Common-mode interference

Under common-mode disturbances are understood in electrical interference voltages and currents on the connecting lines between electrical components or electrical devices which with the same phase angle and direction of current spread (see also a common mode signal ) on both the forward line and the return line between these components.

The analysis and prevention of these disorders take place in the framework of electromagnetic compatibility.

Causes

Electrical components such as a telephone, and a switching system are connected to each other with a reciprocating and a return line. This line pair is part of the circuit for transmission. Common mode disturbances acting on these two lines to the same parts, which interference currents to flow therein in contrast to the Nutzströmen same direction, interference voltages in the two lines have the same amplitude and phase position. Therefore to cause an actual disturbance of the useful signal, a further current path is necessary: This is given mostly by the common reference potential (ground or earth connection ) of the useful signal and noise source as well as from stray capacitance.

The coupling of the common mode noise can be caused among others by capacitive coupling or due to potential differences along the transmission path. In the capacitive coupling radio-frequency signals or pulses arrive simultaneously on both conductors. Potential differences arise, for example by a voltage drop, which can be caused by high currents in ground connections or ground connections.

Ground loops, large-scale electrical or magnetic interference or lightning strikes in the vicinity are typical causes of common mode noise.

Avoidance

Remedy against common-mode disturbances are signaling and transmission paths with high common mode rejection:

• The transmission of differential signals on a pair of lines rather than a reference potential to use (mass / earth).
• Galvanic separation at a location in or between the devices by Transformers
• With optical transmission ( optical coupler, fiber optic cable )

• When compared to the useful signal substantially lower noise frequencies by capacitive coupling (example: antenna cable )
• In the useful signal similar or opposite the desired signal higher frequencies by common mode chokes or chokes. Common mode noise of very high frequencies can be reduced with ferrite sleeves that are inserted or why folded onto the cable (feed and return) ( split ferrites ).

This common-mode interference can not continue to spread.

Examples of the use of differential signals to minimize common mode noise in the data transfers are electrical interfaces such as LVDS, 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, inter alia,

Common mode noise in DC circuits can be suppressed with capacitors to ground / earth. In AC power, this is restricted to keep the leakage current low.

Since common-mode noise can accept such high amplitudes and energies in the electricity network and the telephone network (eg by lightning ) that even destructions occur in devices and systems, is given to a part of the lightning protection system connected to ground and surge arresters.