Single regulator to power mixed signal PCB?
So I've always followed this advice, for all sensitive analog sections of a design.   AGND and DGND tied at a single point together under the ADC, codec, etc.   For a recent high gain, hi-fidelity audio board I just necked down the GND plan under my ADC.  The connection is about 100 mil wide, and 10 mil long.  I've got an MCU and DSP on one side and my ADC on the other and no noise issues to speak of.



My power comes from a single regulator and the analog supply is a filtered version of the digital supply.  In my case it's LC but could be ferrite.  If I had just a few analog pins I would just filter at the pins, but I had a lot of analog circuitry so I have a separate analog plane connected to digital with a filter.

I also like to think about where my return currents will be flowing and try to arrange things so I don't have any digital current flowing near my analog ADC pins.  This was all done with a 4 layer board, and of course no signals ever cross a split in either plane :)