Applications that require a simple, low-cost current sensor with galvanic isolation can employ the circuit described here. If used as drawn, it's possible to sense currents up to 10 A with high precision (usually within %) and wide bandwidth (more than 500 kHz). The use of optocouplers provides 3000 V of isolation between the primary and secondary and 50 dB of CMRR. Transistors T1 and T2 form a coupled pair in current-mirror configuration that's unbalanced by the current (I) to be measured. Once this happens, transistor in OP1 balances the current-mirror biased by T3 that also biases OP2, making its transistor conduct the same current that balances the current-mirror. Because the balancing current is directly proportional to the current being measured, a direct measurement is achieved. It can be easily shown that the gain in current is given by the ratio of the sensing resistor (RS) and R1, producing a proportional voltage drop at the Output terminals.
For the circuit to work properly, it's important that the coupled transistors which form the current mirror share the same substrate and that the optocoupler be a dual version, in order to have matching characteristics. Variations on this idea may lead configurations that can be used in a wide range of applications, such as sensing current in a ground line (inverting the operating idea of the current-mirror using npn transistors) or offering different conditioning at the output (increasing voltage with increasing current). The power supplies needed can be implemented using Zener diodes since only a small amount of power is required—usually less than 50 mW.