Re: PCI - decoupling of unused 3.3V connector pads

[Ted Firlit <firlit@utmc.aeroflex.com>]

burched - tony burch wrote:

> For this design, I am actually not using the 3.3 volt
> from the PCI connector.  However, the above
> implies that I should still decouple these pads.
> 
> Is that correct?  If so, what is the reasoning behind
> needing to decouple those pads if I am not using
> the 3.3 volt supply on this card?

The decoupling is required for return-path currents.  The motherboard
may have a 3.3V plane nearest (in the stackup) to some of the PCI signal
traces.  Thus the return current will flow on this plane under the
signal trace.  When the signal gets to your card, a decoupling cap will
provide a path for the return current to flow onto one of your ground
planes nearest the signal's PCB routing layer.  

FYI, there is more info on this topic both on the web and in books such
as "High Speed Digital Design" by Howard W. Johnson.