TIA Photodiode Bootstrap at 10MHz
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From: George Herold on 6 Nov 2009 08:55 On Nov 5, 11:46 pm, Artist <art...(a)sj.speakeasy.net> wrote: > George Herold wrote: > > On Nov 5, 12:40 pm, Artist<Art...(a)sj.speakeasy.net> wrote: > >> Phil Hobbs wrote: > >>> Artist wrote: > >>>> Phil Hobbs wrote: > >>>>> Artist wrote: > >>>>>> Phil Hobbs wrote: > >>>>>>> Artist wrote: > >>>>>>>> I need to bootstrap a photodiode in a TIA circuit similar to the way > >>>>>>>> it is done as shown on page 18 of: > >>>>>>>>http://cds.linear.com/docs/Datasheet/6244fa.pdf > >>>>>>>> This example is much too limited in bandwidth. I need a 10MHz > >>>>>>>> bandwidth. > > >>>>>>>> The bootstrapping is needed because of the low impedance of the > >>>>>>>> photodiode. This is 150pF in parallel with 1 Kohm. The problem is one > >>>>>>>> of designing a 10MHz unity gain amplifier with high impedance input, > >>>>>>>> low noise, negligible phase change, and unity gain. > > >>>>>>>> Does anyone have any ideas? I am not sure it can be done. > > >>>>>>> One method is to connect the PD directly to the input of a nice quiet > >>>>>>> 50-ohm amplifier. If you have at least 200 uA of photocurrent, this > >>>>>>> will > >>>>>>> work very well--you can get to the shot noise limit that way. > > >>>>>>> At lower photocurrents, life gets a bit harder. Your particular > >>>>>>> problem > >>>>>>> gets quite difficult below about 20 uA--at that point you have to > >>>>>>> start > >>>>>>> trading away SNR or reducing that capacitance. The best Si PIN diodes > >>>>>>> have a capacitance of 40-100 pF/cm**2 when reverse biased, so if > >>>>>>> your PD > >>>>>>> isn't at least a half inch square, you can reduce the capacitance by > >>>>>>> choosing a different PD and/or reverse biasing. > > >>>>>>> So how big a photocurrent are you expecting, and what's your SNR > >>>>>>> target? > > >>>>>>> Cheers > > >>>>>>> Phil Hobbs > > >>>>>> The peak current is expected to be 1 uA. > > >>>>> If there's a way to make that 10 uA, your life will be much easier. > > >>>>>> The latest value for the capacitance I have is now 30pF. > > >>>>>> I do not have a choice on photodiodes. The detector I have been > >>>>>> assigned to make work for this project is not actually a photodiode in > >>>>>> the conventional sense. It is a custom made photoelectromotive force > >>>>>> detector for use in a laser ultrasonics application. This device > >>>>>> cannot be reverse biased like a PIN diode. > > >>>>>> A major concern about the low series resistance is that it will create > >>>>>> a high gain noninverting amplifier with the feedback resistor for the > >>>>>> equivalent input noise on the inverting input. This gain will also > >>>>>> reduce the bandwidth of the opamp circuit. > > >>>>>> The zero the capacitance will make is another reason I am looking to > >>>>>> bootstrap this. > > >>>>> Bootstraps have the same noise multiplication problem as TIAs, for the > >>>>> same reason: they put their own noise voltage across the PD capacitance. > >>>>> With equivalent devices, you can get a 3 dB improvement by using both, > >>>>> but bootstrapping is not a slam dunk. One good thing about it is that > >>>>> you can AC-couple the bootstrap, which means it can be single-ended > >>>>> rather than differential. > > >>>>> You can get the same 3 dB improvement by putting a TIA on each end of > >>>>> the PD. > > >>>>> If it's a photoacoustic measurement, you may not need DC-10 MHz. What's > >>>>> the actual measurement bandwidth? > > >>>>> Cheers > > >>>>> Phil Hobbs > > >>>> The low end of the bandwidth is 100kHz. > > >>>> A bootstrap does not necessarily add noise to the circuit. If the > >>>> bootstrap amplifier has less noise than the op amp its noise is > >>>> swapped for that of the op amp. This measured reduction in noise is > >>>> documented in figures 4b, 5b, and 6b on pages 17 and 18 of: > >>>>http://cds.linear.com/docs/Datasheet/6244fa.pdf > > >>>> Suppose I returned the photoelectromotive detector to ground. Since > >>>> the expected detector current is 1uA, to get some reasonable output > >>>> from this first TIA stage I would need a feedback resistor on the > >>>> order of 1Mohm. Since the series resistance of the detector is 1kOhm > >>>> there is a noise gain of 1001 before the zero from the 30pF > >>>> capacitance kicks in at 5.3MHz. Taking into account the DC noise gain, > >>>> the capacitance on the virtual ground (dominated by detector > >>>> capacitance) and the needed bandwidth of the TIA (10MHz) I would need > >>>> an opamp with an 18.8THz GBW. If it weren't for the noise gain I would > >>>> have needed only 18.8MHz. The only way this is going to be done is by > >>>> means of bootstrapping to reduce the effective impedance of the detector. > > >>> In general you can make the TIA out of the same type of