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 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=92s 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=92s it caused by? I first thought it was perhaps heating of the LED during the step. But I was reading =93Photodetectors=94 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=92m looking to see if I can dig up any blue or white LED=92s. 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.
LED on Photodiode step response
Started by ●April 11, 2011
Reply by ●April 11, 20112011-04-11
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) net http://electrooptical.net
Reply by ●April 11, 20112011-04-11
On Apr 11, 3:32=A0pm, Phil Hobbs <pcdhSpamMeSensel...@electrooptical.net> wrote:> George Herold wrote: > > I looking at the step response of a photodiode (PD). =A0I m stepping > > 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(The to=p> > 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 pictur=e> > 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 the > > LED during the step. =A0But I was reading Photodetectors by S. Donati, > > over the weekend. =A0In 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, =A0(times in the nano-second range.) =A0Th=e> > second smaller set are those generated in the doped region. =A0If they > > are within a diffusion length of the depletion region then these have > > a chance of adding to the photocurrent. =A0Donati says that these have =a> > time constant in the microsecond range! =A0I wonder if this is what I a=m> > seeing? > > > I looked at the step response with a different color LED. =A0(you expec=t> > 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 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.ne=t- 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.
Reply by ●April 11, 20112011-04-11
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.The blue from the LED will mostly be getting absorbed in the epi, so you won't see much of it. Diffusion out of the epi may be what you're seeing on the other LEDs too. Try a 900 or 940 nm LED and see if that makes a difference. Re full depletion: I'd crank it right up to 30V, with an RC to protect it from breakdown. You can probably go significantly higher than that. Most diodes speed up amazingly when they're fully depleted--you may see the bandwidth double between 27 and 30V. Donati has a curve somewhere. (Great book, Donati, but a bit unevenly edited. Still, a sterling effort for an E2L writer.) 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 11, 20112011-04-11
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. Nemo
Reply by ●April 11, 20112011-04-11
On Apr 11, 3:54=A0pm, Phil Hobbs <pcdhSpamMeSensel...@electrooptical.net> wrote:> George Herold wrote: > > 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). =A0I m stepping > >>> 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(The =top> >>> is the voltage step to the LED (through 100 ohms)... AC coupled to ge=t> >>> 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 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. > > >>> What s it caused by? =A0I first thought it was perhaps heating of the > >>> LED during the step. =A0But I was reading Photodetectors by S. Donati=,> >>> over the weekend. =A0In 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, =A0(times in the nano-second range.) =A0=The> >>> second smaller set are those generated in the doped region. =A0If the=y> >>> are within a diffusion length of the depletion region then these have > >>> a chance of adding to the photocurrent. =A0Donati says that these hav=e 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 exp=ect> >>> 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 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. =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 turn- > > on, I wonder if it's just the LED time response that I'm seeing. > > > George H. > > The blue from the LED will mostly be getting absorbed in the epi, so you > won't see much of it. =A0Diffusion out of the epi may be what you're > seeing on the other LEDs too. =A0Try a 900 or 940 nm LED and see if that > makes a difference. > > Re full depletion: > I'd crank it right up to 30V, with an RC to protect it from breakdown. > You can probably go significantly higher than that. =A0Most diodes speed > up amazingly when they're fully depleted--you may see the bandwidth > double between 27 and 30V. =A0Donati has a curve somewhere. =A0(Great boo=k,> Donati, but a bit unevenly edited. =A0Still, a sterling effort for an E2L > writer.) > > 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 -OK 30 Volt bias made no difference in the step response. (I added in two 9V batteries, I put 100 ohms in series, 0.1uF) The lack of a change in the fast response time at 30 V may not be surprising, there's a lot of excess stray capacitance in the circuit. (7 pf or so.) So I may not have noticed. I'm going to try an IR LED. (I thought those were too slow though. ... I'll find out.) George H.
Reply by ●April 11, 20112011-04-11
On Apr 11, 4:05=A0pm, Nemo <N...@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. > > NemoThanks Nemo, I sent an email to OSI (the manufacturer). They may be able to tell me something. (If I get sent to the right person!) George H.
Reply by ●April 11, 20112011-04-11
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. 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.
Reply by ●April 11, 20112011-04-11
On Apr 11, 4:38=A0pm, 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 stepping > >>> 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(The =top> >>> is the voltage step to the LED (through 100 ohms)... AC coupled to ge=t> >>> 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 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. > >>> What s it caused by? =A0I first thought it was perhaps heating of the > >>> LED during the step. =A0But I was reading Photodetectors by S. Donati=,> >>> over the weekend. =A0In 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, =A0(times in the nano-second range.) =A0=The> >>> second smaller set are those generated in the doped region. =A0If the=y> >>> are within a diffusion length of the depletion region then these have > >>> a chance of adding to the photocurrent. =A0Donati says that these hav=e 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 exp=ect> >>> 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 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. =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 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 -
Reply by ●April 11, 20112011-04-11
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 - >Actually, any factsheet will do, to show the relation between capacity and voltage. Google:> http://home.sandiego.edu/~ekim/photodiode/pdtech.htmlshows the relation between voltage and capacity.
