8.4 Differential Signaling, Termination, and Layout Rules

Differential signaling using two wires to simultaneously send the true and complement versions of a signal is a commonly used high-speed interconnect technique. Although it requires twice as many wires, when properly implemented, differential signaling has better noise immunity than single-ended signaling (i.e., signals referenced to a voltage assumed to be common between the transmitter and receiver, such as ground or a reference voltage). In fact, a characteristic of differential signaling is that it allows proper reception even when the transmitter and receiver grounds are not at the same voltage. This has great advantage when signaling across long cables or between cards plugged into a backplane because in both cases there will be a voltage difference between the transmitter and receiver local grounds. And, in both cases crosstalk between signals or noise coupled in from the return path will alter the switching levels, degrading performance. To a certain extent, differential signaling is immune to both of these effects, as discussed next.

8.4.1 Differential Signals and Noise Rejection

Differential signaling uses a differential transmitter and differential receiver arraigned as shown in Figure 8.13.

Figure 8.13: Differential transmitter and receiver.

The transmitter simultaneously sends out voltages corresponding to a true and complement version of the input signal on the V _p and V _n wires, respectively. The signal is seen to swing above and below a common mode voltage V _cm and has a differential voltage V _diff that is the difference between...