[SI-LIST] Re: Split gnd planes - for/against?
- From: <Christopher.Jakubiec@xxxxxxxxxxxx>
- To: <scott@xxxxxxxxxxxxx>, <Sol.Tatlow@xxxxxxxxxxxxxxxxxxxx>
- Date: Wed, 8 Apr 2009 20:25:55 +0200
I suppose the person who was looking for a concrete example can add this to his
course, as he suggested.
Chris
Infineon Technologies
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Scott McMorrow
Sent: Wednesday, April 08, 2009 2:12 PM
To: Sol Tatlow
Cc: wjcsongr@xxxxxxxxxxxxxxxxxxx; Grasso, Charles; si-list;
si-list-bounce@xxxxxxxxxxxxx; steve weir
Subject: [SI-LIST] Re: Split gnd planes - for/against?
Sol
I gave you an example from 9 years ago. We fabricated two 10 GHz organic wire
bond packages. One package used a solid ground plane between the die wire bond
pads and the substrate pads, underneath the bond wires. The other package had
a slot cut into the ground plane between the die wire bonds and the substrate
pads. We made this modification to the package because we understood the
underlying physics involved. Package 2 with the slots performed better and met
jitter performance requirements at 10 GHz.
Engineering is not about running blind A/B tests. Leave that to the soft
sciences. Engineering is about using measurements to confirm theories about
the physical nature of the "things" we think we know.
Eventually we get to the point where we understand the nature of our models and
can substitute calculations or simulations for the real thing.
A good rule of thumb is:
"if you don't know the underlying physics of a complex structure ...such as a
split plane, moat or slot ... you liable to misapply it an make things worse."
Scott
Sol Tatlow wrote:
> Bill, thanks for your time.
> Yes, I appreciate I have had some of the top experts taking time on my
> account, as was clear, hopefully, through my repeated thanks. Still,
> with all respect, they just weren't answering the question, much the
> same as you haven't either - you're 'bottom line' is clear, and I
> agree... it just isn't an answer to the subject I raised, in any of
> its forms. In fact, I AM using engineering principles as I question
> approaches recommended in some datasheets that just don't seem to make any
> sense.
>
> Perhaps the problem really lay in how I originally formulated my
> request(s), but the latest 'incarnation' was, I think, pretty clear and easy
> to answer.
>
> Let's see who, out of those 3,500 subscribers, is up for answering it.
>
> Sol
>
>
> wjcsongr@xxxxxxxxxxxxxxxxxxx schrieb:
>
>> Sol,
>>
>> You have had some of the TOP experts in the SI field taking time to
>> answer your question over and over again. And I think they have
>> answered your question, no matter which way you ask it.
>>
>> The bottom line; it DEPENDS on the situation. One must apply
>> engineering principles, and THINK about the problem to be solved.
>> There's no easy answer; every design is unique. Some rules of thumb
>> apply, but the engineering judgement and experience part of it is
>> WHICH rule of thumb to apply to WHAT problem.
>>
>> I don't know what more anyone can tell you. It seems to me, that you
>> don't want to hear the answer?
>>
>> Regards,
>>
>> Bill
>>
>> William Csongradi
>> Senior Electrical Engineer
>> Rockwell Collins Heads Down Display Center
>> 319-295-7884
>>
>> Mailing Address
>> Rockwell Collins
>> 400 Collins Road NE
>> MS 105-167
>> Cedar Rapids, Iowa 52498-0001
>>
>>
>>
>>
>>
>> Sol Tatlow <Sol.Tatlow@xxxxxxxxxxxxxxxxxxxx> Sent by:
>> si-list-bounce@xxxxxxxxxxxxx
>> 04/08/2009 11:07 AM
>>
>> To
>> steve weir <weirsi@xxxxxxxxxx>, "Grasso, Charles"
>> <Charles.Grasso@xxxxxxxxxxxx>
>> cc
>> si-list <si-list@xxxxxxxxxxxxx>
>> Subject
>> [SI-LIST] Re: Split gnd planes - for/against?
>>
>>
>>
>>
>>
>>
>> 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
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>
>>>>
>>>
>>>
>>
>>
>
>
--
Scott McMorrow
Teraspeed Consulting Group LLC
121 North River Drive
Narragansett, RI 02882
(401) 284-1827 Business
(401) 284-1840 Fax
http://www.teraspeed.com
Teraspeed(r) is the registered service mark of Teraspeed Consulting Group LLC
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