What Is a Ground Loop?

A ground loop is an unwanted current circulating in the ground line when the grounds of two or more devices are connected to each other through more than one path. This circulating current creates small voltage differences between the devices' "zero" (GND/reference) points and couples into the signal as noise (especially "hum" in audio systems).

Why Does It Happen?

Real cables aren't zero-resistance; when two devices are grounded through different outlets/paths there's a small potential difference between the ground points. When the devices are also connected by a signal cable, the ground forms a "loop" (a closed ring) and current flows through it (mains 50/60 Hz or nearby magnetic fields drive this current). The voltage this current creates rides on top of the sensitive signal.

How Is It Prevented?

• Single-point grounding: Joining the grounds at one common point to break the loop (star grounding).
• Galvanic isolation: Electrically separating the two sides' grounds with an optocoupler, isolation transformer, isolated DC-DC converter or isolation amplifier.
• Differential signaling: Differential transmission like RS-485 or balanced audio (XLR) largely rejects common-mode ground noise.
• Cable layout: Separating signal and power return paths and minimizing the loop area.

Where Does It Cause Problems?

It's most common in audio/video systems (50/60 Hz hum in a speaker), measurement/instrumentation circuits and data lines between differently-powered devices. In industrial communication (RS-485, CAN) isolation and differential signaling are standard for this reason.

Things to Watch Out For

• Don't cut the safety ground: Removing the protective (mains) earth to "break the ground loop" is DANGEROUS; solve the problem on the signal side with isolation.
• Isolation in the right place: Put the isolation on the interface the noise crosses (the signal/data line), not on all of the power.
• Single point: Joining grounds at multiple points creates a loop; deliberately prefer a single point.

Frequently Asked Questions

Why does a ground loop cause noise?
Current flowing through the ground loop creates a small voltage difference between the devices' reference (GND) points; this difference couples into the sensitive signal (e.g. as "hum" in audio).

How is it fixed?
With single-point grounding, galvanic isolation (optocoupler/isolation transformer/isolated DC-DC) and differential signaling. Never remove the safety (protective) earth.

For isolation, look at optocouplers and isolation solutions.