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Differential Signal Design, Part 2
October 30, 2013 |Estimated reading time: 1 minute
How should differential pairs be routed?
From the discussion in Part 1 of this series, it can be concluded that tightly coupled differential pairs carry two handicaps. The first is that for a given impedance the traces will have to be narrowed in order to maintain the desired differential impedance. Second, the differential pair must remain tightly coupled along its entire length resulting in routing restrictions that can prove to be a problem. As a result, it is advisable to design a stackup and trace width that meets both the skin effect loss requirements and achieves differential impedance that results in only minor changes in impedance when the pair must be separated to pass through a tight pin field.
The design rule that satisfies the above conditions is a “not closer than” spacing that results in no routing restrictions. The loosely coupled example above is such a “not closer than” case.
A big advantage of routing using the “not closer than” rules for differential pairs is that both single-ended traces that are meant to be 50 ohms can use the same trace width as the differential pairs with a differential impedance specification of 100 ohms.Read the full article here.Editor's Note: This article originally appeared in the September 2013 issue of The PCB Design Magazine.