split plane/micro-island

stefan@pad.incases.com
Thu, 3 Apr 1997 10:41:36 +0200

Hi Farrokh,

I would like to add some remarks concerning the split plane/micro-island
discussion:

>
> What about separating the Vcc plane between an area of a design with a
> noisy ASIC and another area where noise on Vcc can not be tolerated?
> Is it better to isolate the Vcc or to leave it a continous plane? Are
> there any application notes that might help me?
>

I agree to the other replies that splitting the power-plane is an effective
measure for limiting noise-coupling.

There are some application-notes available inside a special edition of the
EDN-magazine, which is called "EDN's DESIGNER's GUIDE TO ELECTROMAGNETIC
COMPATIBILITY", written by Daryl Gerke and Bill Kimmel. This magazine also
has been distributed by the marketing division of our company. You can read
there that it is very important to use a proper filtering when micro-islands
are introduced. Furthermore, some rules are given of how to do the GND/VCC-
split.

We now have released a software-package which principally is able to address
such structures. However, only a limited amount of projects have been performed
up to now. Therefore, I am still interested in exchanging ideas and, if available,
measurement-results for typical micro-island structures.

Additionally, I have written a paper entitled "USING EMI PREDICTION TOOLS
DURING THE PCB DESIGN PROCESS", which focuses on designing GND/VCC-structures.
I will present this paper at an IEE-colloquium at Savoy-Place, London, April
16th. This colloquium is entitled "Achieving electromagnetic compatibility:
accident or design?".

Derek Walton wrote:

>
>Two chaps in MN have published papers on
>this topic at the EMC Symposiums, and I'm sure I have copies somewhere. I
>believe the term is "micro-island". If you need to know more I could try to
>dig them out.
>

Yes, micro-island is the right term, see above. Can you inform me in which
EMC-Symposiums the related papers were published ??

Andy Ingraham wrote:

>
>You also must be careful about WHERE to draw the split between grounds.
>Remember, "no" switching signals should cross over the split.
>

It is very important with divided planes to eliminate any overlap in planes
or traces. Inside the special EDN-edition mentioned above this is expressed
by the "chain-saw-law":

"if you cut through the isolation barrier, you cut through only those
traces sourced from or destined to the isolated plane. All other traces
must not cross the barrier, and the plane must not overlap."

cheers

stefan

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