[SI-LIST] Re: Rembrandt versus Art Barn
- From: wjcsongr@xxxxxxxxxxxxxxxxxxx
- To: steve weir <weirsi@xxxxxxxxxx>
- Date: Thu, 9 Apr 2009 10:16:38 -0500
Steve,
Amen Brother! Especially on the ferrite bead thing...seems like a go to
solution to every problem...by people who don't know what they are talking
about.
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
steve weir <weirsi@xxxxxxxxxx>
Sent by: si-list-bounce@xxxxxxxxxxxxx
04/08/2009 06:38 PM
To
olaney@xxxxxxxx
cc
Sol.Tatlow@xxxxxxxxxxxxxxxxxxxx, si-list@xxxxxxxxxxxxx
Subject
[SI-LIST] Re: Rembrandt versus Art Barn
Ferrites can be incredibly useful tools. More often than not I see them
used inappropriately where they do little or no good and introduce nasty
side effects.
With my apologies to Jeff Foxworthy:
If your IC manufacturer won't tell you the required power rail impedance
versus frequency at the IC mounting pads, the IC manufacturer might just
be a redneck.
If your IC manufacturer says place some number of bypass capacitors per
power pin, the IC manufacturer might just be a redneck.
If your IC manufacturer tells you to use a certain bypass network
without regard to capacitor placement, mounting method, or PCB stack-up,
the IC manufacturer might just be a redneck.
If your IC manufacturer tells you to put special features in your PCB
like moats without specifying the numeric parameters driving that, the
IC manufacturer might just be a redneck.
If your IC manufacturer tells you to add reactive elements like ferrite
beads without specifying the network frequency response, the IC
manufacturer might just be a redneck.
If your IC manufacturer insists that you have to use particular
practices like a linear regulator, but won't tell you the noise
susceptibility of their IC, the IC manufacturer might just be a redneck.
If you blindly follow advice from an IC manufacturer without getting
metrics, you might just be a redneck.
olaney@xxxxxxxx wrote:
> Perhaps the supplier knows the chip better than anyone else, but when it
> comes to putting it on PCB and making it play well with others... let's
> just say that silicon designers often aren't SI or EMC experts by a long
> shot. I've spent enough time showing semi manufacturers their mistakes
> and suggesting improvements that I never take a demo board at face
value.
> Some are empirically tweaked beyond all reason to make them work, and
> sometimes I wonder what they were smoking (a ferrite bead in series with
> the ground pin of an IC???). Playing monkey see, monkey do with these
> layouts can get you into real trouble. Others are clearly works of
> scientific art that I study carefully to see what I can learn. GaAs
> designers seem to have the knack.
>
> The expertise to tell Rembrandt from Art Barn when studying a suggested
> layout is not in everybody's kit bag. I'd be curious to hear the ways
> that others distinguish one from the other. One I use is the
observation
> that a classic response to inadequate understanding is an attempt to
> compensate through overdesign. The result is lots more components in
the
> schematic than experience says should be necessary.
>
> Orin Laney
>
> On Wed, 08 Apr 2009 18:47:26 +0200 Sol Tatlow
> <Sol.Tatlow@xxxxxxxxxxxxxxxxxxxx> writes:
>
>> Steve, thanks, finally, for an answer.
>>
>> As to why one would want to do this, it's very simple:
>> when the chip manufacturer recommends you do something that
>> goes against what you believe is right.
>>
>> Theoretically, he should know his chip better then anyone
>> else (right?!?), and the application note should be there
>> to help 'real-life' engineers implement the chip with
>> minimal effort. Trouble is, experience tells me this is
>> often not the case, as you already confirmed, like, I guess,
>> many others could.
>>
>> The 2 variant 'trick' covers you with minimal risks and
>> costs. This is exactly why I did it in the past, and in all
>> 3 cases, I was vindicated. Simple, and painless, and not
>> really a science experiment at all, with all due respect.
>>
>> My aim is pretty much the same as yours, I think, to get
>> customers' designs realised as best I can, and I believe
>> that in some cases, the 2 variant trick is a handy 'tool' -
>> granted, not in every case, perhaps, but certainly sometimes.
>>
>> Sol
>>
>>
>> steve weir schrieb:
>>
>>> 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
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>
>>>>>>
>>>>>
>>>>>
>>>>
>>>>
>>>
>> --
>> ________________________________________
>>
>> Sol Tatlow, M. Eng. (Oxon)
>> Product Developer
>>
>> Pro Design Electronic GmbH
>> Albert-Mayer-Str. 16
>> D-83052 Bruckmuehl
>> Phone: +49 (0) 8062/808-302
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