Troubleshooting

Noisy or Offset Analog Signal: Ground Loops and How to Break Them

The flow reading wanders by 3% and jumps when the pump starts. If the loop is fine on the bench, it is probably a ground loop — here is how to prove it.

· 8 min read · By TryPLC Team

  • Analog Input
  • Noise
  • Grounding
  • Shielding

The symptom

An analog value (flow, level, pressure) is unstable or carries an offset in the plant, but reads perfectly when the same transmitter is tested on the bench. It gets worse when a pump or VFD starts.

What a ground loop is

If the signal circuit is connected to earth at two places that are at slightly different potentials — say, the transmitter body earthed at the pipe and the PLC's 0 V earthed in the cabinet — a current flows through the signal wiring between them. That current adds to, or subtracts from, the signal.

Step 1 — Prove it

  • Measure the loop current with a series mA meter while the reading is disturbed. If the current itself changes with the pump, the error is real current in the loop.
  • Measure AC and DC volts between the transmitter's negative terminal and local earth, and between the PLC card's 0 V and cabinet earth. A difference of more than a volt or two between earth points indicates potential difference.
  • Temporarily lift the earth connection on one end of the signal circuit (not a protective earth of a mains device). If the reading stabilises, you found the loop.

    • Step 2 — Shield rules

  • Use twisted-pair shielded cable for analog signals.
  • In most process plants, ground the shield at one end — the cabinet — to avoid shield currents. Where high-frequency VFD noise is the issue, 360° shield clamps at both ends with a good equipotential bonding conductor are often specified instead. Follow the plant standard and the device manual.
  • Never use the shield as a signal conductor.

    • Step 3 — Break the loop with isolation

  • Galvanically isolated input cards or channel-to-channel isolated modules remove the second earth path.
  • Signal isolators (4–20 mA in, 4–20 mA out) between field and card. Check the isolator's voltage drop in the loop budget.
  • Check the card's common-mode voltage limit if inputs are not isolated: exceeding it gives wrong readings on all channels.

    • Step 4 — Separation and filtering

    Run analog cables in separate trays from motor cables. Ensure VFD motor cables are shielded and their shields properly terminated. Then, and only then, apply filtering: the card's hardware filter, or a software moving average for slow processes. Filtering a ground-loop error only hides it.

    Step 5 — Verify

    Log the value before and after the fix while the disturbing load cycles. A stable trace with the pump running is the proof. Related: 4–20 mA loop troubleshooting.

    Frequently asked questions

    What causes a ground loop in an analog signal?

    The signal circuit is earthed at two points with different potentials, so a current flows through the signal wiring and adds error.

    Should a shield be grounded at one end or both?

    For analog process signals, one end (the cabinet) is common. For high-frequency VFD noise, both ends with a good bonding conductor is often specified. Follow the plant standard and device manual.