= Differential signal pair
{title2=$V_d=V_+-V_-$}
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 by
$$
V_d=V_+-V_-,\qquad V_c=\frac{V_++V_-}{2}.
$$
Equivalently $V_+=V_c+V_d/2$ 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.<Understanding Common-Mode Signals by Analog Devices>[Definitions of differential-mode signal and the average common-mode input voltage]{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.
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