George Herold wrote:> On Apr 11, 4:38 pm, Sjouke Burry <burrynulnulf...@ppllaanneett.nnll> > wrote: >> George Herold 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 >>>> -- >>>> 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.net-Hide quoted text - >>>> - Show quoted text - >>> 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. >>> George H. >> You could also switch the LED on/off by shorting the LED >> current to ground, thereby discharging the LED capacitance. > > Ohh, maybe that long tail is just the series 100 ohms and C of the > LED? (20nF that can't be it.) I looked at the current through the > LED and that was nice and square. > >> As for the photo diode, I would advise 0.5 times max, 15 volts. >> The factsheet of the photo diode should have a plot of >> capacitance versus voltage. > > If OSI has that information they are keeping it well hidden on their > web site. Perhaps the email will shake something out of them... (More > likely I'll be ignored.) > > George H. > - Hide quoted text - >> - Show quoted text - >Oh, and I forgot to tell you, NEVER USE A WHITE LED!!! They use a fluorescent coating to convert blue light to white light, and that fluorescent keeps on emitting light long after the LED switches of!!!!!! Use a blue one, if you need high energy photons, besides, IR diodes have one bad property, you cant see them working.... (with your own eyes that is.)
LED on Photodiode step response
Started by ●April 11, 2011
Reply by ●April 11, 20112011-04-11
Reply by ●April 11, 20112011-04-11
George Herold wrote:> I=92 looking at the step response of a photodiode (PD). I=92m stepping=> the current into a LED such that the step doubles the photo current > from the PD. (I=92m not starting with the LED off, but on slightly.) > Here=92s a =91scope shot. The bottom trace in the PD response. (The t=op> is the voltage step to the LED (through 100 ohms)... AC coupled to get > rid of a big DC offset.) >=20 > http://img189.imageshack.us/i/tek0024.png/ >=20 > There is this long tail on the step response. Here=92s the same pictur=e> with the timebase slowed down. >=20 > http://img812.imageshack.us/i/tek0025.png/ >=20 > Looks something like a 2-2.5 us tail on the step.Your circuit is slow. While ago I measured the pulse response of a cheap general purpose LED=20 vs moderately good transimpedance photodetector (Fc ~ 500 MHz). The=20 timings were in 50ns range. Vladimir Vassilevsky DSP and Mixed Signal Design Consultant http://www.abvolt.com
Reply by ●April 11, 20112011-04-11
On Mon, 11 Apr 2011 11:39:22 -0700 (PDT), George Herold <gherold@teachspin.com> 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.Would a white LED give an afterglow from the phosphor? Blue be better to try? Grant.> >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.
Reply by ●April 11, 20112011-04-11
On Apr 11, 5:10=A0pm, Sjouke Burry <burrynulnulf...@ppllaanneett.nnll> wrote:> George Herold wrote: > > On Apr 11, 4:38 pm, Sjouke Burry <burrynulnulf...@ppllaanneett.nnll> > > wrote: > >> George Herold 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). =A0I m steppin=g> >>>>> the current into a LED such that the step doubles the photo current > >>>>> from the PD. =A0(I m not starting with the LED off, but on slightly=.)> >>>>> Here s a scope shot. =A0The bottom trace in the PD response. =A0(Th=e 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. =A0Here s the same pi=cture> >>>>> 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? =A0I first thought it was perhaps heating of t=he> >>>>> LED during the step. =A0But I was reading Photodetectors by S. Dona=ti,> >>>>> over the weekend. =A0In Chapter 5 he talks about two sets of photo- > >>>>> generated carriers the majority are made in the depletion layer, an=d> >>>>> travel with the drift speed, =A0(times in the nano-second range.) ==A0The> >>>>> second smaller set are those generated in the doped region. =A0If t=hey> >>>>> are within a diffusion length of the depletion region then these ha=ve> >>>>> a chance of adding to the photocurrent. =A0Donati says that these h=ave a> >>>>> time constant in the microsecond range! =A0I wonder if this is what=I am> >>>>> seeing? > >>>>> I looked at the step response with a different color LED. =A0(you e=xpect> >>>>> 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. =A0I m looking to se=e 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 > >>>> -- > >>>> Dr Philip C D Hobbs > >>>> Principal > >>>> ElectroOptical Innovations > >>>> 55 Orchard Rd > >>>> Briarcliff Manor NY 10510 > >>>> 845-480-2058 > >>>> email: hobbs (atsign) electrooptical (period) nethttp://electrooptic=al.net-Hidequoted text -> >>>> - Show quoted text - > >>> Thanks Phil, > >>> I can try more bias. =A0Is 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 tur=n-> >>> on, I wonder if it's just the LED time response that I'm seeing. > >>> George H. > >> You could also switch the LED on/off by shorting the LED > >> current to ground, thereby discharging the LED capacitance. > > > Ohh, maybe that long tail is just the series 100 ohms and C of the > > LED? =A0(20nF that can't be it.) =A0I looked at the current through the > > LED and that was nice and square. > > >> As for the photo diode, I would advise 0.5 times max, 15 volts. > >> The factsheet of the photo diode should have a plot of > >> capacitance versus voltage. > > > If OSI has that information they are keeping it well hidden on their > > web site. =A0Perhaps the email will shake something out of them... (Mor=e> > likely I'll be ignored.) > > > George H. > > - Hide quoted text - > >> - Show quoted text - > > Actually, any factsheet will do, to show the relation > between capacity and voltage.Goes as the square root of the bias voltage?> Google:>http://home.sandiego.edu/~ekim/photodiode/pdtech.htmlThat's Great! Is it yours? Can I give a link to it? George H.> > shows the relation between voltage and capacity.- Hide quoted text - > > - Show quoted text -
Reply by ●April 11, 20112011-04-11
On Apr 11, 5:17=A0pm, Sjouke Burry <burrynulnulf...@ppllaanneett.nnll> wrote:> George Herold wrote: > > On Apr 11, 4:38 pm, Sjouke Burry <burrynulnulf...@ppllaanneett.nnll> > > wrote: > >> George Herold 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). =A0I m steppin=g> >>>>> the current into a LED such that the step doubles the photo current > >>>>> from the PD. =A0(I m not starting with the LED off, but on slightly=.)> >>>>> Here s a scope shot. =A0The bottom trace in the PD response. =A0(Th=e 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. =A0Here s the same pi=cture> >>>>> 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? =A0I first thought it was perhaps heating of t=he> >>>>> LED during the step. =A0But I was reading Photodetectors by S. Dona=ti,> >>>>> over the weekend. =A0In Chapter 5 he talks about two sets of photo- > >>>>> generated carriers the majority are made in the depletion layer, an=d> >>>>> travel with the drift speed, =A0(times in the nano-second range.) ==A0The> >>>>> second smaller set are those generated in the doped region. =A0If t=hey> >>>>> are within a diffusion length of the depletion region then these ha=ve> >>>>> a chance of adding to the photocurrent. =A0Donati says that these h=ave a> >>>>> time constant in the microsecond range! =A0I wonder if this is what=I am> >>>>> seeing? > >>>>> I looked at the step response with a different color LED. =A0(you e=xpect> >>>>> 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. =A0I m looking to se=e 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 > >>>> -- > >>>> Dr Philip C D Hobbs > >>>> Principal > >>>> ElectroOptical Innovations > >>>> 55 Orchard Rd > >>>> Briarcliff Manor NY 10510 > >>>> 845-480-2058 > >>>> email: hobbs (atsign) electrooptical (period) nethttp://electrooptic=al.net-Hidequoted text -> >>>> - Show quoted text - > >>> Thanks Phil, > >>> I can try more bias. =A0Is 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 tur=n-> >>> on, I wonder if it's just the LED time response that I'm seeing. > >>> George H. > >> You could also switch the LED on/off by shorting the LED > >> current to ground, thereby discharging the LED capacitance. > > > Ohh, maybe that long tail is just the series 100 ohms and C of the > > LED? =A0(20nF that can't be it.) =A0I looked at the current through the > > LED and that was nice and square. > > >> As for the photo diode, I would advise 0.5 times max, 15 volts. > >> The factsheet of the photo diode should have a plot of > >> capacitance versus voltage. > > > If OSI has that information they are keeping it well hidden on their > > web site. =A0Perhaps the email will shake something out of them... (Mor=e> > likely I'll be ignored.) > > > George H. > > - Hide quoted text - > >> - Show quoted text - > > Oh, and I forgot to tell you, NEVER USE A WHITE LED!!! > They use a fluorescent coating to convert blue light to > white light, and that fluorescent keeps on emitting light > long after the LED switches of!!!!!!OK, I'll have to order some blue LEDs. Thanks, George H.> Use a blue one, if you need high energy photons, besides, > IR diodes have one bad property, you cant see them working.... > (with your own eyes that is.)- Hide quoted text - > > - Show quoted text -
