4.7.2 Ground Planes

In many designs, it looks like the ground plane is defined to be “all the copper not used for something else, connected to ground somewhere.” This will not be an effective ground plane.

Note that for a high frequency signal, the return path in a ground plane will be exactly under the track, even if this path is longer than the direct route. This is because the return path will always be the path of least impedance, and for a high-frequency signal, this is the path with the smallest loop, not the path that has lowest DC resistance.

For circuits that include both digital and analog circuits, the ground plane may be divided into an analog ground plane and a digital ground plane. This will reduce the interference between the analog and digital parts of the system.