Preventing ground loops; electromagnetic coupling

Preventing ground loops

A ground loop forms when a circuit is grounded at more than one point, so the ground-potential difference drives noise current. To prevent it:

• Use single-point (star) grounding at low frequency.
• Use isolation: isolation transformer, opto-coupler or differential amplifier to break the loop.
• Ground the cable shield at only one end (low frequency).
• Use balanced differential signalling and twisted pairs, to cancel the induced voltage.
• Reduce the loop area and keep all equipment on one power circuit.
• Use common-mode chokes or a ferrite on cables.

Electromagnetic (radiated) coupling

Electromagnetic coupling occurs when a source emits a radiating wave, and wires, traces or loops of the receiver act as antennas that pick up the energy. It occurs in the far field (distance greater than λ/2π), where electric and magnetic fields travel together with wave impedance of 377 ohm.

• Sources: radio/TV transmitters, mobile phones, switching power supplies, digital clocks, motors with sparking brushes, and lightning.
• Receiver: any conductor of length near λ/4 or longer; the induced voltage is proportional to the field strength and effective antenna length.
• Reduction: enclose the circuit in a grounded metal shield, filter all lines entering the enclosure, reduce lead and trace lengths, use shielded or twisted cables, ferrite beads, increase distance, slow down clock edges.