Ways to Reduce Capacitve Coupling in instrumentation signals ~ Learning Instrumentation And Control Engineering Learning Instrumentation And Control Engineering

Ways to Reduce Capacitve Coupling in instrumentation signals

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If sets of wires lie too close to one another, electrical signals between the wires tend to couple or interfere with one another thereby introducing noise into the analog signal circuitry and corrupting the signals in the process. This can be especially detrimental when the coupling or interference occurs between AC power conductors and low-level instrument signal wiring such as thermocouples or pH sensor cables.

Capacitance is a property intrinsic to any pair of conductors separated by a dielectric (an insulating substance), whereby energy is stored in the electric field formed by voltage between the wires.
The natural capacitance existing between mutually insulated wires forms a conductive path for AC signals to cross between those wires, thereby introducing noise into the signal cables and circuitry. The strength of this conductive path is inversely proportional to the capacitive reactance (XC = 1/2πfC). The noise introduced in the analog signal circuitry will be proportional to both the voltage and the frequency of the AC power.
There are four ways of reducing the noise induced by capacitive coupling. These are:
  • Shielding of the instrument signal wires with low resistance material
  • Separating from the source of the noise - (This is usually done by ensuring that power cables and instrument signal wires don’t pass through the same conduit or cable tray)
  • Reducing the amplitude of the noise voltage (and possibly the frequency)
  • Twisting of the instrument signal wires

Consider the case when a shield is installed around the instrument signal wires. The noise voltage generated currents prefer to flow down the lower impedance path of the shield rather than the instrument signal wires. If one of the instrument signal wires and the shield are tied to earth at one point, then no signal current flows between the instrument signal wires and the shield as shown below:
Note that in the diagram below, noise voltage shown as red arrows goes through the low impedance shield of the instrument cable to ground.


In practice, most instrument signal cables in instrumentation systems are usually shielded (with a material of low resistance) and twisted.