Common-Mode Rejection And Source-Impedance Balance
Definition
Common-mode voltage appears substantially equally on both input conductors. A differential receiver ideally removes it and preserves the difference signal. Common-mode rejection ratio compares differential gain with common-mode gain:
CMRRdB = 20 log10(Adiff / Acommon)The result is incomplete unless frequency, level, source impedances, receiver configuration, bandwidth, temperature, cable, and common-mode range are stated.
Why Source Balance Matters
Interference can couple equally as voltage but still become differential error when the source impedances differ. Receiver input currents, filter capacitance, cable capacitance, leakage, and resistor mismatch then develop different voltages on the two legs.
Verror ≈ Icommon × (Zsource_plus - Zsource_minus)This simplified relationship shows the mechanism; the real result varies with frequency and the complete input network.
Data-Sheet CMRR Is Not Installed CMRR
Precision differential receivers can specify very high CMRR under controlled conditions. Some conventional receivers lose large amounts of rejection as source-impedance imbalance increases, while high-common-mode-impedance architectures are designed to preserve more rejection. The installed result still includes source, cable, protection, RF filtering, PCB layout, reference paths, and common-mode voltage.
Common-Mode Range
Rejection applies only while the receiver remains in its valid operating region. Measure:
- maximum and minimum voltage of each leg to receiver reference
- differential signal and headroom
- receiver supply and bias
- distortion and recovery as common mode changes
A small differential signal can coexist with a common-mode voltage that overloads the input stage.
Measurement Method
- Configure the receiver at the intended gain and load.
- Apply the same test voltage to both input conductors.
- Control and measure residual differential stimulus.
- Measure receiver output over frequency and common-mode level.
- Repeat with representative source imbalance and final cable capacitance.
- Verify the complete interface in vehicle operating states.
The generator and test fixture must not create unequal impedances or an unintended reference path.
Useful Reporting
| Report | Required context |
|---|---|
| CMRR | frequency, common-mode level, gain, source imbalance, load, bandwidth, temperature |
| common-mode range | supplies, bias, differential amplitude, distortion criterion, recovery |
| noise rejection | interference source, spectrum, cable, routing, shield, vehicle state |