Printed Circuit Design & Fab - December 2007 - (Page 32)

NOISE REDUCTION Benefits of Implementing DIFFERENTIAL PAIRS When using differential circuits, specific design rules can maximize the advantages while minimizing design-induced difficulties. by SYED W. ALI As the PCB industry moves toward designing faster and more miniaturized products, data rates are constantly increasing, and printed circuits are shrinking. The need for transmitting the signals cleanly from the driver to the receiver has become of prime importance and has increased the need for using differential signaling. Differential signals involve a pair of traces routed at a uniform distance from each other over its length. One trace carries the positive signal while the other carries the signal that is equal, but opposite in polarity. Differential signaling is not new. It has been around ever since the existence of large-scale computing to move logic signals from one circuit to another. The only hindrance to its extensive use is the fact that every differential signal requires two wires, a set of drivers and a set of receivers. Hence, it is used for signals that require greater immunity to noise, or if the data rates are high and there is a ground offset between the driver and the receiver. However, if certain design rules are not followed, the functionality of the circuit board may be affected. This article describes the different applications of differential signaling and the correct way to implement them. On a PCB, differential signals can be implemented in two ways, as shown in FIGURES 1a and 1b. In these figures: ■ Edge coupled: The differential traces are routed side by side. ■ Broad side: The traces are routed one over each other on consecutive layers. In nine out of 10 cases, edge coupled differential pairs would be used, primarily for two reasons: ■ Lack of control in maintaining uniform dielectric thicknesses. ■ Single-ended impedance mismatch between each member of the broadside differential pair. Differential Signal Applications To correctly implement differential signals on a PCB, a designer must first know the main reason behind using differential signals. The main advantages of using differential circuits are as follows. Once a differential signal is transmitted and the receiver picks up the sig- FIGURE 1. Types of differential pairs on a PCB. 32 nals, it calculates the voltage difference between the two positive and negative signals. Hence, the received voltage is effectively doubled. This facet is very useful for low-level applications where signal to noise ratio is a problem, e.g. LVDS. The second advantage is that once the receiver compares the two signals to determine their logic polarity, the task does not require any reference voltage. Hence, the differential reception is unaffected by ground voltage shifts between the transmitter and receiver, as shown in FIGURE 2. Consequently, the differential circuits are capable of dealing with significant ground offsets that exist between the two ends of the data path. Another advantage is that there is no need to provide a low impedance path to the return, since there is none. This is because the returns of both the +ve and the –ve signals nullify each other. Hence, during layout of a mixed-signal PCB, if an analog signal is transmitted as a differential signal to a digital device, there will be no problem if the signal crosses power planes. The fourth benefit brought about by using differential circuits is that these signals are fairly immune to outside EMI and crosstalk from nearby signals. This is because: the received DECEMBER 2007 PRINTED CIRCUIT DESIGN & FAB

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Printed Circuit Design & Fab - December 2007 Contents Our Line Market Watch Around the World Happenings ROI 2007 PCB Designer Salary Survey Interconnect Strategies Positive Plating RF Design Ad Index Noise Reduction Supply Chain DfM Off the Shelf Marketplace The Signal Doctor

Printed Circuit Design & Fab - December 2007

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