Common-mode rejection is a measure of the change in output voltage when both inputs are changed by equal amounts. These specifications are usually given for a full-range input voltage change and a specified source imbalance. “Common-Mode Rejection Ratio” (CMRR) is a ratio expression while “Common- Mode Rejection” (CMR) is the logarithm of that ratio. For example, a CMRR of 10,000 corresponds to a CMR of 80 dB. In an instrumentation amplifier, ac common-mode rejection is only as good as the differential phase shift. Degradation of ac common-mode rejection is caused by unequal drops across differing track resistances and a differential phase shift due to varied stray capacitances or cable capacitances. In many applications shielded cables are used to minimize noise. This technique can create common-mode rejection errors unless the shield is properly driven. Figures 32 and 33 shows active data guards which are configured to improve ac common-mode rejection by “bootstrapping” the capacitances of the input cabling, thus minimizing differential phase shift.