Re: [SI-LIST] : Split power plane

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From: Ritchey Lee (leeritchey@earthlink.net)
Date: Wed Jun 28 2000 - 18:10:09 PDT


Very good advice.

Lee

Alan Hilton-Nickel wrote:

> Yosi,
>
> As described, your design sounds risky. I would encourage you to keep to
> one ground plane, but partition the board as much as possible into
> Analog and Digital areas. Reason: there are going to be signals from
> parts on the analog plane to parts on the digital plane. If the return
> current is diverted in any way from the lowest inductance path
> (immediately under the signal trace) you will compromise your signal
> integrity and possibly set up conditions for EMI. Many of us on this
> board have taken problem systems and fixed them by removing gaps in the
> ground plane.
>
> If you *must* separate the planes, keep them on the same layer and
> "stitch" them together with small (0.01uF or thereabout) caps. Lots of
> them. Another strategy would be to place the components in one analog
> subsystem as much as possible on one side of the board in one are, and
> digital components of another subsystem on the other side of the board.
>
> Filters between power areas: from one power source, use a ferrite bead
> to connect to additional power planes. Do not daisy-chain the planes.
> Personally, I have not had much use for ferrites, but others here would
> disagree with very good reasons.
>
> Filters between Power areas and AGND: just a sprinkling of the usual
> well-considered decoupling caps.
>
> Distance between power and ground layers should be as small as possible
> to take advantage of any plane capacitance - 0.005" is not unreasonable,
> check with your board house to see what they can do.
>
> Alan
>
> Yosi.Zangiri@ecitele.com wrote:
> >
> > Hi,
> >
> > We have a design with 4 different Power and 2 Grounds.
> > We plan to put all 4 Power and AGND in one plan - layer 2 (close to DGND
> > plan - layer 3).
> > Hi-Speed Signals ( Rise time of 150 ps) are routed in other layers as strip
> > lines.
> >
> > We would like to know:
> > Is there any limitation/ recommendations to the gap between the power
> > areas?
> > Do you recommend any "filter" between power areas?
> > Do you recommend any "filter" between power areas and AGND?
> > What should be the minimum thickness between the Power plane and layer1?
> > What should be the minimum thickness between the Power plane and layer 3?
> >
> > Thanks
> >
> > Yosi Zangiri
> > Design & Manufacturing Technologies
> > ECI Telecom
> > mailto:Yosi.Zangiri@ecitele.com
> > Phone: +972-3-926 8235
> >
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>
> ------------------------------------------------------------------------
>
> Alan Hilton-Nickel <ahilton@transmeta.com>
> Systems Engineer
> Transmeta Corp.
>
> Alan Hilton-Nickel
> Systems Engineer <ahilton@transmeta.com>
> Transmeta Corp. HTML Mail
> 3940 Freedom Circle Building 2;Santa Clara;CA;95054; Fax: (408) 496-0761
> Home: (408) 979-9439
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> Last Name Hilton-Nickel
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