Dave,
I must disagree with this "concept", but do agree with hi-speed lines
crossing splits, but only regarding impedance control.
This "concept" of the "return path" is what I don't agree with. Yes, I
know it is being "taught" now in certain circles, but I don't find any
mention to it in Lee Ritchey's classes (Lee's what the industry considers
the hi-speed guru, and disagrees with the "return path" theory), nor in
James Blankenhorn's SMTPlus hi-speed classes, nor in Rick Hartley's
hi-speed classes.
I did attend a class(es) by Mary Sudgren recently (the Copper
Connection), and she went into the return path theory again, and seemed
like a no-issue (regarding the above mentioned "experts"), and took
away time from some real issues (let's talk NOISE BUDGETS!).
With that said, I believe the real issue regarding split planes is most
recognizing proper power distribution and impedance issues.
BTW, you might be seeing an article on the "return path" theory in the
near future in PC Design magazine authored by Lee Ritchey, so keep
your eyes open.
Mitch Morey
NSI Communications
San Diego, CA
www.nsicomm.com
>>> David Baldwin <[log in to unmask]> 07/21/98 08:38am
>>>
The biggest problem is when high speed lines cross the split in the
planes.
The return currents follow directly under the traces, and if there is a
split in the plane, the currents have to flow around the split causing
emissions problems. There is no real problem splitting the plane if high
speed lines do not cross that barrier.
Dave.
At 10:13 AM 7/21/98 -0700, you wrote:
>At the expense of sounding ignorant, what are the problems caused by
>splitting gnd planes? I have to do this constantly, mainly under
>opto-isolators, to seperate the part of the board that is our product from
>the outside world. I hope you can get some online answers so that
everyone
>has a chance to check the data. Thanks.
>Richard
>
> ----------
>From: dschaefer
>To: DesignerCouncil
>Subject: [DC] !!! SPLIT GROUND PLANES !!!
>Date: Tuesday, July 21, 1998 8:25AM
>
>I'm looking for information regarding SPLIT GROUND PLANES.
>
>Experts on grounding, EMI, EMC are teaching that gaps in ground planes
>create serious problems. Component manufacturers, however,
continue to
>recommend placing gaps under components (such as A/D's and D/A's).
>
>Can anyone:
>
> - point me in the direction of theoretical reference material
> - relate their experiences / practices
>
>Thanks,
> --
>Dave Schaefer Phone
>: (204)631-2298
>Senior PWB Designer FAX :
>(204)631-2475
>NORTEL - Broadband Wireless Access email : [log in to unmask]
>37 Stevenson Road
>Winnipeg, MB R3H 0H9
>
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(602) 539-4400 FAX (602) 539-4500
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