Unstable 4–20 mA Analog Values: Check VFD Noise, Shielding and Grounding First

If an analog value is noisy, adding a software filter first can hide the real cause. In a VFD panel, start with signal routing and grounding.

Start with the noise pattern

If the signal is stable when the VFD is stopped and noisy only while it runs, electromagnetic coupling becomes a stronger suspect. If the noise is always present at a similar amplitude, sensor resolution, transmitter noise and supply quality also deserve attention.

Separate signal wiring from power wiring

Long parallel runs beside motor leads, VFD output cables and contactor power wiring can increase coupled noise. When crossing is unavoidable, keep the shared distance short and cross close to perpendicular where practical.

Shielding includes the termination method

Using a shielded cable is only half the job. Follow the applicable installation guidance, avoid unintended ground loops and do not randomly mix signal reference, 24 V common and protective earth.

Tune filtering after the physical path is healthy

Only after wiring and power integrity are verified should you tune digital filters, moving averages and control intervals. Heavy filtering may make the display look stable while adding unacceptable process delay.

• Compare noise with VFD stopped versus running
• Inspect analog and power cable routes
• Verify shield termination and ground-loop risk
• Check 24 V supply and reference potential
• Tune filters and sampling last