Ground loop in PCB

Ground loop is one of the common problems in the PCB. Ground loop occurs due to the unintended potential difference between the various ground points in the design. Ideally, the ground potential on a PCB must be zero and it must be the same across various sections in the PCB. due to improper grounding techniques used, potential difference across the sections causes non-zero ground potential there by inducing noise to the signal path. While this noise is tolerable in many cases (due to acceptable signal-to-noise ratio) and ground noise gets unnoticed, the radiation that might happen due to this grounding issue may lead to EMI/EMC certification failure. This issue is technically called a ground bounce.

To understand this better, let us assume we have two circuits which need to be tied to the same ground potential. A circuit 1 tied to the ground with small distance and the second circuit tied with a longer distance. These traces have different inductance associated and can cause potential difference between the ground points of circuits. The simplest thing is to have a full ground plane.

If we see the circuit above, there is a circuit on the left driving the signal towards the other circuit. Expectation is that the ground on left part of the circuit must have the same potential as the ground on the right side. If not, noise gets introduced to the signal travelling from source to destination. We all know that the ground must be as close as possible to the signal so that return path is close by. Especially, for the High speed signals this is very crucial. Also for low-level analog circuits, ground loops cause issues.

There are several scenarios which cause ground bounce issues:

• One which we talked above, multiple ground paths due to potential difference between ground points at various ground points of the PCB
• Ground potential difference caused by the chips themselves due to the parasitic inductance of the pins. This can be eliminated by the bypass capacitors in the circuit
• In a system where AC power input is used, if the grounding is made common to the AC ground at several places, it causes ground loops

As designers we have to understand that grounding techniques are not the same for all application scenarios (low frequency digital, high frequency digital, analog).