Filtering for conductive noise coupling

Conductive noise travels along wires (power, signal or ground). A filter passes the wanted signal and blocks the noise. Filters are classified in three ways.

Based on frequency

• Low-pass: passes low frequencies, blocks high-frequency noise. Used on power lines and sensor inputs (RC, LC, or ferrite bead plus capacitor).
• High-pass: blocks DC/low-frequency drift and hum, passes the signal (series capacitor, coupling).
• Band-pass: passes only a band around the signal frequency.
• Band-reject (notch): removes a single frequency such as 50 Hz mains hum.

 Low-pass LC:   in --L--+-- out
                        C
                        |
                       GND

Based on mode

Noise appears in two modes:

• Differential (normal) mode: noise voltage between the two conductors (signal and its return), in opposite directions. Reduced by series inductors and a capacitor across the lines (X capacitor).
• Common mode: noise voltage that appears equally on both conductors with respect to ground, in the same direction. Reduced by a common-mode choke (both lines wound on one core) and capacitors from each line to ground (Y capacitors).

The two modes need different filters, so line filters normally contain both: X capacitor, common-mode choke and Y capacitors.

Based on amplitude

These limit the noise by its size, not frequency:

• Clamping/clipping circuits using zener, TVS or ordinary diodes limit spikes above a set level.
• Varistors (MOV) and gas discharge tubes absorb large surges.
• Amplitude (slicing) filters/Schmitt triggers pass only signals above a threshold, rejecting small noise.

In practice a combination (an LC low-pass for frequency, common-mode choke for mode and a TVS for amplitude) is used at the entry of the system.