Capacitive coupling

Definition

The level of disturbance depends on the voltage variations (dv/dt) and the value of the coupling capacitance between the disturber and the victim.
Capacitive coupling increases with:

• The frequency
• The proximity of the disturber to the victim and the length of the parallel cables
• The height of the cables with respect to a ground referencing plane
• The input impedance of the victim circuit (circuits with a high input impedance are more vulnerable)
• The insulation of the victim cable (εr of the cable insulation), particularly for tightly coupled pairs

Figure R25 shows the results of capacitive coupling (cross-talk) between two cables.

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Fig. R25: Typical result of capacitive coupling (capacitive cross-talk)

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Examples

(see Fig. R26)

• Nearby cables subjected to rapid voltage variations (dv/dt)
• Start-up of fluorescent lamps
• High-voltage switch-mode power supplies (photocopy machines, etc.)
• Coupling capacitance between the primary and secondary windings of transformers
• Cross-talk between cables

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Fig. R26: Example of capacitive coupling

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Counter-measures

(see Fig. R27)

• Limit the length of parallel runs of disturbers and victims to the strict minimum
• Increase the distance between the disturber and the victim
• For two-wire connections, run the two wires as close together as possible
• Position a PEC bonded at both ends and between the disturber and the victim
• Use two or four-wire cables rather than individual conductors
• Use symmetrical transmission systems on correctly implemented, symmetrical wiring systems
• Shield the disturbing cables, the victim cables or both (the shielding must be bonded)
• Reduce the dv/dt of the disturber by increasing the signal rise time where possible

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Fig. R27: Cable shielding with perforations reduces capacitive coupling

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