Earlier today, I had a quick design review with one of my mentors. We focused our attention on the audio ADC and its power regulators. This review changed my perspective on them, with respect to number and placement.
First, we clearly need regulators for the 4 operational amplifiers used for the input buffers. Somehow, the reference design for the AK5397 did not include any, which seems to be a major omission.
Second, while using separate boards for individual regulators might help a little bit, what matters even more is to get the regulator as close as possible to the input it needs to power. Therefore, we might be better off putting our regulators on the SO-DIMM board itself.
Third, while the analog and digital sections of our converter should use separate regulators, the different digital power inputs certainly do not need individual regulators. For the analog ones, the primary consideration is power, and it should be fine to share a regulator across multiple inputs if their cumulative power requirement is well below the maximum supported by the regulator.
Fourth, it seems that using a large tantalum capacitor for the VREF section of the power regulator could do more harm than good, since large capacitors have a tendency to leak current quite a bit. Therefore, we might want to test both options on a breadboard before committing to a particular design.
With all that in mind, it seems that we should be well served with just 4 onboard regulators, one for the operational amplifiers (15V), one for their bias inputs (5V), one for AVDD (5V), and one for DVDD (3.3V). With a bit of luck, everything should fit on one side of our 100-pin SO-DIMM board.