Bidirectional Isolated High-Side Current Sense with 270 V Common-Mode Rejection (CN0240)
This circuit, shown in Figure 1, monitors bidirectional current from sources with dc voltages of up to ±270 V with less than 1% linearity 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 100 mV at maximum load current.
The AD629 amplifier accurately measures and buffers (G = 1) a small differential input voltage and rejects large positive common-mode voltages up to 270 V.
The dual AD8622 is used to amplify the output of the AD629 by a factor of 100. The AD8475 funnel amplifier attenuates the signal (G = 0.4), converts it from single-ended to differential, and level shifts the signal to satisfy the analog input voltage range of the AD7170 sigma-delta ADC.
Galvanic isolation is provided by the ADuM5402 quad channel isolator. This is not only for protection but also to isolate the downstream circuitry from the high common-mode voltage. In addition to isolating the output data, the ADuM5402 digital isolator can supply isolated +5.0 V for the circuit.
The measurement result from the AD7170 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 and negative rail current sense solution with a small component count, low cost, and low power.
Figure 1. High Common-Mode Voltage Bidirectional Isolated Current Monitor (All Connections and Decoupling Not Shown)
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