device as the > >>> bootstrap, which is what I'm assuming. If you have a noise problem, > >>> then reducing the TIA noise is job 1, followed by bootstrapping. Using > >>> the same device type in the TIA and the bootstrap gives a noise level > >>> about 3 dB better than either one alone. > > >>> A quiet bootstrap plus a noisy TIA is a lot better than just the noisy > >>> TIA barefoot, but not as good as a quiet bootstrap and a quiet TIA. > > >>> The resistance of the detector is in series with the capacitance, not in > >>> parallel, so it looks like a lead-lag network. The capacitance makes a > >>> feedback zero, and the resistance puts in a zero. The noise gain may be > >>> 1000 at high frequency, but not at low frequency. > > >>> Cheers > > >>> Phil Hobbs > > >> I have been told by the project engineer who has given me this > >> assignment the capacitance is a parallel one. It is a custom detector > >> device so there is no data sheet I can link to. I have not even been > >> given a hard copy of one. So I can only go by what he has told me. Do > >> you have knowledge of photoelectromotive force detector physics that > >> causes you to disagree? > > >> Right now my main concern is just getting the bandwidth. Once one or > >> more ways to do that are established I can be concerned about minimizing > >> noise. > > >> I have not until now considered using another op amp instead of an amp > >> composed of discrete devices to do the bootstrapping. Doing it this way > >> would reduce added capacitance on the virtual ground. > > >> Regarding my earlier posting that was 18.8GHz not 18.8THz. > > >> -- > >> If you desire to respond directly remove the "sj." from the domain name > >> part of my email address. It is a spam jammer.- Hide quoted text - > > >> - Show quoted text - > > > "I have been told by the project engineer who has given me this > >> assignment the capacitance is a parallel one." > > > That sounds much worse. It's like some photo-resistor.? It will > > always be leaking as you bias it. (and leaking as the light shines on > > it.) ughh. > > > George H. > > Similar, but unlike the passive photoresistor, the electromotive force > detector converts photons into electrical energy. In a TIA circuit it > can be modeled as a current source that does not require a bias. The > current is proportional to the motion of a beam light across it. It is > very good at detecting changes in a laser light's speckled pattern > produced when it reflects off any but the most even of surfaces. > > Right now my plan is get two of the lowest noise JFET or CMOS input op > amps I can find that satisfy the bandwidth requirement. One will be used > for the feedback resistor and the other will do the bootstrap. > > -- > To reply directly remove the sj. from my email address. This is a spam > jammer.- Hide quoted text - > > - Show quoted text - You might look at the ADA4817 from analog devices. They show a TIA circuit that almost meets your bandwidth spec with 50k ohm of 'gain'. At 1uA that would give you 50mV of signal which is starting to get the shot noise above the johnson noise of the resistor. And perhaps you can do better with your bootstrap or using a cascode front end ala Phil H. I've got a sample of one of these in a drawer... whenever I find the time to try it. There is a nasty gain peak out beyond 100MHz. George H.
From: Phil Hobbs on 6 Nov 2009 09:45 Artist wrote: > George Herold wrote: >> On Nov 5, 12:40 pm, Artist<Art...(a)sj.speakeasy.net> wrote: >>> Phil Hobbs wrote: >>>> Artist wrote: >>>>> Phil Hobbs wrote: >>>>>> Artist wrote: >>>>>>> Phil Hobbs wrote: >>>>>>>> Artist wrote: >>>>>>>>> I need to bootstrap a photodiode in a TIA circuit similar to >>>>>>>>> the way >>>>>>>>> it is done as shown on page 18 of: >>>>>>>>> http://cds.linear.com/docs/Datasheet/6244fa.pdf >>>>>>>>> This example is much too limited in bandwidth. I need a 10MHz >>>>>>>>> bandwidth. >>> >>>>>>>>> The bootstrapping is needed because of the low impedance of the >>>>>>>>> photodiode. This is 150pF in parallel with 1 Kohm. The problem >>>>>>>>> is one >>>>>>>>> of designing a 10MHz unity gain amplifier with high impedance >>>>>>>>> input, >>>>>>>>> low noise, negligible phase change, and unity gain. >>> >>>>>>>>> Does anyone have any ideas? I am not sure it can be done. >>> >>>>>>>> One method is to connect the PD directly to the input of a nice >>>>>>>> quiet >>>>>>>> 50-ohm amplifier. If you have at least 200 uA of photocurrent, this >>>>>>>> will >>>>>>>> work very well--you can get to the shot noise limit that way. >>> >>>>>>>> At lower photocurrents, life gets a bit harder. Your particular >>>>>>>> problem >>>>>>>> gets quite difficult below about 20 uA--at that point you have to >>>>>>>> start >>>>>>>> trading away SNR or reducing that capacitance. The best Si PIN >>>>>>>> diodes >>>>>>>> have a capacitance of 40-100 