PCB: digital at one side and analog at another
First of all you must understand that the digital signals will have return signals directly underneath them on the opposite side of the board.  You need to make sure that your analog signal traces are not running parallel to the digital return traces or you will have cross talk.  

Also, you need to make sure that all signals have a solid reference plane on the opposite side of the board or you will have impedance mismatches and excessive inductance and radiated noise since the return currents will be taking paths around any cuts in the planes above/below them.

Ideally you should flood the top and bottom of PCB with copper that is connected to ground, just be sure to keep clearance from unmasked copper to avoid shorts.  You can also drop vias connected to ground along the path of your analog signals to shield them as best as possible.