Common mode interference

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In electrical engineering, common-mode interference is understood to mean interference voltages and currents on the connecting lines between electrical components or electrical components, which propagate with the same phase position and current direction both on the outgoing line and the return line between these components (see also common-mode signal ).

The analysis and avoidance of these interferences are carried out within the framework of electromagnetic compatibility .

causes

Electrical components, such as a telephone and a switching system, are connected to one another with a forward and a return line. This pair of wires is part of the transmission circuit .
Common-mode interference affects these two lines in equal parts, the interference currents flow in the same direction in contrast to the useful currents, interference voltages have the same amplitude and phase position in both lines. In order to actually disturb the useful signal, another current path is necessary: ​​This is usually provided by common reference potentials ( earth or ground connection ) of the useful signal and the source of interference, as well as stray capacitances.

The coupling in of common-mode interference can be caused, among other things, by capacitive coupling or due to potential differences along the transmission path. With capacitive coupling, high-frequency interference signals or pulses reach both conductors at the same time. Potential differences arise, for example, from a voltage drop that can be caused by high currents in ground connections or ground connections.

Ground loops , large-scale electrical or magnetic interference fields or nearby lightning strikes are typical causes of common-mode interference.

Avoidance

Signal and transmission paths with high common-mode rejection are a remedy against common- mode interference :

  • the transmission of differential signals via a pair of cables instead of using a reference potential (ground / earth).
  • a galvanic isolation at a position in or between the devices
  • the creation of a high series impedance that only affects the common-mode interference:
    • with significantly lower interference frequencies compared to the useful signal by means of capacitive coupling (example: antenna cable)
    • in the case of the useful signal, frequencies that are similar to or higher than the useful signal by means of common mode chokes or current-compensated chokes . Common-mode interference at very high frequencies can be reduced with ferrite sleeves that are plugged onto the cable (outgoing and return line) or folded over (hinged ferrite).

This means that common-mode interference cannot spread any further.

Examples of the use of differential signals to minimize common-mode interference in data transmission are electrical interfaces such as LVDS , the CAN bus and other serial digital bus systems, as well as professional analog audio cable connections ( XLR ) and corresponding differential amplifier inputs or coupling transformers for microphones and the like. a.

Common-mode interference in DC voltage circuits can be suppressed with capacitors to ground / earth. In the AC network , this is only possible to a limited extent in order to keep the leakage current low.

Since common-mode interference in the power network and also in the telephone network can have such high amplitudes and energies (for example from lightning strikes ) that devices and systems can even be destroyed , surge arresters connected to earth are also used as part of lightning protection .

See also