Measuring small differential voltages in the presence of large common-mode voltages:In applications such as motor control, power-supply current
monitoring, and battery cell-voltage monitoring, a small differential
voltage must be sensed in the presence of a high common-mode
voltage. Some of these applications require galvanic isolation, others
do not. Some applications use analog control, others use digital
control. Four cases of such measurements will be considered, each
requiring 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 Signals
Figure 1 shows the input of a measurement system. VDIFF
represents the differential voltage, the signal of interest. VCM
represents the common-mode voltage, which contains no useful
information about the measurement and could in fact reduce
the measurement accuracy. The common-mode voltage could
be an implicit part of the measurement system, as in a battery
cell-voltage monitoring application, or it could be created by a
fault condition where the sensor accidentally comes in contact
with a high voltage. In either case, that voltage is unwanted,
and it is the job of the measurement system to reject it, while
responding to the differential-mode voltage.