Re: Low drift OP amp for photodiode circuit

Robert Latest wrote:
Phil Hobbs wrote:

There are lots of nice things about this method. One is that it's much much quieter at very high gains--unlike 500G ohm resistors, capacitors don't have thermal noise.

A 500G resistor has about 0.18fA/rtHz noise, that's about as much noise as a 0.1pA (leakage) current has intrinsically. So if your opamp is (and needs to be) better than that, use the integrator method.

How does the LED reset method work? Using the LED as a photoswitch?

I also like the supply reversal method conceptually but have never used it.

robert

It's worse than that though--don't forget the big noise peak caused by the input capacitance and the amplifier's input noise--with a 500G resistor and a 3 pF input capacitance, the noise starts to rise linearly at 1 Hz! That doesn't exist with charge sensitive amps. And also the bandwidth can be quite different. If you use a capacitor across Rf to reduce the bandwidth to the same value you'd get by charge dispensing, you basically wind up with an integrator with a continuous resistive reset as opposed to a switched one.

Cheers,

Phil Hobbs
.

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