Differential signaling tech-
niques help keep data flow-
ing with high confidence
In real-world applications, there are several best practices
By Clark Kinnaird, Industrial Interface
Systems Engineer, Texas Instruments,
Figure 2. When noise is received equally on A
(purple) and B (blue) signal wires, it is rejected in
the differentially received signal (black = A-B).
Differential signaling is used for noise immunity in Ethernet, RS-485, CAN, and USB. In ideal
cases, all common-mode noise is
rejected. In real-world applications,
there are several design techniques
and component parameters to consider in order to
keep the data flowing with high confidence. Differential
signaling is used in most interfaces, which sends digital
information over cables. Although requiring two signal
wires rather than one, differential signals are much
more immune to noise than single-ended signaling.
The basics of differential signaling are well-known,
taking advantage of the noise rejection which affects
Figure 1. Typical differential signal chain with simplified model of
electrical noise coupling.
both signal wires equally. This is illustrated in Figure 1,
where a balanced differential signal is transmitted on two
twisted signal wires (twisted-pair).
so that the differential voltage signal is:
VA – VB = VSIGNAL
Popular electrical standards such as USB, Ethernet,
RS-485, and CAN use differential signaling and balanced
twisted-pair media to provide reliable high-speed com-
In practice, designers should keep in mind that no real
system has the ideal performance of a theoretical model.
There are several key sources of errors and noise that
should be considered.
Electrical noise from the environment affects both wires
equally, such that the received signals on A and B are:
VA = + ½ VSIGNAL + VNOISE
VB = - ½ VSIGNAL + VNOISE
Line-to-line impedance imbalance
Balanced signal wires are critical to the noise immunity
of differential signaling. Twisted-pair cables specify
the level of imbalance allowed. For example, at low