Hello:
First of all, this my first post on this forum, so if this is posted in the wrong spot, please tell and I will inmediately try and move it to the appropiate spot.
I tried to create a pico ammeter, similar to the ones already discussed in this forum, using a low leakage op amp and a high valued resistor in a TIA configuration. I used a LMC6082, a 1G 10% resistor and a 680pF polystyrine cap to avoid oscillations. I connected the power supplies and such in a protoboard, but lifted the input pin and soldered the resistor and capacitor in air. I then cleaned it all using 98 alcohol and dried it.
Because I dont have any of the specialized test equipment, I was unsure of how to test the thing. What I did at the end was to connect another 1G resistor to a variable power supply and to the input. To measure the current, I connected a Fluke 87V in the mV range in series. I then measured the output voltage, and estimated the gain in V/nA.
What surprised me the most is that the gain is not always the same! In fact, it always decreases with increasing current. I repeated the measurement several times and the result was always consistent. The left column is the input current, and the right one the measured gain in V/nA:
I(nA) G(V/nA)
3.5 0.9
4.4 0.83
5.6 0.8
6.6 0.78
8.7 0.76
10.8 0.73
What could cause this? The whole thing is also inside a cookie tin, and the noise floor is around 20mV).
Many thanks!