Analog Signal Noise and Grounding Issues

The Impact of Analog Signal Noise

Noisy analog signals cause erratic process variable readings, false alarms, unstable control loops, and inaccurate historical data. In industrial environments, electromagnetic interference (EMI) from variable frequency drives, motor starters, welding equipment, and radio transmitters couples into instrument wiring and corrupts the milliamp or voltage signals carrying process measurements. A 1% noise spike on a pressure transmitter reading can cause a PID loop to oscillate or trigger a nuisance high-pressure alarm that shuts down production.

The key to eliminating noise is understanding how it enters the signal path and applying the correct mitigation technique. The three primary coupling mechanisms are conducted noise (through shared power supplies or ground paths), capacitive coupling (electric field from adjacent power wiring), and inductive coupling (magnetic field from nearby motors or transformers).

Identifying Noise Sources

Before applying fixes, identify the noise source and coupling mechanism:

• 60 Hz hum: Steady oscillation at 60 Hz (or 50 Hz) indicates power line coupling. The source is typically adjacent power wiring running parallel to signal cables, or a ground loop between the instrument and the PLC.
• High-frequency spikes: Sharp, irregular spikes correspond to VFD switching (typically 2-16 kHz carrier frequency), motor contactor operation, or welding arcs. These spikes often correlate with specific equipment operation.
• Intermittent noise: Noise that comes and goes with no obvious pattern may be caused by loose connections, intermittent shield grounding, or rotating equipment vibration that periodically moves cables near noise sources.
• Broadband noise: Elevated noise across all frequencies suggests a grounding problem, such as multiple ground points creating a ground loop or a deteriorated grounding electrode connection.

Diagnostic Tools

• Oscilloscope: The definitive tool for noise diagnosis. Connect across the analog input terminals and observe the waveform. A clean 4-20mA signal should be a flat DC level; any AC component is noise.
• Signal simulator: Disconnect the field instrument and connect a precision milliamp source. If the noise disappears, the problem is in the field wiring or instrument. If it persists, the problem is at the PLC input or grounding.
• Clamp-on milliamp meter: Measure loop current without breaking the circuit. The Fluke 771/772 can detect AC noise riding on the DC signal.

Proper Grounding Techniques

Correct grounding eliminates the majority of analog signal noise problems:

• Single-point grounding: Ground the instrument cable shield at one end only — typically at the PLC cabinet. Grounding both ends creates a ground loop that acts as an antenna for 60 Hz noise.
• Instrument ground bus: Install a dedicated instrument ground bus bar in the PLC cabinet, connected to the building ground electrode with a dedicated conductor. Do not share this ground with motor or power circuits.
• Ground electrode testing: Measure ground electrode resistance annually with a ground resistance tester (Fluke 1625 or equivalent). Resistance should be below 5 ohms for instrument grounding. Deteriorated ground rods cause floating ground potential and increased noise.
• Avoid ground potential differences: When instruments and PLCs are in different buildings or structures, ground potential differences of several volts are common. Use signal isolators to break the galvanic connection between grounds.

Cable Shielding Best Practices

Proper shielding provides a barrier against capacitive and inductive coupling:

• Use shielded twisted pair (STP): Always use individually shielded twisted pair cable for analog signals. The twist reduces inductive coupling; the shield reduces capacitive coupling.
• Drain wire connection: Connect the shield drain wire to the instrument ground bus at the PLC end. Leave the field end disconnected and taped back to prevent accidental contact.
• Shield continuity: Verify shield continuity from end to end. Breaks in the shield from junction boxes or cable damage create gaps in the protection.
• Separation from power cables: Maintain at least 12 inches (300 mm) separation between analog signal cables and power cables rated above 120V. Cross power cables at 90-degree angles when separation is not possible.

Filtering and Isolation Solutions

When grounding and shielding improvements are not sufficient, additional filtering or isolation is required:

• PLC input filtering: Most modern analog input modules have configurable digital filters. Increase the filter time constant (e.g., from 10 ms to 100 ms or 500 ms) to average out high-frequency noise. Note that increased filtering adds latency to the measurement.
• Signal isolators: Loop-powered or externally powered isolators break ground loops galvanically and provide filtered output. These are the most effective solution for persistent noise problems that cannot be resolved with grounding or shielding.
• Ferrite chokes: Snap-on ferrite chokes on signal cables near the PLC entry point attenuate high-frequency noise from VFDs and radio transmitters. They are inexpensive and easy to install.
• RC filters: A simple resistor-capacitor filter across the analog input terminals attenuates high-frequency noise. Typical values: 100-ohm resistor and 0.1 µF capacitor for a 16 kHz cutoff.

VFD-Specific Noise Mitigation

Variable frequency drives are the most common source of high-frequency noise in modern industrial facilities:

• Use VFD output reactors or dV/dt filters to reduce high-frequency emissions at the source
• Route VFD motor cables in separate conduit from all signal and control wiring
• Use shielded VFD motor cable with the shield grounded at both ends (this is opposite of signal cable practice)
• Install VFD line reactors or RFI/EMI filters on the VFD input to reduce conducted noise back into the power system
• Verify that VFD carrier frequency is not set higher than necessary — lower carrier frequencies produce less radiated EMI