RE: [SI-LIST] : Connector pin pwr/gnd ratio

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From: Ingraham, Andrew (Andrew.Ingraham@compaq.com)
Date: Tue Nov 14 2000 - 08:17:06 PST


...
> Almost all of the signals are from 3.3v totem pole outputs (lvcmos, lvttl,
> and pci), so I would assume that return currents want to flow on both
> power
> and gnd planes. ...
 
It's not which voltage powers the outputs, but rather what are the planes
that are adjacent to your signal traces.

The high frequency return current hugs the signal trace ... no matter what
voltage is on that plane, no matter what voltage powers the driver IC ...
except in the immediate vicinity of the driver IC.

A little distance from the driver IC, the high frequency return current is,
for all practical purposes, only in the one or two adjacent planes. If the
driver is 3.3V CMOS and the trace runs between 2.5V planes, then the HF
return current tends to be only in those two 2.5V planes.

The return currents do something different at the driver IC, depending on
whether it switches from high to low or from low to high, on the supply
voltage for the output transistors, and on the type of load. Local
decoupling at the driver IC provides the current paths from there to the
planes that are adjacent to your traces.

If there is any significant trace length between the IC and the connector in
question, it doesn't much matter what the supply voltage of the IC was.
It's the layup of your board, and where you routed your traces with respect
to which plane, that matters at the trace/connector interface.

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