Re: gEDA-user: analog/digital partitioning

[gene glick <carzrgr8@xxxxxxxxxxxxx>]

On Thu, Jul 22, 2010 at 8:29 PM, Geoff Swan wrote:

> I came across this (
>
> 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

This is a huge topic, Geoff.  There are a whole lot of "rules of thumb" 
that have been
written to help people get to the finish line without spending too much 
time thinking about it.
In the specific case, the best answer is "it depends", and you have to 
excercise some
brain cells, assuming you understand the basic reasons for making a 
design/layout decistion.
Also, what works at audio doesn't necessarily work at 1 GHz.  Edge rates 
are just as important
as clock rates in the SI world.

For some good info, you can check out stuff from Howard Johnson, Henry 
Ott, Eric Bogatin,
Doug Smith, Lee Ritchey to name just a few (there are many more).

In my case (this thread) I was concerned about slots.  Having been 
taught they are not
good (rule of thumb) I was having a tough time allowing it - even though 
it looks like
a pretty good solution.  Here's some info from Henry Ott on this 
subject:
http://www.hottconsultants.com/techtips/tips-slots.html
Some info from Lee Ritchey apparently backs up the notion as well (I 
don't have
any reference to it though)

good luck

gene


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