Op-Amp buffer circuit for parallel plate capacitor experiment

G^3, Wed Feb 16 2022, 03:27AM

Hey everyone,

I built an op-amp buffer circuit on a breadboard. My goal was to make a demo/experiment similar to *. Where you have a high-impedance voltage meter measuring the voltage across a parallel plate capacitor and then you move the plates apart and the voltage goes up.

The problem is that the output voltage is constantly drifting around. With nothing connect to the input, the voltage will fall to some negative value about 1.5V away from the negative rail.

Op-Amp used *

Thank you,
G
Re: Op-Amp buffer circuit for parallel plate capacitor experiment
klugesmith, Thu Feb 17 2022, 12:50AM

The first ref says: 2. Instead of a capacitance meter, a separate power supply can be used to charge the plates and an electrometer to measure the voltage across the plates.

Your opamp datasheet has some numbers like 10 picoamps. How many pF is your experimental capacitor? How fast will its voltage change with 10 pA of charging or discharging current?
Re: Op-Amp buffer circuit for parallel plate capacitor experiment
G^3, Thu Feb 17 2022, 05:55AM

That's probably it. The C is 10s of pF so 10 pA will swamp that soon.

Looks like I need a new op-amp, maybe the LMC 6081 *

It has input bias current typically of 0.01 pA

Does anyone have suggestions?

Thanks,
G