Common Mode Choke Impedance

Formula

Description

A common mode choke must present enough impedance to suppress common mode noise current below the required limit. This simple relationship helps determine the minimum choke impedance needed at the frequency of concern. Common mode noise appears as identical signals on both conductors of a pair, typically caused by parasitic capacitance coupling to the chassis or ground plane. The choke presents high impedance to common mode signals while passing differential signals with minimal impedance, making it ideal for power line and data cable filtering.

Variables

• Z — Required choke impedance at the noise frequency (Ω)
• V — Common mode noise voltage (V)
• I — Maximum allowable noise current (A)

Practical Notes

Common mode choke impedance is specified at specific frequencies in datasheets. Select a choke whose impedance curve covers your noise frequency range. Typical values range from 100Ω to 10kΩ depending on the application. For USB, Ethernet, and HDMI interfaces, choose chokes with high impedance at the noise frequencies but low insertion loss in the signal passband.