Capacitive coupling: Difference between revisions
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== 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.<br>Capacitive coupling increases with: | The level of disturbance depends on the voltage variations (dv/dt) and the value of the coupling capacitance between the disturber and the victim.<br>Capacitive coupling increases with: | ||
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== Examples == | |||
(see '''Fig. R26''') | (see '''Fig. R26''') | ||
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== Counter-measures == | |||
(see '''Fig. R27''') | (see '''Fig. R27''') | ||
Revision as of 08:47, 17 June 2011
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.
Fig. R25: Typical result of capacitive coupling (capacitive cross-talk)
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
Fig. R26: Example of capacitive coupling
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
Fig. R27: Cable shielding with perforations reduces capacitive coupling
