= Differential signalling
{wiki}
<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.<Understanding Common-Mode Signals by Analog Devices>[Common-Mode Signals Defined and Cables and Noise]{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.
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