Definition

Common-mode signals appear identically on both conductors of a differential pair - noise that couples equally to P and N lines. Differential receivers reject common-mode noise (CMRR), which is why differential signaling handles noisy environments well. Common-mode impedance differs from differential impedance and affects EMI. AC coupling capacitor mismatch converts common-mode to differential, degrading signal quality. Common-mode chokes filter common-mode noise on I/O lines. Keep differential pairs tightly coupled and length-matched to minimize common-mode conversion.