I need to do a current source controlled by voltage, with an excursion in Amp (peak to peak) 20nA. I need to have the load grounding. Using OP-AMPS.
Precision current source.
refer to "OpAmp cook book" and u will find at least 3 ckts for VCCS.
BTW I think the current range u need is so small thus requires precision/low noise opamps like OP-07 and even with them u need to use dynamic offset cansellation of the opamp( only for feedback path) or something like that to measure very low voltages.
also at this range of current u need shield for the ckt to remove EMI.
You can use a current source like these used in HiOhm multimeter circuit. I have designed some years ago this kind of high precision low current source. You need two precision OPamp and N and P jfet.
The file I wrote (in french, sorry) will give you all the informations. Use PTFE stand-off for output connexion.
I tried a OPAMP ckt with the RL (impedance) in the feedback loop, a ckt with positive an negative feedback (OP Amps cookbook, bidirectional current source), but I had problems because the impedance value are between 1 to 6 MOhms, and this configurations can't regulate the current at this RL values.