资 源 简 介
Common-mode rejecTIon is a measure of the change in outputvoltage when both inputs are changed by equal amounts. ThesespecificaTIons are usually given for a full-range input voltagechange and a specified source imbalance. “Common-ModeRejecTIon RaTIo” (CMRR) is a ratio expression while “Common-Mode Rejection” (CMR) is the logarithm of that ratio. Forexample, a CMRR of 10,000 corresponds to a CMR of 80 dB.In an instrumentation amplifier, ac common-mode rejection isonly as good as the differential phase shift. Degradation of accommon-mode rejection is caused by unequal drops acrossdiffering track resistances and a differential phase shift due tovaried stray capacitances or cable capacitances. In many applicationsshielded cables are used to minimize noise. This techniquecan create common-mode rejection errors unless theshield is properly driven. Figures 32 and 33 shows active dataguards which are configured to improve ac common-moderejection by “bootstrapping” the capacitances of the inputcabling, thus minimizing differential phase shift.
