. George H.
Reply by ●April 12, 20112011-04-12
On Apr 12, 3:34=A0pm, AES <sieg...@stanford.edu> wrote:> > > Is there any hidden connection between the insistence of these LEDs o=n> > > continuing to emit light long after they've been turned off, and the > > > insistence of the participants in this thread in continuing to re-emi=t> > > screenfuls and screenfuls of previous posts, before finally adding a > > > one-sentence comment at the end?!?!?! > > Modern newsreaders often run on machines with LED backlights? =A0;) > > Cheers, =A0Phil Hobbs > > This thread really has been off the charts in the unwillingness of > posters to trim out unnecessary cruft when they reply!Sorry AES, "Twas I", that crossed posted to both sci.optics and SED (sci.electronics.design). Next time I want to ask both groups the same question, I'll start separate threads. George H.
Reply by ●April 12, 20112011-04-12
On 4/12/2011 4:36 PM, John Larkin wrote:> On Tue, 12 Apr 2011 12:34:54 -0700, AES<siegman@stanford.edu> wrote:(TRIM)>> posters to trim out unnecessary cruft when they reply! > > Get some scroll bars. They're not expensive. > > John >Good one, John!
Reply by ●April 12, 20112011-04-12
On Apr 12, 3:56=A0pm, Phil Hobbs <pcdhSpamMeSensel...@electrooptical.net> wrote:> George Herold wrote: > > On Apr 12, 10:45 am, Phil Hobbs > > <pcdhSpamMeSensel...@electrooptical.net> =A0wrote: > >> Phil Hobbs wrote: > >>> George Herold wrote: > >>>> On Apr 11, 10:59 pm, John Larkin > >>>> <jjlar...@highNOTlandTHIStechnologyPART.com> =A0wrote: > >>>>> On Mon, 11 Apr 2011 16:35:20 -0700 (PDT), George Herold > > >>>>> <gher...@teachspin.com> =A0wrote: > >>>>>> On Apr 11, 7:04 pm, John Larkin > >>>>>> <jjlar...@highNOTlandTHIStechnologyPART.com> =A0wrote: > >>>>>>> On Mon, 11 Apr 2011 12:46:16 -0700 (PDT), George Herold > > >>>>>>> <gher...@teachspin.com> =A0wrote: > >>>>>>>> On Apr 11, 3:32 pm, Phil Hobbs > >>>>>>>> <pcdhSpamMeSensel...@electrooptical.net> =A0wrote: > >>>>>>>>> George Herold wrote: > >>>>>>>>>> I looking at the step response of a photodiode (PD). I m stepp=ing> >>>>>>>>>> the current into a LED such that the step doubles the photo cu=rrent> >>>>>>>>>> from the PD. (I m not starting with the LED off, but on slight=ly.)> >>>>>>>>>> Here s a scope shot. The bottom trace in the PD response. (The=top> >>>>>>>>>> is the voltage step to the LED (through 100 ohms)... AC couple=d> >>>>>>>>>> to get > >>>>>>>>>> rid of a big DC offset.) > > >>>>>>>>>>http://img189.imageshack.us/i/tek0024.png/ > > >>>>>>>>>> There is this long tail on the step response. Here s the same > >>>>>>>>>> picture > >>>>>>>>>> with the timebase slowed down. > > >>>>>>>>>>http://img812.imageshack.us/i/tek0025.png/ > > >>>>>>>>>> Looks something like a 2-2.5 us tail on the step. > > >>>>>>>>>> What s it caused by? I first thought it was perhaps heating of=the> >>>>>>>>>> LED during the step. But I was reading Photodetectors by S. Do=nati,> >>>>>>>>>> over the weekend. In Chapter 5 he talks about two sets of phot=o-> >>>>>>>>>> generated carriers the majority are made in the depletion laye=r,> >>>>>>>>>> and > >>>>>>>>>> travel with the drift speed, (times in the nano-second range.)=The> >>>>>>>>>> second smaller set are those generated in the doped region. If=they> >>>>>>>>>> are within a diffusion length of the depletion region then the=se> >>>>>>>>>> have > >>>>>>>>>> a chance of adding to the photocurrent. Donati says that these > >>>>>>>>>> have a > >>>>>>>>>> time constant in the microsecond range! I wonder if this is wh=at> >>>>>>>>>> I am > >>>>>>>>>> seeing? > > >>>>>>>>>> I looked at the step response with a different color LED. (you > >>>>>>>>>> expect > >>>>>>>>>> more absorption in the doped region with shorter wavelength li=ght.)> >>>>>>>>>> So here is the pulse response from a 635 nm red and 594 nm Amb=er> >>>>>>>>>> LED. > > >>>>>>>>>>http://img52.imageshack.us/i/tek0026.png/ > >>>>>>>>>> The red LED is the trace that is a bit higher. I m looking to > >>>>>>>>>> see if > >>>>>>>>>> I can dig up any blue or white LED s. > > >>>>>>>>>> Any other ideas how I might determine if that's what I'm seein=g...> > >>>>>>>>>> Oh, the PD is a OSI-optoelectroncs PIN-3CD (3.2mm^2 area) reve=rsed> >>>>>>>>>> biased at ~12V. > > >>>>>>>>>> George H. > > >>>>>>>>> Crank up the bias till the diode is fully depleted, and see. > > >>>>>>>>> Cheers > > >>>>>>>>> Phil Hobbs > > >>>>>>>> Thanks Phil, > > >>>>>>>> I can try more bias. Is there any way to know it's fully deplete=d?> >>>>>>>> (Or do I just take it to the maximum as specified by the maker. =(30> >>>>>>>> volts in this case.) > > >>>>>>>> The white LED showed more of the long tail... but less of a shar=p> >>>>>>>> turn- > >>>>>>>> on, I wonder if it's just the LED time response that I'm seeing. > > >>>>>>> A white LED has a phosphor that is likely slow. > > >>>>>> Yeah, I noticed that.. Thanks to Grant too. > > >>>>>> I didn't save the screeen shot on my flash. (I should hit the save > >>>>>> button more often.) > >>>>>> IR led's are slow too. > > >>>>>>> Try a laser diode. They are (usually) fast. > > >>>>>>> I did encounter some laser diodes that had a weird slow step resp=onse> >>>>>>> when used at low pulse rates. They behaved, electrically and > >>>>>>> optically, as if there were an inductor in series with the diode.