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 R33 shows the results of capacitive coupling (cross-talk) between two cables.

Fig. R33 – Typical result of capacitive coupling (capacitive cross-talk)

Examples

(see Fig. R34)

• 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

Fig. R34 – Example of capacitive coupling

Counter-measures

(see Fig. R35)

• 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

Fig. R35 – Cable shielding with perforations reduces capacitive coupling