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Ground loop and its elimination

Short answer

A ground loop is a closed conductive path formed when multiple ground points have different potentials, injecting noise voltage in series with the signal. Eliminate it by single-point grounding, using isolation (transformers, optocouplers), differential signaling, grounding shields at one end, and reducing loop area.

What is a ground loop? How can we eliminate them?

Ground loop: a closed loop formed when two or more points of a system that should be at the same potential are connected to ground by multiple paths. Because the ground conductors have impedance, the ground currents (and magnetic fields cutting the loop) create a voltage difference between the grounds. This voltage appears in series with the signal and causes hum and noise.

 Equip A --signal--> Equip B
   |                   |
  GND_A ------------ GND_B   <- loop; Vnoise between grounds

Elimination

  • Single-point grounding: connect each circuit to ground at only one point.
  • Isolation: use isolation transformers or opto-couplers to break the conductive path, or differential amplifiers to reject the ground-voltage difference.
  • Reduce loop area / twisted pair, and use balanced differential lines.
  • Ground the cable shield at one end only (at low frequency).
  • Use common-mode chokes or a ground-lift (for the signal, not for safety earth).
  • Keep all equipment on the same power outlet or distribution branch.

✓ Answer checked by Claude Sonnet 5.5

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