500 V Common-Mode Voltage Current Monitor (CN0218)
The circuit shown in Figure 1 monitors current in systems with high positive common-mode dc voltages of up to +500 V with less than 0.2% error. The load current passes through a shunt resistor, which is external to the circuit. The shunt resistor value is chosen so that the shunt voltage is approximately 500 mV at maximum load current.
Figure 1: High Common-Mode Voltage Current Monitor (All Connections and Decoupling Not Shown)
The AD8212 accurately amplifies a small differential input voltage in the presence of large positive common-mode voltages greater than 500 V when used in conjunction with an external PNP transistor.
Galvanic isolation is provided by the ADuM5402 quad channel isolator. This is not only for protection but to isolate the downstream circuitry from the high common-mode voltage. In addition to isolating the output data, the ADuM5402 digital isolator can also supply isolated +3.3 V for the circuit.
The measurement result from the AD7171 is provided as a digital code utilizing a simple 2-wire, SPI-compatible serial interface.
This combination of parts provides an accurate high voltage positive rail current sense solution with a small component count, low cost, and low power.
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