Low Cost, Level Shifted Low Side Current Monitor for Negative High Voltage Rails

The measurement result from the AD7171 is provided as a digital code utilizing a simple 2-wire, SPI-compatible serial interface. The entire circuit operates on a single +3.3 V supply. Optional galvanic isolation is provided by the ADuM5402 quad channel isolator. In addition to isolating the output data, the ADuM5402 digital isolator can also supply isolated +3.3 V for the circuit. The ADuM5402 is not required for normal circuit operation unless galvanic isolation is needed.

This combination of parts provides a accurate high voltage negative rail current sense solution with a small component count, low cost, and low power. The accuracy of the measurement is primarily determined by resistor tolerances and the accuracy of the band gap reference, and is typically better than 1.