On Tue, 12 Apr 2011 07:38:50 -0700 (PDT), George Herold <gherold@teachspin.com> wrote:>On Apr 12, 9:50�am, George Herold <gher...@teachspin.com> 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 >> >> > >> >> -- >> > >> >> 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-Hidequotedtext - >> >> > >> >> - 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. >> >> > >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. >> >> > >I just tried a laser diode (lamda= 785nm). Exactly the same tail... >It's some circuit thing? > >No, A current pulse through a resistor gives a nice square output.... >Grrr. > >I'm going to order a blue LED and set this aside for a bit. (Unless >someone has some idea?) >It's your receiver circuit. Try just the photodiode and a resistor. Signal average on the scope if you need to. John
LED on Photodiode step response
Started by ●April 11, 2011
Reply by ●April 12, 20112011-04-12
Reply by ●April 12, 20112011-04-12
On Apr 12, 10:30=A0am, Phil Hobbs <pcdhSpamMeSensel...@electrooptical.net> 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 stepping > >>>>>>> the current into a LED such that the step doubles the photo curre=nt> >>>>>>> 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 to=p> >>>>>>> is the voltage step to the LED (through 100 ohms)... AC coupled t=o 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 pic=ture> >>>>>>> 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 th=e> >>>>>>> LED during the step. But I was reading Photodetectors by S. Donat=i,> >>>>>>> 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.) Th=e> >>>>>>> second smaller set are those generated in the doped region. If th=ey> >>>>>>> are within a diffusion length of the depletion region then these =have> >>>>>>> a chance of adding to the photocurrent. Donati says that these ha=ve 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 ex=pect> >>>>>>> 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) reverse=d> >>>>>>> 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://electroopt=ical.net-Hidequotedtext -> > >>>>>> - Show quoted text - > > >>>>> Thanks Phil, > > >>>>> I can try more bias. =A0Is 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 sharp t=urn-> >>>>> 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, =A0I noticed that.. Thanks to Grant too. > > >>> I didn't save the screeen shot on my flash. =A0(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 respons=e> >>>> 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? =A0I've never looked at the step response > >> >from our laser diode, but we've modulated it up to ~150 MHz. =A0There='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. =A0At 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 t=he> >>>> 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. =A0There'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. =A0Does the light output=go> >>> up as an LED gets hotter? =A0If 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, =A0If it's due to more current because of a drop in forwar=d> > voltage, then I should see that in the current. =A0(Which I don't... > > sigh) > > > OK let me try a totally different idea. =A0Can the stray capacitance of > > 'stuff' have a long tail like that. =A0Charges 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? =A0If 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. =A0(PSR is specified with respect to the > amplifier _inputs_.) > > Another possibility is phase funnies in the open loop gain. =A0Composite > amps are notorious for that sort of thing. =A0The 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.I removed the PD and sent the (50ohm terminated) voltage pulse through a 100k ohm resistor into the inverting input. Beautiful square looking step at the output.> > Try touching the summing junction with your finger and see what happens > to the step response.Whee... after turning off the room lights, the 'laying on of fingers' did nothing for the slow tail. It did slow down the fast response. George H.> > 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 -
Reply by ●April 12, 20112011-04-12
On Apr 12, 10:19=A0am, John Larkin <jjlar...@highNOTlandTHIStechnologyPART.com> wrote:> On Tue, 12 Apr 2011 06:50:16 -0700 (PDT), George Herold ><Big snip>> > >Thanks John, =A0If it's due to more current because of a drop in forward > >voltage, then I should see that in the current. =A0(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. =A0Can the stray capacitance of > >'stuff' have a long tail like that. =A0Charges 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 -
Reply by ●April 12, 20112011-04-12
George Herold wrote:> On Apr 11, 5:10 pm, 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). 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-Hidequoted 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. > > Goes as the square root of the bias voltage? > >> Google:>http://home.sandiego.edu/~ekim/photodiode/pdtech.html > > > That'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 - >Not mine, just from google.
Reply by ●April 12, 20112011-04-12
On Monday, April 11, 2011 7:31:38 PM UTC-7, John wrote:> On 4/11/2011 9:23 PM, Phil Hobbs wrote:> >>>>> George Herold wrote: > >>>>>> I looking at the step response of a photodiode (PD).> > LEDs have long tails on their falling edges due to the relatively long > > carrier lifetime. That rising edge was when the light turned on, right? > > Because the falling edges always look like that.> What would happen if they were reverse biased by about a volt or so at > turn-off time? Would that help?It probably would, and if your driver can sink as well as source current, it's just a matter of putting a resistor-capacitor series pair in parallel with the current limit resistor.
