In theory, this means that the user may take as much
gain in the front end as desired (as determined by RG)
without increasing the common-mode gain and error.
That is, the differential signal will be increased by
gain, but the common-mode error will not, so the ratio
(Gain (VDIFF)/(VERROR CM)) will increase. Thus, CMRR
will theoretically increase in direct proportion to gain—a
very useful property.
Finally, because of the symmetry of this configuration,
common-mode errors in the input amplifiers, if they track,
tend to be canceled out by the output stage subtractor.
This includes such errors as common-mode rejection
vs. frequency. These features explain the popularity of
this configuration.