Ground plane separation must be done very carefully and with understanding of its side effects. It's not an end-all, be all. You must have an idea of the magnitude of the currents flowing in the "digital" or "noisy" ground plane, the resulting noise voltage, its distribution on the ground plane, and its impact (if any) on the analog circuitry.
Same goes for positive supply planes.
Essentially, think that I ask you: "If you connect a differential voltage probe between those two points on this plane, what RMS voltage do you expect at bandwidth X." The answer to this question, for bandwidths relevant to operation of the analog circuit, is necessary. In most cases an approximated order of magnitude is plenty, I certainly don't expect everyone to do a finite element analysis here. You should be able to form a judgement, and to simply demonstrate why you think it's correct. If unsure, try injecting magnified test DC currents between points on a real test board, and measure the resulting DC voltages. While this ignores the imaginary impedance, it should be a good first step, especially if the planes are still continuous and each power plane is adjacent to a ground plane (thus as low-impedance as they're going to get).
Note that any "isolated" ground planes are providing a return path for the digital signals bridging the analog and the digital domains. If you're not careful, you can increase the level of interfering signals seen on the A/D converter's digital lines, and those can couple into the internal ADC circuitry, potentially wreaking havoc.
Having "split" or "isolated" planes is only crucial if you do it right. If you have no feel for it, it's safest to use continuous planes. The use of "split" planes without some engineering analysis is unlikely to fix any issues you're trying to fix, and is likely to introduce issues that weren't there before you split the planes. You can't treat it as if it were a cargo cult, where you repeat some incantations/rituals without being able to explain to a rubber ducky why they work on your circuit :)