IEEE was Re: Photodiode readout without transimpedance amplifier
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From: JosephKK on 12 Jan 2010 07:57 On Sun, 10 Jan 2010 22:24:03 -0800 (PST), "miso(a)sushi.com" <miso(a)sushi.com> wrote: >On Jan 10, 10:12 pm, "JosephKK"<quiettechb...(a)yahoo.com> wrote: >> On Fri, 8 Jan 2010 20:32:39 -0800 (PST), "m...(a)sushi.com" <m...(a)sushi.com> wrote: >> >On Jan 8, 4:33 pm, "JosephKK"<quiettechb...(a)yahoo.com> wrote: >> >> On Wed, 6 Jan 2010 01:11:34 -0800 (PST), "m...(a)sushi.com" <m...(a)sushi.com> wrote: >> >> >On Jan 5, 7:49 pm, Phil Hobbs <pcdhSpamMeSensel...(a)electrooptical.net> >> >> >wrote: >> >> >> On 1/5/2010 10:23 PM, m...(a)sushi.com wrote: >> >> >> >> > On Jan 5, 6:50 pm, Phil Hobbs<pcdhSpamMeSensel...(a)electrooptical.net> >> >> >> > wrote: >> >> >> >> On 1/5/2010 9:40 PM, m...(a)sushi.com wrote: >> >> >> >> >>> On Jan 5, 6:38 am, "renning"<enn...(a)nano.mavt.ethz.ch> wrote: >> >> >> >>>> Hello all, >> >> >> >> >>>> I'm trying to develop a photodiode readout pre-amp without using a >> >> >> >>>> current-to-voltage converter; the post-processing steps that I want to do >> >> >> >>>> require a current as a signal. The best performance (SNR and bandwidth) >> >> >> >>>> that I have come up with is to feed the photocurrent directly into the base >> >> >> >>>> of an emitter follower NPN, but I'm thinking that there are better methods >> >> >> >>>> than this in terms of SNR. Does anyone have any ideas? >> >> >> >> >>>> Regards, >> >> >> >>>> Raoul >> >> >> >> >>> For bandwidth, you probably want to feed the diode into a low >> >> >> >>> impedance. Generally you only want the bandwidth you need, since noise >> >> >> >>> is proportional to the square root of bandwidth. >> >> >> >> >>> Graeme's book goes into photo diode preamps in painful detail. Linear >> >> >> >>> Tech has some bootstrap circuits in their app notes. There is Phil >> >> >> >>> Hobbs website. I don't think the answer to your question will be a >> >> >> >>> simple usenet post. >> >> >> >> >> You might have a look at an article of mine from some years back:http://electrooptical.net/www/frontends/frontends.pdf. >> >> >> >> >> 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 nethttp://electrooptical.net >> >> >> >> > While I've got your ear, do you have a write up on NEP? Why would the >> >> >> > noise be specd in power instead of current. >> >> >> >> > Here is my non-expert interpretation of NEP. If I had a photodiode amp >> >> >> > with a low impedance input (known value), then I could convert the >> >> >> > power to currrent with P=I^2R, with R the impedance of the amp. This >> >> >> > should also work for a TIA with the R being that of the resistor. But >> >> >> > say you hooked the diode up to a high impedance amp. Then there really >> >> >> > is no power transfer in the ideal case. >> >> >> >> > I've noticed the NEP is larger as the device gets larger. This is >> >> >> > counterintuitive to chip design, where large generally means low >> >> >> > noise, at least for active elements. However, I could see a diode >> >> >> > putting out more noise if it has more area. So it would seem that less >> >> >> > is more here, but if you consider the larger diode also can gather >> >> >> > more light, it might be a break even situation. >> >> >> >> Noise equivalent power is the optical power required to get a SNR of >> >> >> unity (0 dB). There's a lot of confusion generated by the operation of >> >> >> square law photodetectors (i.e. all of them). >> >> >> >> The energy deposited by N photons per second is always h*nu*N, whereas >> >> >> once they're detected, it's I**2*R, i.e. (eN)**2*R. Thus the electrical >> >> >> power goes like the square of the optical power. Signal and noise are >> >> >> always uncorrelated, so when you square a noisy current, the SNR is >> >> >> squared as well. (The cross term 2*<S*N> is zero by hypothesis.) >> >> >> >> Most photodetectors have capacitance, leakage, and a little Johnson >> >> >> noise. Electrically the noise power generally goes as the