Choosing the correct Ground or Power net for internal copper planes in a 4-layered design
It's perfectly ok to connect digital and analog circuitry returns  to the same copper plane (in fact it is a requirement for certain high performance ADCs) but one has to segregate the analog and digital traces on the top layer or layer underneath the plane  and keep them spatially separated until they converge on a common point, usually at the connector supplying power to the board.
Google up anything written by Henry Ott or get a copy of the book &quot;grounds for grounding&quot;