Mixed signal Microcontroller grounding
Manufacturers recommend that you use separate planes because they don't want switching noise from the uP (which shows up as ground bounce) commuted into the sampling of the ADC (which shows up as noise in the LSB). Generally speaking, if you have a single ground plane and reasonable decoupling on the chips consuming the power, you'll be fine. and if you are getting some lsb noise you can stick an front end signal conditioner to push your input signal out of the noise floor (granted at the expense of some dynamic range).

If you can't do either of those things, and you really really want to have separate planes, then the typical suggestion is to tie the ground planes with an inductor that will block the switching noise from showing up but leave your DC bias levels the same.

I'd be interested to know what your application is that requires that level of noise suppression.