So I'm doing another nanoamp photoreceiver.
Normally in this sort of circuit, the dominant noise source is the amplifier's input noise being impressed across the photodiode capacitance, resulting in a noise current density
i_N = e_N * 2 pi f C.
This "eNC noise" is insignificant at low frequency but at high frequency it rises to dominate the total noise.
I've measured the photodiode capacitance with a Boonton 72, and it matches the datasheet value for the part I'm using (Osram SFH206K), i.e.
10 pF @ 24V.With an NP0 cap of that value substituted for the photodiode, the noise floor of the circuit is just where I calculate it to be, so no worries there--the amplifier is doing just what it's supposed to be doing.
However, with a photodiode, it's 3-4 dB higher than that, even in the dark. It's not leakage or shunt resistance, because it rises with frequency just like the other eNC contribution.
It looks like it's the series resistance of the photodiode, which appears in series with the photodiode capacitance and creates current noise just the same way as the amplifier's noise, except that it's only the PD capacitance that enters, not the total capacitance at the amplifier input.
Usually you can just ignore the series resistance unless you're running at zero bias, but not when you're down in the nanoamps!
Sooo, I'm spending time with the manufacturers' apps guys looking for information about the series resistance of 2-3-mm class PIN photodiodes in reverse bias.
It's always something.
Cheers
Phil Hobbs