[SI-LIST] Re: Split gnd planes - for/against?

We use split planes all the time. 
When you have circuits of *vastl8 different noise floors co-existing
on one board - it's the only way to go.

Chas

-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
On Behalf Of steve weir
Sent: Tuesday, April 07, 2009 10:11 AM
To: Sol Tatlow
Cc: si-list
Subject: [SI-LIST] Re: Split gnd planes - for/against?

Sol, unfortunately there is not a single answer.  In most cases moating 
is a bad idea, particularly if one does not understand the caveats and 
how to deal with them.  It's not just the moats:  It's the placement, 
clearances, stitching, and routing that all need to be considered.

Steve

Sol Tatlow wrote:
> I know this subject has been raised before, countless times in one
> guise or another. I have also googled plenty. I'm not looking for
> theoretical opinions, either, about whether or not, or when, they
> should be used (specifically not, "it depends", unless you've got
> REAL-LIFE examples, for and against!!!).
>
> This subject raised its head for me in this case due to using
> 2 A/Ds as well as 2 D/As, both from Analog Devices, where one
> specifies a split plane, the other specifies no split. Now, I am
> all too wary of relying simply on evaluation boards, where, in
> general, one layout is done, and if it works, that's how everyone
> should do it (_without_ comparing 2 different approaches).
>
> I personally have 3 concrete cases where split gnds had no positive
> effect on SI, but significantly worsened EMC results (despite
> sticking to all the usual guidelines, like no tracks over the
> splits, etc.), but I have no concrete case FOR split ground planes.
>
> So, what I'm interested in is: does anyone have CONCRETE examples
> which they would like to share for/against split planes? The kind
> of thing I mean would be like in one of the cases I had, where I
> wanted to go against the suggested approach of using a split gnd,
> and persuaded my customer to pay for 2 variants of the same board
> on the same manufacturing panel, one with split ground, one with
> solid ground. Both variants were assembled and tested, with regards
> to both SI as well as EMC: both were functionally satisfactory; at
> EMC testing, however, the split-plane bombed out big time, while
> the non-split sailed through. I like to think that it wasn't due
> to any screw-ups on my side, that the split ground failed - I am
> not a newbie to PCB layouts, and, while for sure no professional
> expert on all areas of SI, I believe I avoided the typical blunders
> often present in split ground layouts.
>
> Anyway, my customer was more than happy, but not everyone has the
> money/time/desire to do as I suggested. So, any 'war stories' to
> support one or the other approach would be appreciated to help
> expand my knowledge and understanding of this subject - obviously,
> we all respect confidentiality, so I'm not looking for circuits,
> layouts and so on, but I figure many of you must have stories that
> can be related regarding this subject. Or perhaps some good links
> to non-confidential 'real-life' examples/studies?
>
> Regards,
> Sol
>
>   


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