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Differential signaling techniques help keep data flowing with high confidence: in real-world applications, there are several best practices to consider.

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 both signal wires equally. This is illustrated in Figure 1, where a balanced differential signal is transmitted on two twisted signal wires (twisted-pair).

Electrical noise from the environment affects both wires equally, such that the received signals on A and B are:

[V.sub.A] = + 1/2 [V.sub.SIGNAL] + [V.sub.NOISE]

[V.sub.B] = - 1/2 [V.sub.SIGNAL] + [V.sub.NOISE]

so that the differential voltage signal is:

[V.sub.A] - [V.sub.B] = [V.sub.SIGNAL]

Popular electrical standards such as USB, Ethernet, RS-485 and CAN use differential signaling and balanced twisted-pail media to provide reliable high-speed communication.

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.

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-frequencies CAT 6A is specified with 40 dB of transverse conversion loss. This means that a 1-V transient coupled to both signal lines (common-mode) creates only 10 mV of differential-mode signal. Lower grades of cable allow higher fractions of common-mode to differential-mode conversion.

Imbalance in the differential path can be caused by components added to provide protection against transients. For example, transient voltage suppression components are sometimes prescribed as a means to prevent damage due to electrostatic discharge, voltage surges, or electrical bursts.

To view the expanded online version of this article, visit:

By Clark Kinnaird, Engineer, Texas Instruments,

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Title Annotation:Industrial Focus
Author:Kinnaird, Clark
Publication:ECN-Electronic Component News
Date:Jan 1, 2013
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