Define common-mode rejection ratio (CMRR) and explain its significance in differential sensing.

• A. CMRR is the ability to reject common-mode signals; higher CMRR means better rejection of noise that appears equally on both inputs.
• B. CMRR stands for current modulation rejection ratio.
• C. CMRR is the ratio of input to output voltage.
• D. CMRR is irrelevant for differential sensing.

Answer: (C)

CMRR measures how well a differential sensing system suppresses signals that appear on both inputs. In differential measurements you usually have a small signal of interest riding on a large common-mode voltage (noise or interference that is the same on both lines). The amplifier’s differential gain (how much of the difference it passes) and its common-mode gain (how much it lets the same signal on both inputs show up at the output) determine CMRR, typically defined as Ad/Ac (or in decibels, 20 log10(|Ad|/|Ac|)).

A higher CMRR means the device more effectively cancels out noise that is common to both inputs, so the small differential signal is preserved with less distortion or error. In practice, CMRR isn’t infinite; finite CMRR lets some common-mode noise appear at the output, especially as frequency changes, which is why specifications list CMRR across frequencies.

The other descriptions misstate what CMRR measures (for example, confusing it with current modulation or simply an input-to-output voltage ratio) or incorrectly claim it’s irrelevant to differential sensing.