[SI-LIST] Re: Split gnd planes - for/against?
- From: "Aubrey Sparkman" <asparkman@xxxxxxxxxxxxx>
- To: "'Sol Tatlow'" <Sol.Tatlow@xxxxxxxxxxxxxxxxxxxx>, "'steve weir'" <weirsi@xxxxxxxxxx>, "'Grasso, Charles'" <Charles.Grasso@xxxxxxxxxxxx>
- Date: Wed, 8 Apr 2009 11:32:16 -0500
Sol,
I'm not clear what you are trying to accomplish.
If your question is does anyone have A vs B systems, I counted at least 3
who responded yes.
If your question is will you share a real live example, I think you can
assume the deafening silence to be a polite NO.
Perhaps I didn't understand your question either.
Aubrey Sparkman
Aubrey.K.Sparkman@xxxxxxxx
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Sol Tatlow
Sent: Wednesday, April 08, 2009 11:08 AM
To: steve weir; Grasso, Charles
Cc: si-list
Subject: [SI-LIST] Re: Split gnd planes - for/against?
Steve, Charles, thanks for ta,king time to respond - your comments are all
clear and understood... but you still haven't answered the question, or
rather, given any real-life examples!
Let me rephrase the question once more - and this is not just for you, but
for all of those 3,500 people out there who are subscribed to this list, of
whom so few responded:
Have you ever had 2 variants of the same layout manufactured on the same
panel, where the only difference between the two is that one has a solid
ground plane, and the other has some form of moating and/or ferrite (or
otherwise) isolated ground islands, where one of the variants could be
indisputably shown to perform better, with respect either to functionality
or EMI? If so, which one?
There... that's hopefully now in a form where anyone can easily and quickly
give a more or less simple 'yes' or 'no' answer ;)!!!
Sol
P.S. Troublingly, it seems that a lot of the emails on this subject haven't
even reached me... so, sorry if I didn't respond to anyone, assume in that
case I didn't get the mail, and try sending it again.
(yes, I looked in the 'spam' folder - no sign of them there either)
steve weir schrieb:
> 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.
>
> Your comments about vagueries in appnotes is well founded. There is
> no 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
>>>>
>>>>
>>>
>>
>>
>
>
--
________________________________________
Sol Tatlow, M. Eng. (Oxon)
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Pro Design Electronic GmbH
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