Re: gEDA-user: pcb, howto partition power planes?

[Larry Doolittle <ldoolitt@xxxxxxxxxxxxxxx>]

DJ -

On Tue, Oct 28, 2008 at 03:09:37PM -0400, DJ Delorie wrote:
> > App notes and example designs are special cases: there is only
> > one chip straddling the analog and digital divide.  If you have
> > more than one (e.g., both an ADC and a DAC) all those ideas
> > pretty much go out the window, and you're better off with a
> > single ground plane.
> 
> I have 16 of these chips, all talking to a single MCU.  The 8 on one
> side have their own analog power/gnd, and the 8 on the other side have
> their own.

Split power supplies: almost always good.
Split ground planes: almost always bad.

Two layer boards are a special challenge, because you never quite
get a full ground plane.

Your board looks decent.  What kind of voltage resolution are you
looking for (e.g., what IC are Uxx0)?  The first thing that leaps
out at me is the large loop for power supply filtering on those
chips (both AGND to AVdd, and DGND to DVdd).  Oh, and do you really
want REF filtered to digital ground?

If you have noise troubles with that layout, I can offer suggestions
to improve it.  Hint: it will involve more closely approximating a
real ground plane over the whole board.

One major trouble with split ground planes is when tracks cross
the split, and the return path becomes high inductance.  Your board
doesn't suffer too badly from that disease.  Most cases that look
like that are actually differential signals, where the plane is
not part of the circuit.

   - Larry


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