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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.

A differential signal pair carries information in the electric potential difference between two conductors. For conductor potentials $V_{+}$ and $V_{-}$ relative to a stated reference, define the differential and common-mode voltage components byEquivalently $V_{+}=V_c+V_d/2$ and $V_{-}=V_c-V_d/2$. A signal pair need not have zero common component; the receiver uses the difference while tolerating a specified range of common input potentials.^[ref]

Equal added disturbances cancel in the ideal subtraction: replacing both potentials by $V_{\pm }+n$ leaves $V_d$ unchanged. For example, inputs at $1.8$ and $1.2$ volts have $V_d=0.6$ volts. Adding $0.2$ volts to each changes $V_c$ but preserves the difference.

Real receivers have finite common-mode rejection ratio, and unequal coupling or mismatched impedances can convert a common disturbance into a differential one. Thus differential signalling is a system property involving the source, both paths and the receiver. An original test applies the same disturbance to both inputs and compares the output with a deliberately unbalanced path; it separates ideal cancellation from sensitivity to mismatch.