area, because >> >> >> each square millimetre contributes the same as all the others, and the >> >> >> contributions are uncorrelated. Optically, this means that the noise >> >> >> equivalent power goes as the diameter (square root of area). >> >> >> >> Thus detector families are often compared using the figure of merit D* >> >> >> (D-star), which is defined as >> >> >> >> D* = sqrt(Area)/NEP(1 Hz BW), >> >> >> >> at some convenient modulation frequency that's out of the 1/f region >> >> >> (often 1 kHz). Because of the optical vs electrical units, D* is quoted >> >> >> in cm*sqrt(Hz)/W--which is a pretty weird unit. >> >> >> >> D* isn't much use in the visible and near IR, where it's basically the >> >> >> capacitance that sets the noise floor--the intrinsic noise sources of >> >> >> silicon PDs are generally negligible. The capacitance sets a limit on >> >> >> how high a feedback resistor you can use for a given bandwidth, and also >> >> >> causes horrible noise gain due to its differentiating action. Well >> >> >> within the feedback bandwidth, hanging a capacitance C on the summing >> >> >> junction of an op amp produces a noise current >> >> >> >> iN_Vamp = 2*pi*C*e_NAmp >> >> >> >> where e_NAmp is the 1-Hz noise voltage of the amplifier. This usually >> >> >> dominates for low light, large PDs, and unimaginative designs. ;) >> >> >> >> Since the NEP goes as the diameter, for a constant light level you win >> >> >> by going to larger areas, but again that isn't usually the problem with >> >> >> silicon PDs. Big PDs are expensive enough that you usually want to do >> >> >> optical things to avoid them where possible. >> >> >> >> For audio-range jobs you can do fun things by putting a common base >> >> >> stage on a solar cell--3 square inches of sensitive area and 20 kHz of >> >> >> bandwidth. >> >> >> >> 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 nethttp://electrooptical.net >> >> >> >I was doing good up to this point: >> >> >"D* isn't much use in the visible and near IR, where it's basically >> >> >the >> >> >capacitance that sets the noise floor--the intrinsic noise sources of >> >> >silicon PDs are generally negligible. " >> >> >> >How does the spectrum effect D*? If anything, the sensor peaks at near >> >> >IR, so that would be optimal SNR. >> >> >> >The rest of the stuff I got since it is explained in painful detail in >> >> >Graeme's book. >> >> >> OK another book for my collection. ISBN please? >> >> >ISBN-10: 007024247X >> >You can get the details from amazon. I got mine for $30 used with free >> >shipping from half.com. >> >> Merci beaucoup. >> >> >> >> >When you click on it on Amazon, it suggests the book "Building >> >Scientific Apparatus" or something like that. I came across that in a >> >used book store, It would have been a cool book if it wasn't so out >> >dated. >> >> >Berkeley and a few other bay cities have a chain called Half Price >> >Books. The often have cool stuff in their engineering section. I get >> >the old "classic" electronics books there from time to time. Schwartz, >> >Terman, etc. I got Papolius on Stochastics today. These are books when >> >engineers were engineers, not bit jockeys. Palo Alto used to have >> >great geek used book shops. >> >> >I'm dreading the day when all the books are ebooks. >> >> The day is coming when the best technical libraries are digital and require >> membership in a relevant society (Say IEEE, ISME, ASTM). > >Yep. I would go to Terman on the weekends and read journals. Now they >are all on line and you need an account. I don't know if you can get >one without being a student. > >Since the reproduction cost is nil, you would think journals would >just be free. WRONG! Look at the IEEE. You get paid nothing for peer >reviewing articles. There is a fee to get the paper published. So just >why does it cost anything to get these papers? Simple. IEEE has become a publisher, not a professional society, nor a technical society. It is another path to publishing, like Irwin Feerst said, for the publish or perish segment of our society. |