Design007 Magazine

PCBD-Nov2016

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40 The PCB Design Magazine • November 2016 When common sense fails, tap into your un- common sense. While common sense is consid- ered conventional wisdom, uncommon sense is a re-examination of that conventional wisdom. Basically, common sense teaches us that the way it has always been done is the right way, and that's just how things are. Following com- mon sense is usually the safe way to go. But the people who are really making a difference in the world are usually the people who try something new. Tapping into our uncommon sense allows us to take a deeper look at things we often take for granted. It is remarkable that with all of today's high- performance systems, in which very complicat- ed electromagnetic effects play a dominant role, many of us still hold misconceptions about the fundamental nature of how signals interact with interconnects. In this month's column, I will look at the contemporary ways of address- ing an old issue (déjà view, as I call it) and go beyond the design of PCBs. There is always a debate regarding how a dif- ferential pair should be routed. Conventional wisdom tells us that since the two halves of the pair carry equal and opposite signals, a good ground connection is not required as the return cur- rent flows in the opposite signal. And tight coupling between the signals is bet- ter than loose coupling as it reduces undesirable cou- pling/crosstalk from aggres- sor signals. And let's face it, it is easier to route a pair together so that we logical manage the planes, aggres- sor signals and matched de- lay simultaneously particu- larly in complex designs. Some argue that beyond the fact that differential pairs transfer equal and op- posite signals, there are no special requirements that need to be considered when using differential pairs. They should be treated as two single ended signals. The signals of a differential pair don't need to be rout- ed together, should not be tightly coupled and are not required to be routed to the differential impedance. by Barry Olney IN-CIRCUIT DESIGN PTY LTD / AUSTRALIA Uncommon Sense BEYOND DESIGN

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