Re: [SI-LIST] : PCB Pwr Planes

Hemant Shah (hemants@xynetix.com)
Fri, 20 Mar 98 08:09:00 EST

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From: Robert Holmes
Sent: Thursday, March 19, 1998 6:39 PM
To: Hemant Shah
Subject: RE: [SI-LIST] : PCB Pwr Planes

Yes, with respect to the interaction between signals on traces and the
plane. There is a difference
with respect to the consequences of voltages induced on the plane by the
signals. For example,
ground bounce induced on the Gnd power plane can affect input switching
thresholds of devices which
are referenced to that plane, whereas the corresponding effect on the Vcc
plane can modulate
the output drive and switching behaviour of devices which rely on that
plane for their power.

A note of caution - the "servicing" of a trace by one plane is not
easily transferable to another plane,
even if it runs between both planes for a distance. The reason for this
is that there is usually not
a good pathway for the "shadow" return current to flow between the two
planes. Unless there is a
bypass capacitor which is good at all frequencies and harmonics of
interest which goes between
both planes at a point near the trace, you have a problem. The planes
themselves might
create enough capacitance between themselves, but this depends on the
distance the trace
runs between the planes.

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From: Hemant Shah
Sent: Thursday, March 19, 1998 2:14 PM
To: Robert Holmes
Subject: FW: [SI-LIST] : PCB Pwr Planes

Bob,

This should be very easy to answer for you.

Hemant

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From: owner-si-list
Sent: Thursday, March 19, 1998 3:55 PM
To: Sig-Integ
Subject: [SI-LIST] : PCB Pwr Planes

I'd like to verify an assumption of mine:
Is a Vcc power plane really the same as a Gnd power plane at high freq's
(assuming good bypass cap's)?
Thanks for your input,
Jim
Jim Peterson
jfpeterson@space.honeywell.com
Honeywell, Space Systems Division, M/S 934-5
13350 US 19 N., Clearwater, FL, 34624
813-539-2719