[SI-LIST] Re: Split gnd planes - for/against?
- From: steve weir <weirsi@xxxxxxxxxx>
- To: "Grasso, Charles" <Charles.Grasso@xxxxxxxxxxxx>
- Date: Tue, 07 Apr 2009 10:05:18 -0700
So true and particularly in your circumstances where S/N ratios must be
very high and board layer count must be very low. Given that your
products work properly, you also take the other engineering measures
that are needed to make the moating work where you use it. The problem
is that if applied in a vacuum as it were, moating often creates more
problems than it solves. For the record I use moating when and where it
makes sense.
Best Regards,
Steve.
Grasso, Charles wrote:
> We use split planes all the time.
> When you have circuits of *vastl8 different noise floors co-existing
> on one board - it's the only way to go.
>
> Chas
>
> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
> On Behalf Of steve weir
> Sent: Tuesday, April 07, 2009 10:11 AM
> To: Sol Tatlow
> Cc: si-list
> Subject: [SI-LIST] Re: Split gnd planes - for/against?
>
> 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
>>
>>
>>
>
>
>
--
Steve Weir
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