Differential Pair PCB Routing

Many technologies like SATA and USB use differential signaling instead of single ended. This helps reduce noise on the nets by way of the cancellation effect the two signals have on each other. In a differential pair, one signal is high while the other is low and vice versa. Differential pair pcb routing can be tricky, the designer needs to define them in the software as a pair then set the trace widths and gaps according to the desired impedance. Depending on the technology, the typical impedance for a differential pair can be 100 ohms or 90 ohms. To hit the desired impedance target, the pcb layer stack needs to be set properly using a reference plane for the pairs. Avoid routing differential pairs over splits in the reference plane as this will cause an impedance mismatch which may lead to signal integrity issues. Pairs are routed as one and should be as clean as possible. Differential pairs also need to be length matched between each other so the designer usually has to tune them after routing. If vias are needed to change layers, it is a good idea to add ground vias near the transition vias to help keep them coupled to their reference plane.

Author:

Kenneth Wood

Owner - Saturn PCB Design, Inc.

www.saturnpcb.com

Cover photo source: Wikipedia - https://en.wikipedia.org/wiki/Differential_signaling