Signal isolation and ground loops
Why analog signals between different systems need galvanic isolation, how ground loops and common-mode voltage corrupt measurements, and when to use isolating amplifiers, isolated inputs or barriers.
The problem
A 4–20 mA loop or a 0–10 V signal connects two devices. If both reference the signal to their own ground and the grounds are at different potentials, a current flows through the signal conductors — a ground loop. Result: offset, noise at mains frequency, or a reading that changes when another machine starts.
Voltage between the signal reference and the module's reference is common-mode voltage; non-isolated inputs tolerate only a few volts.
Galvanic isolation
Isolation interrupts the conductive path while passing the signal (transformer, optocoupler, capacitive coupling):
| Solution | Where |
|---|---|
| Isolated analog input modules (channel-to-channel and channel-to-bus) | Signals from different sources into one PLC |
| Signal isolators / isolating amplifiers (DIN-rail) | Between systems: DCS ↔ PLC, drive reference, long cables between buildings |
| Loop-powered isolators | 4–20 mA without extra supply |
| Intrinsic safety barriers / isolators | Signals from hazardous areas |
Isolators often also convert ranges (0–10 V → 4–20 mA) and split one signal to two receivers.
Practical rules
- Treat every analog signal that crosses cabinet, building or supplier boundaries as a candidate for isolation.
- Drives: the analog reference input and the PLC output are usually referenced to different grounds — isolate or use a fieldbus.
- Do not connect the 0 V of a 4–20 mA loop to PE at several points.
- Check the isolation voltage rating (e.g. 1.5 kV, 3 kV) against the installation.
Connections
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Sources
Updated 2026-10-02