Reply by ●April 11, 20112011-04-11
On Mon, 11 Apr 2011 12:46:16 -0700 (PDT), George Herold <gherold@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 >> >> -- >> 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.net- Hide quoted text - >> >> - Show quoted text - > >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. 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. 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. John
Reply by ●April 11, 20112011-04-11
On Mon, 11 Apr 2011 23:17:18 +0200, Sjouke Burry <burrynulnulfour@ppllaanneett.nnll> wrote:>George Herold wrote: >> On Apr 11, 4:38 pm, Sjouke Burry <burrynulnulf...@ppllaanneett.nnll> >> wrote: >>> George Herold 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 >>>>> -- >>>>> 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.net-Hide quoted text - >>>>> - Show quoted text - >>>> 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. >>>> George H. >>> You could also switch the LED on/off by shorting the LED >>> current to ground, thereby discharging the LED capacitance. >> >> Ohh, maybe that long tail is just the series 100 ohms and C of the >> LED? (20nF that can't be it.) I looked at the current through the >> LED and that was nice and square. >> >>> As for the photo diode, I would advise 0.5 times max, 15 volts. >>> The factsheet of the photo diode should have a plot of >>> capacitance versus voltage. >> >> If OSI has that information they are keeping it well hidden on their >> web site. Perhaps the email will shake something out of them... (More >> likely I'll be ignored.) >> >> George H. >> - Hide quoted text - >>> - Show quoted text - >> >Oh, and I forgot to tell you, NEVER USE A WHITE LED!!! >They use a fluorescent coating to convert blue light to >white light, and that fluorescent keeps on emitting light >long after the LED switches of!!!!!! >Use a blue one, if you need high energy photons, besides, >IR diodes have one bad property, you cant see them working.... >(with your own eyes that is.)I can see an 850 nm laser diode! John
Reply by ●April 11, 20112011-04-11
On Mon, 11 Apr 2011 21:05:00 +0100, Nemo <Nemo@nocannedmeatproducts.nosirree> wrote:>Yes, some photodiodes have response times on the order of 1-2 >microseconds. One I use, a physically large, high capacitance one >(~400pF under bias) is quoted as 2.5us by its manufacturers, who refer >to the charge carrier transit time phenomenon you mention. > >I have no idea if the charge carrier transit delay is dependent on >current levels, but it probably isn't as photodiodes are such perfect >transducers. I mention it to stimulate musing from someone else. > >NemoIncreasing the pd bias voltage will sweep the carriers out faster. He could try that. John
Reply by ●April 11, 20112011-04-11
On Apr 11, 5:42=A0pm, Vladimir Vassilevsky <nos...@nowhere.com> wrote:> George Herold wrote: > > I=92 looking at the step response of a photodiode (PD). =A0I=92m steppi=ng> > the current into a LED such that the step doubles the photo current > > from the PD. =A0(I=92m not starting with the LED off, but on slightly.) > > Here=92s a =91scope shot. =A0The bottom trace in the PD response. =A0(T=he 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. =A0Here=92s the same pict=ure> > with the timebase slowed down. > > >http://img812.imageshack.us/i/tek0025.png/ > > > Looks something like a 2-2.5 us tail on the step. > > Your circuit is slow.Yeah, I've got swithces, terminal blocks and IC sockets hanging on the inputs, about 7pF over the opamp(~1-3pF) and photodiode (~12pF).> > While ago I measured the pulse response of a cheap general purpose LED > vs moderately good transimpedance photodetector (Fc ~ 500 MHz). The > timings were in 50ns range.Was there any long tail at a few micro-seconds? Perhaps too long for you to have looked?> > Vladimir Vassilevsky > DSP and Mixed Signal Design Consultanthttp://www.abvolt.com- Hide quoted =text -> > - Show quoted text -
Reply by ●April 11, 20112011-04-11
On Mon, 11 Apr 2011 16:06:37 -0700 (PDT), George Herold <ggherold@gmail.com> wrote:>On Apr 11, 5:42�pm, Vladimir Vassilevsky <nos...@nowhere.com> 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. >> >> Your circuit is slow. > >Yeah, I've got swithces, terminal blocks and IC sockets hanging on the >inputs, about 7pF over the opamp(~1-3pF) and photodiode (~12pF). >Yikes! Clean it up! John
