Measuring small differential voltages in the presence of large common-mode voltages:In applications such as motor control, power-supply currentmonitoring, and battery cell-voltage monitoring, a small differentialvoltage must be sensed in the presence of a high common-modevoltage. Some of these applications require galvanic isolation, othersdo not. Some applications use analog control, others use digitalcontrol. Four cases of such measurements will be considered, eachrequiring unique considerations. They are:1) galvanic isolation with analog output;2) galvanic isolation with digital output;3) no galvanic isolation, analog output;4) no galvanic isolation, digital output.Differential Signals Versus Common-Mode SignalsFigure 1 shows the input of a measurement system. VDIFFrepresents the differential voltage, the signal of interest. VCMrepresents the common-mode voltage, which contains no usefulinformation about the measurement and could in fact reducethe measurement accuracy. The common-mode voltage couldbe an implicit part of the measurement system, as in a batterycell-voltage monitoring application, or it could be created by afault condition where the sensor accidentally comes in contactwith a high voltage. In either case, that voltage is unwanted,and it is the job of the measurement system to reject it, whileresponding to the differential-mode voltage.