[sdiy] Ground planes for mixed signal designs - to split or not to split

[Seb Francis]

I've been having a discussion off-list about splitting (or not) ground 
planes for mixed signal designs.  I'd like to open the discussion to the 
list as this seems to be a tricky area where there is not always a clear 
optimal solution.

The specific case I'm looking at is my Digi-Mod design which can be 
summarised as follows:

Has precision analog audio & CV circuitry
Has high frequency digital circuitry
The 3 main places where these domains meet are:
- Audio CODEC (which has both an AGND and DGND pin)
- CV DAC (which has a single GND pin)
- dsPIC built in ADC (the dsPIC has both AGND and DGND pins)

My current thinking is as follows:

Have a separate ground plane for AGND and DGND, with a separate power 
connector pin, connected back at the PSU (off-board).
Have the audio CODEC and CV DAC on the boundary between the 2 planes
Use the AGND plane for both A&D CODEC ground connections and the CV DAC 
ground.
The dsPIC needs to be in the digital zone, but with AGND connection 
routed from the AGND plane.

I'm using this split ground approach because it seems right to isolate 
the analog and digital as much as possible, and also other manufacturers 
such as MOTM use this approach, but I do have concerns with it:

On the dsPIC the AGND and DGND pins could be potentially quite different 
as there may be a lot of inductance between the 2 ground planes.  Will 
this degrade the performance of the dsPIC ADC?

I've read that routing signals across ground planes can cause problems, 
but I am having to route the CV input signals across to the dsPIC, and 
also potentially some digital signals unless I'm very careful to have 
the CODEC and DAC exactly straddling the 2 ground planes.  And maybe the 
MIDI UART signals will have to cross zones because of the position where 
the MIDI headers are.

Another thing is that my 'analog' 3.3V power is made from filtering the 
digital 3.3V via an inductor and capacitor (makes sense to do it this 
way round as the 'analog' 3.3V is very low current so a small, but high 
inductance SMD inductor can be used).  However the 3.3V regulator is 
referenced to DGND, whereas the analog 3.3V devices have ground 
reference AGND.

So it gets me wondering if splitting the grounds is really the right 
thing to do.  And what are the best practice alternatives to avoid 
digital noise in the analog domain without splitting the ground planes?

Seb

P.S. The circuit in question:
http://burnit.co.uk/sdiy/stuff/Digi-Mod.pdf