 Differential signalling

Differential signalling encodes a signal in the electric potential difference between two conductors, $V_d=V_{+}-V_{-}$. Writing $V_{\pm }=V_c\pm V_d/2$ separates this information from the common-mode voltage $V_c$. A receiver responds primarily to the difference, with finite common-mode rejection ratio in practice.^[ref]

For a test, drive one conductor to $1.1$ V and the other to $0.9$ V, giving $V_d=0.2$ V. Adding the same 0.3 V disturbance to both preserves $V_d$. If unequal coupling instead adds 0.30 V and 0.29 V, the receiver sees a 0.01 V differential error. This calculation explains why balanced path impedances matter: common exposure alone does not ensure common coupling. A useful bench measurement compares a shared disturbance with a deliberately unbalanced input path, recording both inputs and the output. Differential transmission also need not mean that either conductor is at zero potential relative to the receiver's reference.