Wednesday, September 3, 2008

AC Coupling Capacitors for PCI Express

PCI Express requires AC coupling between transmitter and receiver. The AC coupling capacitors for both differential pair signals must be the same value, same package size, and have symmetric placement. If possible, TX traces should route on the top layer.

The capacitor value must be in the range of 75 nF to 200 nF (100 nF is best). The 0402 package size is preferred, and 0603 is acceptable. C-pack is not allowed.

The breakout into and out of capacitors should be symmetrical for both signal lines in a differential pair. The trace separation for routing to pads must be minimized in order to optimize tight coupling between the signal pairs.

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