CMRR in Differential Amplifiers
A differential amplifier should amplify the difference between two inputs. It should also reject signals that appear equally on both inputs. That rejected signal is called common mode signal. Common mode rejection ratio shows how well this rejection works.
Why CMRR Matters
High CMRR helps remove hum, sensor cable pickup, ground shift, and supply noise. It is important in instrumentation circuits, bridge sensors, audio front ends, and data converters. A low value can turn unwanted common noise into output error. This error may hide small differential signals.
Gain Based Evaluation
The basic method divides differential gain by common mode gain. Differential gain is the wanted gain. Common mode gain is the unwanted gain. The ratio is usually converted to decibels. A higher decibel value means better rejection. For example, a ratio of 10,000 equals 80 dB.
Voltage Test Method
A practical test can use measured voltages. First, apply a known differential input and measure the output. That gives differential gain. Then apply the same signal to both inputs. Measure the remaining output. That gives common mode gain. The calculator can process those test values directly.
Design Interpretation
CMRR is affected by resistor matching, device balance, input impedance, layout, and frequency. Precision resistor networks usually improve performance. Long traces and unequal source impedances can reduce real rejection. The value may also change at higher frequency. So a bench result should mention test frequency and input level.
Using the Result
Use the ratio for quick comparison. Use the decibel value for reports and datasheets. Check output error when the common mode signal is large. This helps decide whether filtering, shielding, trimming, or a better amplifier is needed. Always compare the result with the required signal accuracy. This makes the design margin easier to judge.
Good Measurement Practice
Use stable sources and accurate meters. Keep leads short during sensitive tests. Record both input and output units. Avoid saturating the amplifier during either test. Repeat the measurement at the operating frequency. If the common mode output is near zero, instrument resolution matters. In that case, report a minimum CMRR value rather than a false exact number. This keeps results honest and useful during design review and troubleshooting later work.