3.1.2 Implementation Example

The above figure shows a solid GND plane on layer 2 of the four-layer PCB. This solid ground plane's purpose is to serve as a low impedance return path for the power planes and also as a reference plane and return path for high speed signals. All these factors reduce the board EMI generation.

The only exception to this rule can be seen in the upper right corner of the board where a small plane serves as AGND (analog ground) plane for an audio application. Its existence is application-dependent and only serves as model of how an acceptable split plane looks like. This is acceptable because there are no other signals routed over that area that could capture undesired external ElectroMagnetic Interference (EMI).