Selection of ferrite bead for analog and digital supply isolation
Separating the planes with a ferrite bead can prevent high frequency noise on the digital ground from conducting into the analog ground, but it has a serious drawback. If the analog plane has changing currents (which it probably will because of ADC's or DAC's) then any current traveling back to the digital plane will create common mode noise through the inductor (because it is blocking current and any current through the inductor will create a short term voltage between the two planes).
A better way would be to tie the planes together and not have separated planes (in almost every case this is better but not all) and have a sufficient ground, usually problem current's from digital electronics can be avoided by proper placement of the components, as currents from ground pins return through the plane back to the source (usually a cable on the board or a regulator) so don't place the analog components on this path (or any current path on ground with a switching load). This scheme may not work if there are many cables that interface with the board so keep that in mind (because of ground loops, analog connectors\shields that connect to ground on the PCB and another location could carry currents, so either break the ground loop on one end of the shield OR...)
Isolate between digital and analog ground, if the digital environment is especially noisy and the analog measurements are sensitive then the best way to avoid noise is to isolate, then no current can flow between analog and digital sections.