OP90 Single-Supply Current Monitor
This is Single-Supply Current Monitor circuit. Amplifying the voltage drop across a resistor placed in a series with the current to be measured is the essential of current monitoring. However it is difficult because with low precision op amps this greatly limits the overall resolution and only small voltage drops can be tolerated. Here is the schematic diagram of Single-Supply Current Monitor :
The resolution of this circuit is 10 µA and can used to monito 20mA of current. The range of this circuit depends on the current sense resistor R1. The supply current of the current monitor which bypasses the current sense resistor, maybe necessary to be included in final result, when measuring total system current.
The offset trim potentiometer R2, is used to measured and calibrated the current (together with the residual offset). A deliberate offset is depends on temperature. But, the supply current of the OP90 is also proportional to the two effects tend to track and temperature. Current in R5 and R4, which also bypasses R1, can be accounted for by a gain trim. [Circuit’s schematic diagram source: analog.com]