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From: firstname.lastname@example.org (Paul Mathews)
Subject: Re: Ground pours on top and bottom layers?
Date: 6 Jan 2003 09:28:34 -0800
NNTP-Posting-Date: 6 Jan 2003 17:28:34 GMT
"Jim boy" wrote in message news:...
> "John Larkin" wrote in
> message news:email@example.com...
> > On Sun, 05 Jan 2003 02:21:59 GMT, 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.
> > On a 4-layer, it probably won't make any difference.
> It'll radiate more effectively if you have large copper areas that act as
> parasitic elements, won't it?
> > And why split the ground plane? (begin religious war now)
> > John
Anything that you can do to increase capacitance between power and
ground and decrease inductance within the power and ground layers
themselves can be beneficial. This includes pouring otherwise vacant
areas of signal planes with power and/or ground areas. However,
observe the following caveats:
1) Increased capacitance between signal traces and power or ground can
slow signal transitions.
2) Power and ground pours that are not well 'stitched' (by means of
vias) to their underlying power and ground planes can worsen EMI
problems by acting as large area radiators.
In our layouts, with DSPs running at external clock rates of 200 MHz
and 1ns rise/fall times, we stitch pours with a via density of at
least 1 per square centimeter. Also, we always place at least 1 via
near the end of any 'flags' in a pour. A 'flag' is simply any part of
the pour that projects or extends from the main body of the pour. In
some cases, this technique has allowed us to use a 4 layer board
rather than a more expensive 6 or 8 layer board.
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