The second design to measure a current for negative power supply is to use an isolated amplifier. Figure 3-2 shows a typical circuit. TI had released lots of isolated amplifier to meet market requirements to sense voltage or current. AMC1202 is an isolated amplifier for current sensing with ±50mV input range and 1000V working isolated voltage. The isolation barrier separates parts of the system that operate on different common-mode voltage levels and protects the low-voltage side from hazardous voltages and damage. With this character, the designer can measure the sensing resistor on the high common voltage rails.

Figure 3-2 Current Sensing for Negative Power Supply Based Isolated Amplifier

The gain of AMC1202 is 41. Assume the sense resistor is Rs, output voltage of AMC1202 is Vo. Then, the current of negative power supply is:

An isolated power supply is necessary for power high side circuit of AMC1202. TI has several isolated power supply with compact size that can meet this application. Among these modules, UCC12040 is one of the parts with good performance, small size, and low cost.