Re: gEDA-user: analog/digital partitioning

[Geoff Swan <shinobi.jack@xxxxxxxxx>]

   I came across this
   ([1]http://www.tentlabs.com/InfoSupport/page35/files/Supply_decoupling.
   pdf) some time ago. I would be interested to hear peoples thoughts as
   there are clearly many differing views on correct grounding and supply
   decoupling. The article certainly made a lot of sense to me and until
   proven otherwise it's the approach I follow. I understand why multiple
   ground planes seem attractive with the idea of somehow partitioning
   different current flows - but I have yet to see an implementation where
   this worked as intended. I have debugged circuits where there were as
   many as 4 separate ground planes and this certainly did not help the
   noise problems. I recognise that this is not enough to rule out the
   approach - just that the person designing didn't understand what they
   were doing.

   If someone has a design/layout that has *correctly* implemented split
   grounds etc I would be keen to have a look. Better yet if the design
   approach can be explained. This is one of those elements of practical
   electronic design that seems to be glossed over as assumed knowledge,
   and not necessarily very well taught.

   regards,



   Geoff

   On Fri, Jul 23, 2010 at 3:56 AM, myjunk stuff
   <[2]carzrgr8@xxxxxxxxxxxxx> wrote:

   On Thu, Jul 22, 2010 at 12:37 PM, DJ Delorie wrote:

   One idea to consider is to start with a solid plane, and cut slots
   around the sensitive analog parts, like big C shaped moats, squares
   open on one side.  You retain the "big ground plane" conductivity, but
   you prevent stray currents from using your analog area as a short-cut.

   Each analog "chunk" can have it's own moat this way, too.  If they're
   near the edge, just cut a thin slot from the edge in.

   Hmmm... this reminds me of the cubicle I used to work in :-P

     Yeah, Dilbert calls them 'anti-productivity pods' :D
     Seriously though, I like your idea except that sloted planes are
     generally frowned upon in the SI world - there's an opportunity
     for eddy currents to flow around the slot, and that will radiate.
     As long as no signals cross the slots on any other layers, maybe
     this is ok.

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References

   1. http://www.tentlabs.com/InfoSupport/page35/files/Supply_decoupling.pdf
   2. mailto:carzrgr8@xxxxxxxxxxxxx
   3. mailto:geda-user@xxxxxxxxxxxxxx
   4. http://www.seul.org/cgi-bin/mailman/listinfo/geda-user


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