Printed Circuit Board Design and Layout Questions

5. Should I always gap the ground plane between analog and digital circuits?

The answer to this questions is a definite "NO." Many of the worst circuit board layouts we've seen have resulted when someone attempted to gap the ground plane between analog and digital circuits. This practice has its roots in the days when audio board designers first started incorporating digital circuits in their design. They found that low-frequency (0 - 20 kHz) noise from the digital circuits was picked up by the sensitive audio amplifiers on the same circuit board. At that time, there was very little concern about high-frequency (30+ MHz) radiation and designers found that gapping the ground plane between their audio analog and digital circuits solved their crosstalk problems. This was a convenient solution at the time, though not the only solution. It is a very poor design practice in situations where radiated emissions are a concern.

The safest rule-of-thumb is to provide one solid plane for returning all signal currents. In situations where you expect that a particular low-frequency signal is susceptible or is capable of interfering with the circuitry on your board, use a trace on a separate layer to return that current to its source. In general, never split, gap or cut your board's signal return plane. If you are convinced that a gap is necessary to prevent a low-frequency coupling problem, seek advice from an expert. Don't rely on design guidelines or application notes and don't try to implement a scheme that "worked" in someone else's "similar" design.