From: Alan Hilton-Nickel (firstname.lastname@example.org)
Date: Wed Jun 28 2000 - 14:52:16 PDT
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
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.
> 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
> We would like to know:
> Is there any limitation/ recommendations to the gap between the power
> 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?
> Yosi Zangiri
> Design & Manufacturing Technologies
> ECI Telecom
> Phone: +972-3-926 8235
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