How to Control Noise in Instrumentation Circuits (Part 1)

4 Common Sources of Noise that Affect Instrumentation Circuits

It can be a real challenge to select the right thermocouple wire. You not only have to determine the type of thermocouple elements to use, but also the temperature range, calibration accuracy and insulation type  for your application. With thousands of different configurations, it’s still far less complicated than thermocouple and instrumentation cable circuits. Regardless, it is important to know how to control noise in instrumentation circuits.

When considering cables for applications, engineers have to make sure that the cables they specify make sense, not only in mechanical performance but also in signal or electrical properties performance.  The type of signal transmitted by the sensor is related to its sensitivity to noise: The lower the voltage level and the higher the impedance of a circuit, the greater the circuit's sensitivity to noise of all types.

I hope the degree of detail provided here helps both seasoned and fledgling engineers working with instrumentation circuits to select and specify the correct cable into their application.

To get us started, there are four types of major noise that we must understand and some commonly accepted solutions for each noise type.

 Common Mode 

• Problem: Different ground potential at each location in a process plant causes common mode noise to be a problem. Two different grounds in an instrument circuit mean a current will flow between them, causing noise to be added to the signal being transmitted.
  
  
• Solution: Using a receiver which has a high common mode rejection ratio will control this type of common mode interference.
  
  
• Problem: A second type of common mode interference will occur even when a high-quality receiver is being used and is a particular problem in thermocouple extension cable circuits. Most thermocouples used are the "grounded” type. That is, the couple is connected physically and electrically to the well in which it is installed.  When a thermocouple circuit shield (or any nearby metallic object, such as conduit, tray, building frame, etc.) is at different potential than the couple, charging currents flow in the extension wire, causing interference to be superimposed on the thermocouple signal.
  
  
• Solution: Grounding the shielded circuit at the couple and only at the couple will eliminate noise problems from common mode. Multipair cables used with thermocouples must be individually shielded, isolated pair shield type so that the shield circuit may be maintained at the individual couple ground potential all the way back to the control room.

• Problem:  Cross talk occurs with AC instrument signals, especially pulse-type signals, where more than one circuit is carried in the same cable. It is the tendency for a signal to be coupled from one pair to another within the cable, resulting in noise being superimposed on a circuit.
  
  
• Solution: Cross talk noise may be eliminated by the use of cables with individually shielded, isolated pair shields.  The pair shield protects against noise picked up from adjacent pairs, as well as reducing noise by the pair it surrounds.

Static

• Problem:  Static interference is caused by the electric field radiated by a voltage source being coupled capacitively into the instrument circuit.
  
  
• Solution: The best way of fighting static noise is to place the circuit inside a total coverage shield which isolated the pair of wires from the outside influence. The grounded shield intercepts static interference and carries it off to ground. The shield must be grounded in order to reduce static noise; an ungrounded shield will not reduce noise.

 Magnetic

• Problem: Magnetic noise is produced by currents flowing through conductors and pieces of electrical equipment such as motors, generators, etc. As the current flows through equipment, a magnetic field is radiated around the conductor. As this field passes through the space between the conductors in a circuit, a current is set up in the circuit to oppose the magnetic field (transformer action). This current causes a noise to be superimposed on the signal in the instrument circuit.
  
  
• Solution: The best way of compensating for this type of noise is to twist the wires in the instrument circuit. Twisting causes the noise to be cancelled in adjacent sections of the wire. This is the least expensive, most effective way of combatting magnetic noise.

Next, we’ll discuss the appropriate installation considerations for the different noise levels based on the environments in which the instrumentation circuits must perform.

See also:

How to Control Noise in Instrumentation Circuits (Part 2)
3 Basic Steps to Selecting the Right Thermocouple Wire for Your Application
Everything You Wanted to Know About Thermocouple Theory and Chemistry (but Were Afraid to Ask)