Instrumentation amplifiers like the AD621 offer high CMR
which is a measure of the change in output voltage when both
inputs arc changed by equal amounts. These specifications are
usually given for a full-range input voltage change and a specified
source imbalance.
For optimal CMR, the reference terminal should be tied to a
low impedance point, and differences in capacitance and resistance
should be kept to a minimum between the two inputs. In
many applications shielded cables are used to minimize noise,
and for best CMR over frequency the shield should he properly
driven. Figures 13 and 14 show active data guards that are configured
to improve ac common-mode rejections by “bootstrapping”
the capacitances of input cable shields, thus minimizing the
capacitance mismatch between the inputs.