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Current monitor compensates for errors

17 Apr 2015  | Chau Tran, Paul Mullins

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There are times when you need to measure load currents as large as 5A in the presence of a common-mode voltage as high as 500V. To do so, you can use Analog Devices' AD8212 high-voltage current-shunt monitor to measure the voltage across a shunt resistor. You can use this circuit in high-current solenoid or motor-control applications. Figure 1 shows the circuit, which uses an external resistor and a PNP transistor to convert the AD8212's output current into a ground-referenced output voltage proportional to the IC's differential input voltage. The PNP transistor handles most of the supply voltage, extending the common-mode-voltage range to several hundred volts.

Figure 1: An external PNP transistor lets you operate the circuit at high voltages.

An external resistor, RBIAS, safely limits the circuit voltage to a small fraction of the supply voltage. The internal bias circuit and 5V regulator provide an output voltage that's stable over the operating temperature range, yet it minimises the required number of external components. Base-current compensation lets you use a low-cost PNP pass transistor, recycling its base current, IB, and mirroring it back into the signal path to maintain system precision. The common-emitter breakdown voltage of this PNP transistor becomes the operating common-mode range of the circuit.

The internal regulator sets the voltage on COM to 5V below the power-supply voltage, so the supply voltage for the measurement circuit is also 5V. Choose a value for the bias resistor, RBIAS, to allow enough current to flow to turn on and continue the operation of the regulator. For high-voltage operation, set IBIAS at 200µA to 1 mA. The low end ensures the turn-on of the bias circuit; the high end is limited, depending on the device you use.

With a 500V battery and an RBIAS value of 1000 kΩ, for example,

IBIAS=(V+–5V)/ RBIAS=495V/1000 kΩ=495µA.

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