Introduction
In industrial and electronic applications, voltage and current signals are transmitted through conductors in two distinct forms: common mode and differential mode. Various electrical devices, such as power lines, communication cables, and instrument connections, use at least two conductors to transmit signals. Additionally, a third conductor, known as the ground wire, is often present.
Interference in analog signals generally falls into two categories:
Differential mode interference: When the two conductors serve as the outgoing and return paths for the signal.
Common mode interference: When both conductors carry the signal in one direction and the ground serves as the return path.
The accuracy and reliability of analog measurement instruments are critical in industrial environments. However, due to external and internal interference sources, unwanted signals often mix with the measured signal, leading to errors or even malfunctions. This article explores the causes of interference and provides practical measures to mitigate them.
Sources of Interference
Interference can originate from both internal and external sources. External sources include high-power electrical devices, transformers, and power lines, while internal sources consist of power transformers, coils, relays, switches, and wiring within the instrument itself. These interference signals enter the instrument through multiple pathways:
1. Magnetic Coupling Interference
When signal transmission cables pass through a high-power transformer, AC motor, or power line, they are exposed to strong alternating magnetic fields. These fields induce unwanted voltages in the measurement circuit, especially when the sensor is located far from the display instrument. This is a major cause of differential mode interference.
2. Capacitive Coupling Interference (Electrostatic Interference)
Interference can occur due to electrostatic coupling, where electric fields induce unwanted voltages between conductors. These common-mode interferences do not directly superimpose on the signal but generate leakage currents that flow through the measurement circuit, ultimately affecting instrument readings. This type of interference is commonly associated with power lines, transformers, and high-voltage equipment.
3. Ground Loop Interference
In industrial environments, ground potential differences exist at various points. When multiple grounding points are introduced unintentionally, voltage differences appear between the ground points, leading to common mode interference. This is especially prevalent in areas near high-power equipment where grounding resistances fluctuate.
4. High-Temperature Measurement Interference
In high-temperature environments, such as furnaces, thermocouples may experience interference due to conductive leakage currents passing through insulating materials. Over time, materials like ceramic insulators degrade, allowing unwanted currents to reach the measurement circuit and introduce noise.
5. Electrical Noise from Power Supplies
Power supplies themselves can introduce interference into analog circuits, especially in switching power supplies that generate high-frequency noise. Additionally, frequency converters, rectifiers, and other nonlinear loads can create harmonic distortion, affecting signal integrity.
Types of Interference
1. Differential Mode Interference (Series Mode Interference)
This interference occurs between the two conductors carrying the signal. It typically originates from power supply fluctuations, electromagnetic fields, and crosstalk between adjacent circuits.
How to Measure: A digital multimeter (AC mV mode) can measure differential mode interference by connecting its probes across the two signal lines. Typical values range from a few millivolts to tens of millivolts.
2. Common Mode Interference (Ground Loop Interference)
Common mode interference appears between each signal conductor and ground. It is often caused by ground loops, capacitive coupling, and electromagnetic radiation.
How to Measure: A high-impedance voltmeter or a digital multimeter in AC mode can be used to measure common mode voltage. Probes should be placed between each signal line and ground. Common mode interference typically ranges from a few volts to tens of volts.
Methods to Reduce Interference
To ensure accurate measurements and minimize noise, different techniques can be applied based on the type of interference.
Reducing Differential Mode Interference
Proper Cable Routing: Signal cables should be kept away from power lines, transformers, and high-frequency equipment.
Use Shielded Cables: Enclosed signal wires within shielded cables and connect the shield to a single ground point.
Filtering Circuits: Use low-pass filters at the instrument input to remove high-frequency noise.
Digital Filtering: Intelligent measurement systems can use software-based digital filters to eliminate transient noise.
Reducing Common Mode Interference
Floating Measurement Circuits: For sensitive applications like thermocouple measurement, keeping the circuit electrically isolated from ground can reduce noise.
Use of Differential Amplifiers: Differential amplifiers help reject common mode voltage while retaining the desired signal.
Reliable Grounding: Ensure that the instrument’s protective ground and signal ground are properly separated.
Isolation Transformers: If power line interference is a concern, isolation transformers can prevent unwanted noise coupling.
Installing Bypass Capacitors: Placing small capacitors (e.g., 0.1 µF) between signal lines and ground can help divert high-frequency noise.
Conclusion
Interference in analog signal transmission is a persistent challenge in industrial environments. By understanding the sources and pathways of interference, engineers can implement targeted solutions such as proper cable management, shielding, filtering, and grounding techniques. Applying these methods will significantly improve measurement accuracy, ensuring reliable operation of industrial control systems and instrumentation.