I am working on a circuit where i have to measure currents from -500uA to 5mA. For this i am using 100ohm resistor for converting current to voltage and then this voltage is sent to OP07 and output of this is given to HP7840 isollator whose gain is 8. circuit is working correctly for higher currents but i want to measure .05uA. But since output of this circuit is fluctuating when i am taking readings in 5 decimal points, due to this my readings are not coming accurate. I have no idea why their is fluctuations upto 0-100uV which is not desired. OP07 thermal noise is 0.2uV/`C and HP-7840 have only 0.1% nonlinearity.