How do you choose an inductor for connecting separate ground planes?
Another school of thought is to forget the inductor and split ground plane. Instead use a solid plane, but use careful placement and routing to ensure the digital/analogue signals stay in their respective portions of the PCB.  



According to the figure below, you can see (for high speed signals) most of the return current flows very close to the trace (x is distance from centre of trace, h is trace height above plane):



No time to go into more detail right now (will try and add more later) but here are a couple of excellent links on the subject:

Henry Ott - Mixed Signal Layout
TI - High speed Layout Guidelines