I have two identical circuits (see attached schematic for example) residing on one PCB. Each circuit amplifies the signal from a silicon pin photodiode VEMD5080X01 . The diodes are sitting in a black enclousure without the possibility of light entering that enclousure. The whole arrangement is placed inside a temperature chamber for a temperature cylce (almost 24 h) from 25 °C up to 70 °C down to -30 °C and back to 25 °C again (black curve in the diagrams). Both "DIFF" signals (see circuit at Cf8) are sampled and logged by an 12 bit ADC with 2.9 V reference -> q = 700 uV). The red curve is the signal from circuit1 and the green one from circuit2 (both identical as already stated). What seems strange to me is that the circuitry seems to behave strongly different. The red curve is dropping down from 140 mV when temperature goes up and the green one is rising slightly with temperature.
Has anyone have a clue if that is related to some instable operating point of the OPAmps used, or even a condition which is out of specification fot that particular OPAmp AD8692?
I'd appreciate any hint in order to track down the reason of that behaviour.
EDIT1: @jonk, thanks for the hint I'll calculate that. According to reverse bias: No It is not reversed biased. I guess for the sake of dynamic range they gave up linearity and decided not to reverse bias. (I am not the initial "designer" of that circuit and have little to none design documentation for that.
@Tony Stewart EE75, thanks first of all for your hints.
- "Unbalanced source.." do you mean that I should AC-terminate each differential signal "PD1" and "PD2" to GND?
- "Open circuit voltage.." that I have noticed as well. Signal "Diff" at the end of the chain is drifting in the order of same magnitude as the -3 mV/K would suggest. Should I consider the PD as beeing in open circuit conditions with that much of resistance connected (high impedance amplifier inputs)?
- "The PD's capacitance.." So it is, but how would it be linked to the behaviour observerd?
Will consider your suggestions (need time foor that) though it means changing much of the circuit. First of all I need to understand them first.
EDIT2: @jonk, the PD is operated with almost 0 V across it, in order to minimize dark current and noise. That's what I was able to investigate. I still need to look for some web ressources to support that statement.
EDIT3: Further thoughts: 200 digits signal correspond to approx. 80 pA current (calculating back through the amplifier chain), which indeed could be dark current. Imagine that the PD is not held at 0 V because the OPamps (TIA) contribute to a Voltage by having certain input offset voltages. Now, if the resulting voltage across the PD is positive I will get current flow in the corresponding direction. But if the resulting voltage is negative the current will also reverse. Is that reasonable?