How to Implement an Analog Ground Plane
First, you probably don't need isolated grounds. It is used when return current produces a problematic offset (high currents), wide parallel data busses, quick rise/fall times (closing eye diagrams), and antenna[-like] copper pours. Use good decoupling practices and don't worry about it until it breaks.

Failing that, the next thing to try is a direct connection, with no extra impedances/filtering. Connect AGND and GND at only one point with relatively thick (i.e.: low inductance) traces, at the power supply. This is sometimes called a star ground when connecting several isolated grounds. It ensures that the return currents from some components don't contribute to an offset voltage for other components. Noise comes not only from the sink, but also the source: if bothering to isolate grounds, isolate, filter, and connect the respective voltage rails, too. This is as simple as decoupling (to the correct ground -- remember that caps transmit noise) with caps to ground, and, if needed, ferrite beads or inductors between equipotential rails.

Study the return current geometry before further modifying the ground system.