差模电压(differential mode voltage)：一对导线上电压的差值；
共模、差模正是“输入信号”整体的属性，差分输入可以表示为 vi = (vi+ , vi-); 也可以表示为vi = (vic, vid)c 表示共模，d 表示差模。两种描述是完全等价的。只不过换了一个认识角度，就像几何学里的座标变换，同一个点在不同座标系中的座标值不同，但始终是同一个点。
Signal-Mode Definitions
Electrical signals carried on cables can be described as normal mode, differential mode, or common mode.
- A normal-mode signal is any type (other than common mode) that appears between a pair of wires, or on a single wire referenced to (or returned through) the earth, chassis, or shield. Normal-mode signals are read between two wires in a balanced or unbalanced transmission path. (For a balanced 2-wire path, one wire is driven positive while the other is driven negative by an equal amount, both with respect to a static or no-signal condition in which both lines assume the same voltage level relative to circuit common.)
- A differential-mode signal appears differentially on a pair of wires in an ungrounded cable configuration.
- A common-mode signal appears equally (with respect to local circuit common) on both lines of a 2-wire cable not connected to earth, shield, or local common. Usually, but not always, this is an unwanted signal that should be rejected by the receiving circuit.
Common-mode voltage (VCM) is expressed mathematically as the average of the two signal voltages with respect to local ground or common: Vcm = (Va＋Vb)/2
Figure shows a 3V differential-mode signal riding on a 2.5V common-mode signal. The DC offset is typical of differential-mode data transmitters operating from a single supply. The common-mode voltage can be AC, DC, or a combination of AC and DC. (Figure represents the simplest case, a DC common-mode voltage with no AC component.)
