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

• From: "Grasso, Charles" <Charles.Grasso@xxxxxxxxxxxx>
• To: "steve weir" <weirsi@xxxxxxxxxx>, <Sol.Tatlow@xxxxxxxxxxxxxxxxxxxx>
• Date: Wed, 8 Apr 2009 09:27:18 -0600
```Sol - You keep asking for concrete answers. There are NO
absolutes - thats what make engineering fun! Moating is
a technique that can be used (and we use it) to isolate
RF circuits from "dirty" logic. Now bear in mind another
factor that come into play - integration. As more and
more circuits are integrated into one chip the need for
esoteric pwb design diminishes greatly. So you will
see designs that "breaks" rules - but what has happened is
that the noise sources are diminished. One other point to
keep in mind is that other types of designs (low noise
measuring devices etc..) have different design issues
than high speed designs and moating again is a technique
that can be used to help.
Steve has explained the physics way better than I ever could
so take what he has described and apply it to your project!

Chas

-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx on behalf of steve weir
Sent: Wed 4/8/2009 6:56 AM
To: Sol.Tatlow@xxxxxxxxxxxxxxxxxxxx
Cc: si-list
Subject: [SI-LIST] Re: Split gnd planes - for/against?

Sol there are too many variables to give you a single answer or a simple
closed formula.  If we have enough distance to work with we can obtain
arbitrary isolation.  Moating comes into play when the amount of
distance we have to work with is insufficient.  For anything more
complicated than the kind of simple demonstration I proposed one is
stuck doing the engineering work.  If you are looking for a benchmark, a
crude one that I can offer is that if you need more than 60dB isolation,
you should be questioning whether this can be packaged compactly without
a split.

substitute for doing the actual engineering.

Steve.
Sol Tatlow wrote:
> Steve, no question, I understand and agree. Actually, I was hoping
> for some real-life examples of when moating (or ferrite connected
> analog/digital grounds) really has been more or less PROVEN to be
> a necessary and good idea... and preferably not just eval boards,
> but 'proper' boards.
>
> As I said, I don't expect a full dissection of anybody's private
> work - it would be something if I could just hear from a handful
> of people that they had 2 variants of the same board made at the
> same time, on the same panel, one with a split ground, and one with
> a solid ground, where it was found that one was better (in whatever
> way) than the other.
>
> It's a simple technique, but my suspicion is that noone is going to
> be able to give me a good CONCRETE case FOR a ground split,
> particularly not with regards to EMI... although I would love to be
> proved wrong, to know 100% certain that all those painstaking
> efforts I have made in the past on so many boards with split planes
> really were necessary :)!
>
> Of course, producing and assembling 2 variants of the same board is
> coupled with higher costs; particularly with prototypes, I can also
> understand why people, if they only have 5 good chips, don't want
> to 'risk' even one of them in this way.
>
> Nevertheless, it really irks me to have to follow some app note
> which seems to have little to do with the real world, simply
> because everyone in the design chain was/is too worried of having
> problems... kind of "Well, the last chip we did was on an eval
> board with a split plane, and THAT worked, so let's do it the same
> way again", the main target being that the eval board looks great
> and the chip performs perfectly!
>
> So, let me reformulate my original 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
>
>
> steve weir schrieb:
>
>> 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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