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EDITED BY MARTIN ROWEdesignideasAND FRAN GRANVILLEREADERS SOLVE DESIGN PROBLEMSCurrent monitor compensatesfor errorsChau Tran and Paul Mullins, Analog Devices, Wilmington, MA↘You sometimes need to measureload currents as large as 5A in thepresence of a common-mode voltage ashigh as 500V. To do so, you can use AnalogDevices’ (www.analog.com) AD8212high-voltage current-shunt monitor tomeasure the voltage across a shunt resistor.You can use this circuit in high-currentsolenoid or motor-control applications.Figure 1 shows the circuit, whichuses an external resistor and a PNP transistorto convert the AD8212’s outputcurrent into a ground-referenced outputvoltage proportional to the IC’s differentialinput voltage. The PNP transistorhandles most of the supply voltage, extendingthe common-mode-voltagerange to several hundred volts.An external resistor, R BIAS, safely limitsthe circuit voltage to a small fractionof the supply voltage. The internalbias circuit and 5V regulator provide anoutput voltage that’s stable over the operatingtemperature range, yet it minimizesthe required number of externalcomponents. Base-current compensationlets you use a low-cost PNP passtransistor, recycling its base current,I B, and mirroring it back into the signalpath to maintain system precision.The common-emitter breakdown voltageof this PNP transistor becomes theoperating common-mode range of thecircuit.The internal regulator sets the voltageon COM to 5V below the power-supplyDIs Inside48 Buck regulator handleslight loads51 Sense multiple pushbuttonsusing only two wires52 Tricolor LED emits lightof any color or hue▶What are your design problemsand solutions? Publish themhere and receive $150! Sendyour Design Ideas to edndesignideas@cancom.com.voltage, so the supply voltage for themeasurement circuit is also 5V. Choose avalue for the bias resistor, R BIAS, to allowenough current to flow to turn on andcontinue the operation of the regulator.HEAVYLOADR SHUNTV AD8212SENSE1k811kV PCURRENT MIRRORA 1BIASALPHA65I OUT3COM2I BR LI CV OUTR BIASI BIASFigure 1 An external PNP transistor lets you operate the circuit at high voltages.SEPTEMBER 9, 2010 | EDN 47