Logic PCB Ground Plane

- ground plane techniques used for providing effective grounding for logic / digital printed circuit boards, PCBs

Logic / Digital Design Guidelines Includes:
Design guidelines overview Supply decoupling PCB groundplane usage Unused gates

One area where issues with noise and spurious switching can occur is when the grounding of the integrated circuits on a PCB is poor.

Having an effective PCB ground plane for the logic circuits is key to the reliable operation of the overall board.

The use of an effective PCB ground plane can only be achieved if this is planned from the earliest stages of the design.

Ground circuits

The ground circuit on a logic board is a key to the successful operation of a logic circuit or board. If poorly configured it can give rise to significant levels of noise. Conversely a board with a well-designed ground circuit is far more likely to give trouble free operation.

An ideal ground circuit is able to provide an equi-potential plane. If this were possible it would mean that the whole of the ground would be at exactly the same potential regardless of the current flowing at any point.

Unfortunately this is not possible because resistance and inductance are major issues and cause voltages to change when current levels vary.

The key for any successful logic circuit design is to minimise the ground potential changes that occur as the current levels vary.

Key logic ground circuit design guidelines

There are many aspects to good ground circuit design. Some of the more important guidelines are included below:

Use one layer of PCB for ground: With many logic circuits being constructed using multiple layer printed circuit boards, it is good practice to use one complete layer for the ground circuit. In this way both resistance and inductance are minimised and it is very easy to get a very short connection to the ground plane.

Spreading the ground plane across the board significantly aids in dissipating noise and other forms of unwanted signal.
Ground plane fill: It is best if the ground plane can cover a complete plane in the board. However sometimes there are spaces caused by large components such as high pin count integrated circuits, or as a result of tracks needing to run on the ground plane. Every effort should be made to maintain the integrity of the ground plane. Fill any spaces and to ensure that voids are well connected to ground and not left floating, or connected via small tracks.
Separate analogue and digital ground planes: It is always good practice to separate the ground return paths for analogue and digital signals. The analogue signals may possibly include audio and this will be very sensitive to the noise generated by the logic circuits.

While it will be necessary to connect the two planes at some stage, this should generally be done as close to the power supply as possible. In this way no return noise is looped onto the analogue board.

By adopting a complete ground plane and implementing it so that it is separated from any analogue grounds, this can save a significant amount of time consuming fault finding later to resolve grounding issues.