[SI-LIST] Re: Split gnd planes - for/against?
- From: steve weir <weirsi@xxxxxxxxxx>
- To: Sol.Tatlow@xxxxxxxxxxxxxxxxxxxx
- Date: Wed, 08 Apr 2009 09:28:46 -0700
Sol, no and aside from a burning desire to conduct science experiments I
don't know why anyone else would.
I don't what is more real life than real physics. The experiment I
proposed to Lee can be implemented as the one panel experiment you seek.
Steve.
Sol Tatlow wrote:
> Steve, Charles, thanks for taking 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
>>>>>
>>>>>
>>>>>
>>>>
>>>>
>>>
>>>
>>
>
>
--
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