Hi i m designing insulation resistance tester for the capacitor. insulation resistance value is in the range from 30000Mega ohms and above. how to detect such a large resistance and/or small current ? I m able to detect up to 500Meg Ohms
100V, 30GOhms: I = 3.3nA.
Not impossible. But needs careful circuit and PCB design. I recommend guarding of sensitive signals.
Maybe consider to measure with positive as well as negative voltage. With this you can cancel out offset errors.
Use analog and/or digital filter to supress noise.
Integrating/averaging (digitally) for the time of a full wave of mains will cancel out mains pickup.