Ground Loop (Cable Shield)
A closed conductive loop formed when the same circuit is grounded at two different points. The small voltage difference between the points drives a circulating current around this loop, and the current can couple into signal lines as noise. Grounding a shielded cable at both ends is the most common example.
The loop also behaves like a loop antenna, inducing voltage from varying magnetic fields around it. The switching current in the motor cable or the 50 Hz field from the mains can corrupt analogue references this way.
To avoid it:
- Ground the shield at one end only: preferably at the driver end, which is the noise source. The other end is left free and insulated.
- Single-point grounding: bring all negative connections to a common busbar and avoid daisy-chaining.
- Separate power and signal: run motor cables at least 50 mm away from analogue and RF cables, or cross them at 90 degrees.
Diagnosis is simple: when you have a noise problem, disconnect one end of the shield and see whether the behaviour changes. Drift in the analogue speed reference, random triggering of digital inputs or a drop in remote range are often related to a ground loop.
Context where this term is used: DC Motor Driver Selection Guide