Explain the concept of common-mode rejection ratio (CMRR) in differential amplifiers and why it matters.

• A. Ratio of differential gain to common-mode gain; higher CMRR reduces noise and interference common to both inputs; important in sensor interfaces.
• B. Ratio of common-mode gain to differential gain; higher CMRR increases noise.
• C. It measures the noise at the output due to power supply ripple only.
• D. It is not relevant in sensor interfaces.

Answer: (B)

CMRR measures how well a differential amplifier rejects signals that come equally on both inputs while still amplifying the difference between the inputs. It’s defined as the ratio of differential gain to common-mode gain (Ad divided by Acm). A large CMRR means the amplifier strongly passes the differential signal you care about but greatly attenuates anything that's common to both inputs, which is exactly the noise and interference that often accompanies sensor signals.

This is crucial for sensor interfaces because sensors connected by cables pick up a lot of common-mode noise from EMI, grounding issues, and power-supply ripple. If you have a high CMRR, these common-mode disturbances have less influence on the output, so your measurement reflects the true differential signal more accurately. In practice, CMRR is usually expressed in dB as 20 log10(|Ad|/|Acm|), and it can vary with frequency, so real-world performance depends on both the static value and how it behaves across the signal band.

The idea that CMRR is the ratio of common-mode gain to differential gain or that higher CMRR would increase noise is incorrect. The correct concept is that we want a large Ad relative to Acm, yielding a high CMRR and better suppression of common-mode noise.