emax0198 -
Remember that the noise in high frequencies can propagates between two parallel plates. So a common GND cannot bring the noise from the digital circuits to the analog circuits. If however, you use the common GND and VDD plates then high frequency noise can propagate between the plates even from one side of the board to the other.
There are dedicated simulation tools for "ground bounce" and resonances (such as Ansoft SIwave). This simulation tool allows you to simulate your board's power planes including the lumped (R,L,C) elements and measure the isolation between the digital and analog power planes.
Regards,
Itai Frenkel