Reference material:
Partitioning and Layout of a Mixed-Signal PCB http://www.hottconsultants.com/pdf_files/june2001pcd_mixedsignal.pdf
Grounding in mixed-signal systems demystified, Part 1 and 2
http://www.ti.com/lit/an/slyt499/slyt499.pdf
http://www.ti.com/lit/an/slyt512/slyt512.pdf