Ground Loop
Understanding Ground Loops
Ground loops are insidious because they can couple interference from remote sources into sensitive circuits through the ground wiring itself. The 'ground' is not a perfect zero-impedance reference; it has finite resistance and inductance that allow voltage differences to develop between different ground points.
Ground Loop Formation
- Equipment A is grounded at point 1.
- Equipment B is grounded at point 2 (some distance away).
- A cable connects A to B, creating a second ground path through the cable shield.
- Current flows around the loop (ground 1, cable, ground 2, building wiring, back to ground 1).
- The loop current creates voltage drops that appear as noise at the signal connection.
Ground Loop Prevention
- Single-point grounding: All equipment grounds connect to one point (star topology).
- Isolation: Break the loop with transformers, optical isolators, or balanced connections.
- Differential signals: Reject common-mode ground noise by design.
Frequently Asked Questions
What is a ground loop?
A ground loop is a closed circuit formed when two ground connections are linked by multiple paths. Current flowing in the loop creates unwanted voltage differences that appear as noise or interference. It is the most common source of hum and low-frequency noise.
How do you fix a ground loop?
Break the loop by removing one of the redundant ground connections, using isolation transformers, optical isolators, or balanced/differential connections. Single-point star grounding prevents loops from forming in the first place.
Why are ground loops a problem in RF measurement?
Ground loops between test equipment and DUT can inject interference that masks small signals. At low frequencies (60 Hz and harmonics), ground loop currents can be several milliamps, creating microvolt-level noise that limits measurement accuracy.