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Protecting Signals from EMI: Introduction and Capacitive Coupling



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Industrial electrical equipment, ordinary elecronic devices, power and signal lines all produce ample amounts of EMI (or "noise"). This noise can reduce the fidelity of important measurement and control signals. Using appropriate grounding and shielding techniques can help minimize or completely eliminate noise, thus assuring signal integrity.

Noise problems are said to exist if there is a:

  • source creating noise
  • receiver affected by the noise
  • coupling channel between the source and receiver

The goal of EMC is to reduce, divert or eliminate one of these three elements.

Corruption of Signals due to Capacitive Coupling

In equipment or wiring, an electric charge (aka potential or voltage) can develop. If the charge changes with time (dVn/dt), a changing electric field is created which can capacitively couple to other electrical equipment or wiring. Noise due to capacitive coupling is shown in the diagram below. This noise, modeled here as a current, becomes pronounced when a circuit or termination has a high impedance. Reason: noise voltage, Vn, created at the receiver is noise current, in, multiplied by the receiver impedance, Zin.

Noise due to capacitive coupling may be reduced by using shielded cables. The shield is a Gaussian or equi-potential surface on which can electric fields may discharge and retrun to ground without affecting the cables internal conductors.

Capacitive coupling favors transfer of the higher-frequency components of a signal, whereas inductive coupling favors lower frequency components, and conductive coupling favors neither higher nor lower frequency components.

Capacitive coupling equivalent circuit
above: Capacitive coupling equivalent (modeling) circuit

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NEXT: Corruption of Signals due to Inductive Coupling


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Updated: Friday, 2007-11-16 17:43 PST