In instrumentation amplifiers, what does high CMRR help to achieve?

• A. Rejection of common-mode signals while preserving the differential signal.
• B. Increased output swing.
• C. Lower input impedance.
• D. Higher sampling rate.

Answer: (A)

The main idea is how well an instrumentation amplifier rejects signals that appear on both inputs (common-mode) while preserving the signal that comes from the difference between the inputs. High CMRR means the amplifier mostly ignores common-mode signals and only amplifies the difference between the two inputs. This is crucial in practice because sensors and long wiring can pick up noise or interference that shows up equally on both inputs; with a large CMRR, most of that unwanted noise is removed at the output, leaving the true differential measurement intact. CMRR is the ratio of how much the amplifier gains the differential signal to how much it gains the common-mode signal; a higher ratio means better suppression of common-mode noise. The other aspects—output swing, input impedance, or sampling rate—are influenced by different design factors and are not what CMRR directly measures.