Short answer
Watch the value while changing one condition at a time: stop the nearby drive, isolate the actuator, or temporarily compare the signal at the transmitter and input. The change that makes the noise disappear is more useful than a guess.
What to look for
Check the cable route and the shield termination against the installation guidance. Keep signal and power conductors separated and look for a reference that is shared through an unexpected path.
If the measured signal is clean at the transmitter but noisy at the PLC input, focus on the cable and input side. If it is noisy at the source, focus on supply quality, sensor configuration, and the physical measurement.
When the symptom appears in the plant
A paper mill upgraded a large fan motor to a variable frequency drive (VFD). Immediately, a nearby 4-20mA duct pressure transmitter began reading erratically, causing the control dampers to hunt. The instrument cable ran parallel to the unshielded VFD motor leads for 30 feet in a shared cable tray. The high-frequency switching (PWM) of the VFD induced noise into the analog loop. Relocating the instrument cable to a dedicated, separate conduit resolved the noise.
Five checks before replacement
- Identify the noise source by turning off suspect equipment (like VFDs, large contactors, or welders) one by one and observing the signal.
- Verify proper physical separation between low-voltage analog cables and high-voltage power or motor cables.
- Confirm the analog signal cable shield is grounded at one end only (typically the panel) and continuous throughout the run.
- Check if twisted-pair cable is used; the twisting helps cancel out electromagnetically induced noise.
- Consider installing a signal isolator or conditioner to break ground loops and filter out high-frequency noise.
VFD cables are notorious noise generators; always use shielded VFD motor cable and keep signal wires far away.
Adding a small filter time constant in the PLC logic can smooth out minor noise, but it shouldn't replace fixing the physical cause.
Never run 24V or analog signals in the same conduit as 480V power.
Bench checklist
- Confirm the supply at the device terminals.
- Confirm the meter is in the correct mode and position.
- Confirm the input range matches the signal type.
- Confirm the reference/common is intentional.
- Record the reading before changing the wiring.
Reference families
- IEEE 518
- Drive Manufacturer Installation Guidelines
Always verify the current edition and the exact device manual for the installation.
Official references
These links point to standards bodies, industry organizations, or manufacturer documentation. Standards landing pages may require purchase or institutional access.
- Electromagnetic compatibility IEC
Official EMC overview covering EMI, shielding, filtering, emissions, and immunity.
- Conditioning and protection of analog process signals Phoenix Contact
Technical guidance on interference, frequency-converter noise, shielding, filtering, and isolation.
- Analog-Signal Data Acquisition in Industrial Automation Analog Devices
Application article on 4–20 mA noise tolerance, differential inputs, and EMI.
- What Is a 4-20 mA Current Loop? Fluke
Explains why current signals preserve integrity over long industrial cable runs.