Ground Loop
A closed loop formed when a circuit is connected to ground at two or more points. Even a potential difference of a few millivolts between those points drives a current in the loop, and that current induces noise into the signal conductors. It is among the most frequent noise sources on analogue measurement and communication lines.
Ground points are assumed to be at equal potential, but in reality they are not. The difference between two points gives rise to a current in the closed loop. The loop also behaves like a frame antenna and induces voltage from the surrounding magnetic fields.
- Typical symptoms: oscillation in an analogue sensor reading, interference that does not disappear despite screened cable, a rising error counter on a communication line.
- Prevention: grounding the shield at one end only, collecting all negative returns at a single point, using optical or galvanic isolation between devices with different supplies.
- Measurement: measure DC and AC voltage in the millivolt range between two ground points. A difference of tens of millivolts read while the loads are running points to a likely loop.
Running an analogue signal cable between two electronic units bonded to different points on the same vehicle is the classic loop trap. Carrying the signal reference from the source device as well and isolating the measurement input at the receiving end solves the problem at its root.
Context in which this term is used: Installation, redundancy and field continuity