Earth Loops and Shield Termination in Instrumentation

Add a second earthed instrument to a measurement chain and a clean signal can acquire mains-frequency hum or a slow wander. The cause is almost always an earth loop.

How the loop forms

Two points on an earthing system are rarely at exactly the same potential — currents flowing in the earth network create small differences. Connect a screen or a signal return to earth at both ends and you have made a closed loop between those two points. Current flows around it, and because it shares a conductor with your signal return, it appears as noise on the measurement.

Practical measures

• Earth the screen at one end only for low-frequency analogue signals — conventionally at the receiving or controlling end. This drains interference without closing a loop.
• Keep signal returns separate from power returns. Sharing a return conductor puts the supply’s current through your measurement reference.
• Use a single star point where the system permits, rather than earthing at each piece of equipment.
• Break the loop with isolation where you cannot avoid two earths — an isolated DC-DC converter, an isolating amplifier or an opto-isolated interface removes the galvanic path entirely.

A caution

Protective earthing is a safety function and must never be removed to cure noise. Lifting an earth to quieten a signal is not an acceptable fix. Isolate the signal path instead, and leave the safety earth alone.

Note also that at radio frequencies the single-point rule can work against you, since a long unearthed screen behaves as an aerial. Analogue instrumentation and high-frequency work genuinely pull in different directions here.

Isolated DC-DC converters are often the cleanest way to break a loop — ask us for a suitable part.