Ground loops in audio and digital electronics

EDN magazine articele Ground loops by Howard Johnson (Signal Integrity) article starts with question why in audio circuit designers avoid ground loops at all costs, yet in digital products you normally see a solid ground plane holding hundreds of circuits. This is a good question and the article gives answers to that.

Audio designers like single-point ground networks. Such a network of ground connections has the topology of a tree, with one main trunk and many branches and sub-branches. None of the branches touch, so it contains no loops. In this type of network there exists only one ground path between any two devices.

Single-point ground networks provide isolation between devices, but only only when communications remain localized to isolated sections of the network. Good audio equipment uses a single-point ground system and keeps disparate circuits confined to isolated sections of the tree.

In the digital world digital circuitry easily tolerates few tens of millivolts of noise, so in most cases we simply do not need the complication of single-point grounding for ordinary digital logic to work. In addition, all electronic systems suffer mutual-inductive coupling whose severity grows in proportion to the bandwidth of the signals involved. We must have a solid ground plane and power plane to control inductive crosstalk in digital products.

I can agree this writing. The description is well written and easy to understand. Here is some more material on ground planes and grounding.

If you need to have both digital and analogue signals on same circuit board, a preferable way to proceed is to use a single, system-wide 0V plane and carefully lay out the board so that digital noise is effectively segregated from low-level analogue circuits. Also, lay out the analogue circuits so that the critical nodes are subject as little as possible to ground noise voltages, that is, keep sensitive signals free of common ground impedance. Remember that in the centre of a plane inductive effects are minimal and the plane impedance is dominated by resistance, but towards the edges ringing magnetic fields are created and the inductive impedance of the plane rises.

The ground plane arrangement is also needed in many RF circuits. A continuous ground plane on the back side of a printed circuit board as an ideal arrangement, the RF signal traces should be well terminated and kept short.

Remember also correct chassis grounding. Chassis ground is important for RF. Usually you need to consider signal loop more than ground loop. A good circuit designers considers the effect of both of them.