Precise current measurement upto .05uA

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.


The input noise of a OP7 is about 11nV/√Hz. The bandwidth (unless your circuit reduces it) is about 1MHz, so the expected input noise is about 10uV. With a gain of 8 the output noise is about 80-90uV.

You need to reduce the bandwidth. Work out how often you actually need a measurement, and add low-pass filtering with a roll-off at about double the measurement rate.

Depending on your exact requirements, you might find a transimpedance amplifier a better fit than a series resistor & voltage amplifier.

Category: op amp Time: 2016-07-28 Views: 0
Tags: op amp

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