[SI-LIST] Re: Split gnd planes - for/against?
- From: Sol Tatlow <Sol.Tatlow@xxxxxxxxxxxxxxxxxxxx>
- To: "Grasso, Charles" <Charles.Grasso@xxxxxxxxxxxx>
- Date: Wed, 08 Apr 2009 14:26:12 +0200
Chas, thanks for taking the time to respond... if I may, I would
like to probe a little deeper: in what way do you mean "it's the
only way to go"?
Does this mean you have actually done something like I did, with 2
variants of the same placement and routing, where the only
difference was the ground plane? Or have you 'simply' never had any
failures, and this is attributed, at least partially, to the fact
that you always have split planes? I guess what I mean is: how can
you be so sure? Is the certainty based more on theory, or on real-
life 100% definite and indisputable examples?
Please don't misunderstand the above questions - I am simply trying
to get a hold on exactly what concrete proof there is for/against
the various theories in this area; I ask myself whether a well
placed and routed board ever needs split planes, if the layouter
knows what he/she is doing; whether split planes don't just cause
more problems than they solve, specifically with regards to end
products and their certification (where, say, a small decrease in
functional performance would be happily accepted in place of a
significant improvement in EMI performance).
At any rate, the internet is full of theories, but real-life tests
or studies, even relatively simple ones, seem to be pretty thin on
the ground. So, perhaps I should reformulate my question:
Have you any real-life examples where the correct use of moating
or split DGND/AGND planes (as opposed to one solid ground) on an
otherwise well placed and routed board, was 100% shown to "make or
break" a product? A simple "no" is of course also a good answer :)!
Sol
Grasso, Charles schrieb:
> 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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Product Developer
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