Reply by ●April 12, 20112011-04-12
On Apr 12, 10:45=A0am, 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 steppin=g> >>>>>>>> the current into a LED such that the step doubles the photo curr=ent> >>>>>>>> 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 t=op> >>>>>>>> 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 t=he> >>>>>>>> LED during the step. But I was reading Photodetectors by S. Dona=ti,> >>>>>>>> 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.) T=he> >>>>>>>> second smaller set are those generated in the doped region. If t=hey> >>>>>>>> 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 ligh=t.)> >>>>>>>> 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) revers=ed> >>>>>>>> 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-Hidequotedtext - > > >>>>>>> - 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. (3=0> >>>>>> 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 respon=se> >>>>> when used at low pulse rates. They behaved, electrically and > >>>>> optically, as if there were an inductor in series with the diode. A=t> >>>>> 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 TI=A> >>>> 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 g=o> >>>> up as an LED gets hotter? If it's linear with delta T, I can't chang=e> >>>> 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. =A0That 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. =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, (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.> > 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 -
Reply by ●April 12, 20112011-04-12
On Apr 12, 11:12=A0am, John Larkin <jjlar...@highNOTlandTHIStechnologyPART.com> wrote:> On Tue, 12 Apr 2011 07:38:50 -0700 (PDT), George Herold > > > > > > <gher...@teachspin.com> wrote: > >On Apr 12, 9:50 am, George Herold <gher...@teachspin.com> 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 ste=pping> >> > >> >> > 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 slig=htly.)> >> > >> >> > Here s a scope shot. The bottom trace in the PD response. (T=he top> >> > >> >> > is the voltage step to the LED (through 100 ohms)... AC coup=led 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 sam=e 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 ph=oto-> >> > >> >> > generated carriers the majority are made in the depletion la=yer, 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 t=hese have> >> > >> >> > a chance of adding to the photocurrent. Donati says that the=se 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. (y=ou 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 A=mber> >> > >> >> > LED. > > >> > >> >> >http://img52.imageshack.us/i/tek0026.png/ > >> > >> >> > The red LED is the trace that is a bit higher. I m looking t=o 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 see=ing...> > >> > >> >> > Oh, the PD is a OSI-optoelectroncs PIN-3CD (3.2mm^2 area) re=versed> >> > >> >> > 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://electr=ooptical.net-Hidequotedtext-> > >> > >> >> - Show quoted text - > > >> > >> >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 microsecond > >> > >range and longer its due to heating, at shorter times it drops by a=n> >> > >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 T=IA> >> > >with OPA134 (GBW~8MHz, 1-3pF Cin) and 10kohm feedback resistor, wit=h> >> > >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 chan=ge> >> > >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. > > >I just tried a laser diode (lamda=3D 785nm). =A0Exactly the same tail... > >It's some circuit thing? > > >No, A current pulse through a resistor gives a nice square output.... > >Grrr. > > >I'm going to order a blue LED and set this aside for a bit. =A0(Unless > >someone has some idea?) > > It's your receiver circuit. > > Try just the photodiode and a resistor. Signal average on the scope if > you need to. > > John- Hide quoted text - > > - Show quoted text -Oh, I did that Friday. The PD is reversed biased 12 V, then through 10k ohm with a buffer looking at the top of the 10k. Here's the fast response... slower than without the TIA. http://img535.imageshack.us/i/tek0018.png/ And here's the slow time.... The long tail remains! http://img859.imageshack.us/i/tek0019.png/ George H.
Reply by ●April 12, 20112011-04-12
On Apr 12, 11:12=A0am, John Devereux <j...@devereux.me.uk> wrote:> George Herold <gher...@teachspin.com> writes: > > 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. > > > What=92s it caused by? =A0I first thought it was perhaps heating of the > > LED during the step. =A0But I was reading =93Photodetectors=94 by S. Do=nati,> > 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=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. > > 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), > > -- > > John Devereux- Hide quoted text - > > - Show quoted text -Thanks John, I've got lotsa laser diodes. VCSELs sound expensive. Who sells them? George H.
Reply by ●April 12, 20112011-04-12
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?
Reply by ●April 12, 20112011-04-12
> > 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 HobbsThis thread really has been off the charts in the unwillingness of posters to trim out unnecessary cruft when they reply!
