From: John Popelish
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Subject: Re: Ground pours on top and bottom layers?
Date: Sun, 05 Jan 2003 02:35:35 GMT
NNTP-Posting-Date: Sat, 04 Jan 2003 21:35:35 EST
Andy Peters wrote:
> Hi, all,
> I'm just wondering. What's the current thinking regarding grounded
> copper pours on the top and bottom layers of boards that already have a
> power and ground plane?
> My application is a DSP board with an analog audio front-end, a CODEC
> and a processor. My board has four layers: top and bottom are signal,
> layer two is ground, layer three is power.
> The ground layer has a split to separate the analog and digital grounds.
> The power layer has splits to accomodate the different supply voltages.
> Any pros/cons of the added ground pours on the signal layers are welcome.
I doubt that just pouring any big, unused areas on the signal layers
with ground is likely to do much that is useful, and has a few
possibilities of making something worse. I suggest that you reserve
the extra signal layer space to build intentional grounded or guarded
structures that have specific shielding purposes, (with the point that
any part of this structure is referenced to given careful thought) is
a better way to use this area. You may realize that you can use
several signal ground areas that have to pass through each other to
minimize noise and cross talk better than a single plane of signal
ground that also has power supply and signal currents passing through
it, in unplanned ways.
Every ground connection or shield should have a well thought out
purpose, in relation to other ground connections or shields. For
example: is it a reference voltage, a supply return, a signal return,
or a shield? What will be the effect of connecting these grounds of
various purpose together in various ways?