pF/cm**2 when reverse biased, so if >>>>>>>> your PD >>>>>>>> isn't at least a half inch square, you can reduce the >>>>>>>> capacitance by >>>>>>>> choosing a different PD and/or reverse biasing. >>> >>>>>>>> So how big a photocurrent are you expecting, and what's your SNR >>>>>>>> target? >>> >>>>>>>> Cheers >>> >>>>>>>> Phil Hobbs >>> >>>>>>> The peak current is expected to be 1 uA. >>> >>>>>> If there's a way to make that 10 uA, your life will be much easier. >>> >>>>>>> The latest value for the capacitance I have is now 30pF. >>> >>>>>>> I do not have a choice on photodiodes. The detector I have been >>>>>>> assigned to make work for this project is not actually a >>>>>>> photodiode in >>>>>>> the conventional sense. It is a custom made photoelectromotive force >>>>>>> detector for use in a laser ultrasonics application. This device >>>>>>> cannot be reverse biased like a PIN diode. >>> >>>>>>> A major concern about the low series resistance is that it will >>>>>>> create >>>>>>> a high gain noninverting amplifier with the feedback resistor for >>>>>>> the >>>>>>> equivalent input noise on the inverting input. This gain will also >>>>>>> reduce the bandwidth of the opamp circuit. >>> >>>>>>> The zero the capacitance will make is another reason I am looking to >>>>>>> bootstrap this. >>> >>>>>> Bootstraps have the same noise multiplication problem as TIAs, for >>>>>> the >>>>>> same reason: they put their own noise voltage across the PD >>>>>> capacitance. >>>>>> With equivalent devices, you can get a 3 dB improvement by using >>>>>> both, >>>>>> but bootstrapping is not a slam dunk. One good thing about it is that >>>>>> you can AC-couple the bootstrap, which means it can be single-ended >>>>>> rather than differential. >>> >>>>>> You can get the same 3 dB improvement by putting a TIA on each end of >>>>>> the PD. >>> >>>>>> If it's a photoacoustic measurement, you may not need DC-10 MHz. >>>>>> What's >>>>>> the actual measurement bandwidth? >>> >>>>>> Cheers >>> >>>>>> Phil Hobbs >>> >>>>> The low end of the bandwidth is 100kHz. >>> >>>>> A bootstrap does not necessarily add noise to the circuit. If the >>>>> bootstrap amplifier has less noise than the op amp its noise is >>>>> swapped for that of the op amp. This measured reduction in noise is >>>>> documented in figures 4b, 5b, and 6b on pages 17 and 18 of: >>>>> http://cds.linear.com/docs/Datasheet/6244fa.pdf >>> >>>>> Suppose I returned the photoelectromotive detector to ground. Since >>>>> the expected detector current is 1uA, to get some reasonable output >>>>> from this first TIA stage I would need a feedback resistor on the >>>>> order of 1Mohm. Since the series resistance of the detector is 1kOhm >>>>> there is a noise gain of 1001 before the zero from the 30pF >>>>> capacitance kicks in at 5.3MHz. Taking into account the DC noise gain, >>>>> the capacitance on the virtual ground (dominated by detector >>>>> capacitance) and the needed bandwidth of the TIA (10MHz) I would need >>>>> an opamp with an 18.8THz GBW. If it weren't for the noise gain I would >>>>> have needed only 18.8MHz. The only way this is going to be done is by >>>>> means of bootstrapping to reduce the effective impedance of the >>>>> detector. >>> >>>> In general you can make the TIA out of the same type of device as the >>>> bootstrap, which is what I'm assuming. If you have a noise problem, >>>> then reducing the TIA noise is job 1, followed by bootstrapping. Using >>>> the same device type in the TIA and the bootstrap gives a noise level >>>> about 3 dB better than either one alone. >>> >>>> A quiet bootstrap plus a noisy TIA is a lot better than just the noisy >>>> TIA barefoot, but not as good as a quiet bootstrap and a quiet TIA. >>> >>>> The resistance of the detector is in series with the capacitance, >>>> not in >>>> parallel, so it looks like a lead-lag network. The capacitance makes a >>>> feedback zero, and the resistance puts in a zero. The noise gain >>>> may be >>>> 1000 at high frequency, but not at low frequency. >>> >>>> Cheers >>> >>>> Phil Hobbs >>> >>> I have been told by the project engineer who has given me this >>> assignment the capacitance is a parallel one. It is a custom detector >>> device so there is no data sheet I can link to. I have not even been >>> given a hard copy of one. So I can only go by what he has told me. Do >>> you have knowledge of photoelectromotive force detector physics that >>> causes you to disagree? >>> >>> Right now my main concern is just getting the bandwidth. Once one or >>> more ways to do that are established I can be concerned about minimizing >>> noise. >>> >>> I have not until now considered using another op amp instead of an amp >>> composed of discrete devices to do the bootstrapping. Doing it this way >>> would reduce added capacitance on the virtual ground. >>> >>> Regarding my earlier posting that was 18.8GHz not 18.8THz. >>> >>> -- >>> If you desire to respond directly remove the "sj." from the domain name >>> part of my email address. It is a spam jammer.