Differential Amplifier Metric
Ability to ignore voltage fluctuations that appear simultaneously on both input terminals ensures high signal integrity in noisy environments. Within sensitive measurement circuits, common-mode rejection prevents external electrical interference from corrupting the intended data. The ratio compares the gain of the desired differential signal against the gain of the unwanted noise.
Input Balance
Symmetry between the two signal paths determines the effectiveness of the suppression. If one wire has slightly higher resistance or capacitance, the noise no longer appears identical at the amplifier. This imbalance converts part of the common noise into a differential signal that the system cannot filter out.
High precision resistors and careful circuit board layout are required to maintain this balance.
Operational Frequency
Performance typically decreases as the frequency of the interference increases. While a circuit might provide excellent suppression at 60Hz, it may fail to block high frequency noise from a variable speed drive. Data sheets specify the rejection ratio across a range of frequencies to help engineers predict behavior in the field.
Circuit Isolation
Galvanic barriers or optical couplers provide a physical break in the electrical path to stop common mode currents. Isolation prevents ground potential differences between the sensor and the receiver from creating large common mode voltages. Modern isolation amplifiers achieve very high rejection ratios by digitizing the signal before crossing the barrier.