=At> >>>>>>> high rep-rates, 1 MHz maybe, the effect disappeared. > > >>>>>> Bring out the big guns ehh? I've never looked at the step response > >>>>> >from our laser diode, but we've modulated it up to ~150 MHz. There='s> >>>>>> a reported change in the modulation sensitivity, at the microsecon=d> >>>>>> range and longer its due to heating, at shorter times it drops by =an> >>>>>> order of magnitude and is due to (index of refraction?) carrier > >>>>>> density effects. At very high frequency you can get an enhancement > >>>>>> due to relaxation oscialltion in the LD... about 6 GHz, in the LD'=s I> >>>>>> know. > > >>>>>>> Try looking at the voltage across the LED, and see if it's a clea=n> >>>>>>> step. > > >>>>>>> What's the signal path from the photodiode to the scope? Since al=l the> >>>>>>> LEDs seem to have similar waveforms, maybe the detector is the > >>>>>>> problem. > >>>>>> Yeah sorry I didn't save all the crummy 'scope shots. > >>>>>> But I'm certainly open to some silly measurement error. There's a =TIA> >>>>>> with OPA134 (GBW~8MHz, 1-3pF Cin) and 10kohm feedback resistor, wi=th> >>>>>> 3.3pF parallel capacitance (plus some extra stray... call it ~2 pF=,> >>>>>> perhaps a bit less). > > >>>>>> I think I'm seeing a LED temperature effect. Does the light output=go> >>>>>> up as an LED gets hotter? If it's linear with delta T, I can't cha=nge> >>>>>> drive level and make it go away. > > >>>>> LEDs are less efficient when they're hot. But the forward drop goes > >>>>> down. So there is usually some source resistance where the effects > >>>>> cancel. > > >>>> Thanks John, If it's due to more current because of a drop in forwar=d> >>>> voltage, then I should see that in the current. (Which I don't... > >>>> sigh) > > >>>> OK let me try a totally different idea. Can the stray capacitance of > >>>> 'stuff' have a long tail like that. Charges moving around on pieces > >>>> of plastic/fiber glass or something? > > >>>> I could air wire the circuit and see what that does... > > >>>> George H. > > >>>>> John- Hide quoted text - > > >>>>> - Show quoted text - > > >>> How about posting the schematic? If the edges are symmetrical, it's > >>> probably the TIA--the LED would be cooling down when it wasn't emitti=ng> >>> light, so you wouldn't see the thermal tail. > > >>> Op amps have really terrible supply rejection when it comes to fast > >>> transients--depending on the feedback, you can easily get _gain_ from > >>> the supply pin to the output. (PSR is specified with respect to the > >>> amplifier _inputs_.) > > >>> Another possibility is phase funnies in the open loop gain. Composite > >>> amps are notorious for that sort of thing. The long-time settling > >>> behaviour is dominated by the lowest-frequency pole or zero in the op=en> >>> loop transfer function--lead-lag networks can cause this sort of thin=g> >>> even though the closed-loop response looks nice on a spectrum analyze=r.> > >>> Try touching the summing junction with your finger and see what happe=ns> >>> to the step response. > > >>> Cheers > > >>> Phil Hobbs > > >> Rereading, it looks like you have something like a 20 pF summing > >> junction capacitance and a 2 pF feedback cap. =A0That means you have a=n> >> open-loop pole/zero pair separated by a factor of 10 in frequency, up > >> near the GBW of the op amp. =A0That will lead to some entertaining > >> settling behaviour, which at this point would be my best guess as to > >> what you're seeing. > > > Yeah perhaps more like 3.3 pF across the feedback. > > > The pulse response through a 'big' resistor looks just fine, =A0(no PD > > or LED). =A0I went back and added the 'missing' 12 pF to ground, just i=n> > case...(That's the removed PD capacitance, the strays and opamp C are > > still there.) > > > Hey this might be some 'real' physics and not just a circuit screw- > > up. > > > George H. > > Well, it does look like the photodiode, which suggests that the > aforementioned diffusion tail mechanism is doing it. =A0 =A0I'm surprised > that jacking up the bias didn't fix it. =A0 =A0It isn't usually as bad as=that.> > Cheers > > Phil Hobbs > > -- > Dr Philip C D Hobbs > Principal > ElectroOptical Innovations > 55 Orchard Rd > Briarcliff Manor NY 10510 > 845-480-2058 > > email: hobbs (atsign) electrooptical (period) nethttp://electrooptical.ne=t- Hide quoted text -> > - Show quoted text -Yeah, but I'm still confused. I tried it with an adjustable bais (from 0-12V) late today. There was the expected change in the short time response. But none in the long tail. I had a nice chat with someone at OSI. (sent pic's and schematics to them.). I'll have to try a different PD. George H.