- Hide quoted text - >>> >>> - Show quoted text - >> >> "I have been told by the project engineer who has given me this >>> assignment the capacitance is a parallel one." >> >> That sounds much worse. It's like some photo-resistor.? It will >> always be leaking as you bias it. (and leaking as the light shines on >> it.) ughh. >> >> >> George H. >> >> > > Similar, but unlike the passive photoresistor, the electromotive force > detector converts photons into electrical energy. In a TIA circuit it > can be modeled as a current source that does not require a bias. Photodiodes and photoconductors are completely different. The > current is proportional to the motion of a beam light across it. It is > very good at detecting changes in a laser light's speckled pattern > produced when it reflects off any but the most even of surfaces. I'd never heard of a photoelectromotive detector, but Google had. Interesting. From what I gather from the free literature, it's basically a GaAs 1-D lateral effect photodiode run at zero bias, with the output taken across the ends of the top layer. I haven't read any really detailed papers, because they all cost $30, but it relies on the carrier density responding slowly to changes in the illumination pattern. It's made of GaAs rather than silicon, which I'm guessing is because the hole mobility in GaAs is so very low, which essentially glues the charge density in place. (Silicon would respond much faster.) For static illumination, the slow carrier diffusion makes the junction E field flatten out locally, so the charges all recombine, and the output current is zero. When the illumination changes, though, you have photocarriers being generated in regions where the net current is suddenly nonzero. These carriers produce a displacement current immediately (not when they arrive at the electrodes), and the carriers still flowing in the previously illuminated region produce a net displacement current of the opposite sign. The two sum to zero, but they're spatially offset from each other, which produces a net current from the outputs. Like everything else in nature, it's linear for small displacements. It's a cute technique, which reminds me a bit of a photorefractive phase conjugate mirror, in which the beam interference pattern forms a slowly-varying hologram that corrects for phase and pointing errors. A few things I'm not clear on: (1) There has to be a junction in there somewhere, because it works at zero bias and can produce current in both directions--which in a symmetric structure requires a third electrode. (2) Splitting the photocarriers between the ends seems to require a high-resistance epi layer, which would tend to make the response slow or else very noisy, like a lateral effect cell--the full Johnson noise current of the epi layer appears in the output. (3) The diffusion and recombination currents both have full shot noise, and the forward-biased regions will have a very low resistance. This will seemingly contribute a lot of noise to the output--the noise will be that of the full beam's photocurrent, not merely that of the small difference current. So in general this doesn't sound like a low-noise device, although it's certainly convenient for speckle measurements--the usual alternative method is TV interferometry, in which you cross-correlate the speckle patterns before and after some change (e.g. pressurizing a tank). TV interferometry is slow, since it takes a minimum of two video frame times. > Right now my plan is get two of the lowest noise JFET or CMOS input op > amps I can find that satisfy the bandwidth requirement. One will be used > for the feedback resistor and the other will do the bootstrap. It would be worth finding out what the noise level of the device itself is. If I'm mistaken and it's actually quiet, you'd be better off making the bootstrap out of a single-ended BF862 JFET rather than an op amp. Otherwise a couple of op amps is probably fine (one of my favourites is the ADA4817-1). Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal ElectroOptical Innovations 55 Orchard Rd Briarcliff Manor NY 10510 845-480-2058 hobbs at electrooptical dot net http://electrooptical.net
From: George Herold on 6 Nov 2009 12:17
On Nov 6, 9:45 am, Phil Hobbs <pcdhSpamMeSensel...(a)electrooptical.net> wrote: > Artist wrote: > > George Herold wrote: > >> On Nov 5, 12:40 pm, Artist<Art...(a)sj.speakeasy.net> wrote: > >>> Phil Hobbs wrote: > >>>> Artist wrote: > >>>>> Phil Hobbs wrote: > >>>>>> Artist wrote: > >>>>>>> Phil Hobbs wrote: > >>>>>>>> Artist wrote: <Big snip> Thanks for the hint Phil, A search for (photo-emf detector) brings up a nice article from Newport. George H. |