Photodiode series resistance -> dominant noise source!

Isn't there a way to measure that along with the capacitance using a network impedance analyzer? Keeping the excitation really low, of course.

Probably. I don't have one, though, and anyway it's just as easy to cram it into the circuit and measure the noise, which is what I care about anyway.

Thanks

Phil Hobbs

"Gotcha" is the daily event :-( ...Jim Thompson

I usually sim a pd with a current source and a capacitance, so now you have made my life more complex.

The fiberoptics detectors that we use are small so have pretty short RsCj time constants, 10s of ps maybe, so the noise will be way up in the spectrum. But even a 100 ps time constant could add jitter to a fast receiver circuit. But we launch milliwatts, which makes things a lot easier.

In a free-space app, might multiple small detectors and amps be better than one giant one?

What effect, if any, does bias voltage have on Rs?

Yup, me too.

A little matter of 100 dB or so better. ;)

Probably. Even just one amp with several parallelled detectors would be likely to reduce series resistance.

Using multiple amps works as long as the amps' input capacitance is sufficiently smaller than the individual PD capacitance--otherwise you just add more noise power from the intrinsic eNC noise of the amps.

It's mostly silicon resistance, which should drop as the bias increases. Many PDs speed up amazingly when you fully deplete the silicon, which is either a drop in Rs or eliminating diffusion delay, or probably both.

Cheers

Phil Hobbs

Yeah, more voltage speeds things up even after the capacitance levels out.

Incidentally, I have a lot (embarrasingly a lot) of First Sensor PS13-5B-TO5 UV-enhanced photodiodes.

Nice part if you have a bit of light--they're about the best UV PDs I know of, but their capacitance levels off at about 60 pF, which is about

BTW by switching to Osram BPW34s (same area) and jacking the bias up to

I'll get some Vishay BPW34s and try them out. They're slower, so maybe they have thicker epi with lower resistance.

Cheers

Phil Hobbs

That's interesting. Would it help if I order one and measure it with an HP 4194?

Sure, that would be interesting. Thanks!

So far an Osram BPW34 at 30V is down to 1.6 dB worse than the 10 pF cap, so maybe I can find the right PD for the job.

Cheers

Phil Hobbs

OTOH, while a network analyzer is certainly nice, if you only need to measure C and ESR, an LCR meter might do. Maybe not one of the really cheap ones for dry electrolytics, but a halfway decent LCR meter that goes at least to 100 kHz should have a chance to "see" the ESR.

Dimitrij

? The thermal current noise of the series resistance? Does that mean the series resistance is higher than than the feedback R? That doesn't sound right... I'm probably not understanding.

George H.

Is it possible to calculate or estimate approximately how much resistance, are you seeing tens ohms, hundreds ohms or kilo-ohms etc?

piglet

If you imagine that the TIA itself is noiseless, it puts exactly zero volts AC across the photodiode. But the PD isn't a pure capacitance, it's a seri es RC. Since there's zero volts across the series combination, the resistor 's Johnson noise voltage appears across the capacitor, leading to e_NC curr ent noise in the external circuit.

In my circuit this dominates when R_s is above about 60 ohms.

Cheers

OK - will do.

What is the physical mechanism of this resistance? Just the usual diode "bulk" resistance that all PN junctions have or is it something specific to photodiodes?

Is it at all frequency dependent?

You should be able to TDR it pretty easily.

McGuver method: Make an resonant circuit out of it with a coil that has very low losses. The resonant frequency will reveal the capacitance and the -3dB points will tell you the Q from which you can calculate what the resistance would be. This assumes that all other losses are insignificant compared to the contribution of the series resistance.

This could be done with an RF generator, a FET probe, and a scope. The exact capacitance of the FET probe needs to be known but it usually is. If not, run it sans PD and log the frequency (and then note it in the manual for next time). The Q in that case must be more than 2x better than with PD. If it isn't you need a better inductor. No kidding, I have polished coils in the past to get the Q up. Wenol paste is really great for that but don't get that on your skin or clothes.

ts AC across the photodiode. But the PD isn't a pure capacitance, it's a se ries RC. Since there's zero volts across the series combination, the resist or's Johnson noise voltage appears across the capacitor, leading to e_NC cu rrent noise in the external circuit.

Got it, Thanks.

GH

Would the series resistance be any different forward biased? Maybe just an I-V curve will show it?

George H.