My question is what is a ballpark number for the capacitance of the ESD diodes inside the OPA1642 op-amp.
I'm using the OPA1642 as a transimpedance amplifier for a photodiode. TIA gain (the TIA is a capacitive TIA, with a voltage gain stage following it) has a transimpedance of about 2 GOHm, so the input currents are quite small, the dynamic range is 2 pA to about 5 nA. Bandwidth of the TIA is 17 Hz to 4.3 kHz.
I see 60 Hz at the output of the TIA.
I'm wondering if the 60 Hz could be coupling into the OPA1642 from the supply pins through the capacitance of the ESD diodes.
I performed a simulation with an op-amp model (a Universal Op-amp model in a competitor's simulation tool) and with 10 pF of capacitance between the supply pin and the input pin and with 10 mV, 60 Hz power supply ripple, I see in the simulation the output 60 Hz ripple which matches what I see on the scope.
I completely guard the inverting input of the OPA1642 with a trace from the non-inverting input (which is at ground potential).
