I want to measure the current that goes into a noninverting opamp input. I've setup my experiment as follows:
I can't measure the voltage drop across the 10M (R1) resistor directly since my DMM has an input impedance in the megaohm range. So, i have to measure the voltage on the ouput of the opamp. R2 is a potentiometer, so the gain is not exactly 4, and V1 is 1.19V.
When I connect the input to voltage source of 1.19V, I get 4.04V on the opamp's output. When I connect the input via the 10M resistor (R1), I get 3.64V on the output.
Now, my gain is:
$$ 4.04/1.19 = 3.39 $$
so, voltage that the opamp sees when connected via the 10M resistor is
$$ 3.64/3.39 = 1.07V $$
this means, I have a voltage drop across the resistor R1:
$$ 1.19 - 1.07 = 0.12V $$
so, the current into opamp's input is
$$ 0.12V/10M = 12nA $$
Am i doing it right?
Another question is: Could 12nA leakage current go thru a home made PCB that was vigorously cleaned from flux residue?
LMC6001 has a stated input current in femtoampere range, it might be that it was overheated while soldering and went kaput. But first, I want to be sure that I'm doing the measurements the right way.