Common mode rejection

One method to measure a small differential signal riding on a large common mode voltage is to make the differential gain much larger than the common mode gain. With the standard difference amplifier configuration, we can have large differential gain with common mode gain determined by the common mode rejection ratio of the opamp. But one of its drawbacks is the low impedance inputs. If we add unity-gain buffers to get high impedance inputs, we would add common mode offset to the signals. One way to reduce the common mode influence is to raise only the differential gain at the input buffer stage. One arrangement is to take the non-inverting amplifier configuration, but instead of referencing to the ground, referencing to the common mode voltage by eliminating grounding connection from the ground resistors and connecting them together. Now only differential current flows through these resistors and gets amplified and the common-mode offset stays the same. The signals then are put through the difference amplifier to remove the common-mode voltage. This is the classic three-opamp instrumentation amplifier configuration.