and the undesired common-mode signal Vcm = (Vin+) - (Vin-)
The common-mode signal is undesired noise that was summed into the signal-carrying wires.
This causes the common-mode signal to be time-varying, but that should not occur at the output of the amplifier--the start of the cable.
One servo adjusts the two outputs to ground since the servo is trying to bring the common-mode signal to ground.
LCL is measured by applying a common-mode signal to the cable.
Like LCL, LCTL applies a common-mode signal to the cable pair and the resultant differential signal is measured.
Second, balanced signal lines have that extra dimension of freedom: the common-mode signal.
FIGURE 5 shows two possible filter and attenuator configurations and details the differences between their effects on common-mode signals and differential mode signals.
The common-mode signals
can be cancelled out in the virtual short plane while the differential-mode bandgap could help generating higher notch attenuation.
Achieving length matching (for optimum timing margins, preventing common-mode signals
can be detected by hosts or devices whose receivers are in low-power mode, unlike the high-speed differential signals which require power hungry termination and biasing.
Furthermore, the AD8220's 80 dB CMRR (G=1) over frequency ensures that only the desired signals are monitored and unwanted common-mode signals