Common Mode Chokes: Attenuation, Impedance, and Selection

What is Common Mode Attenuation?

This refers to the value that attenuates (varies the amplitude of) common mode signals that are mostly noise components.

How to Measure Impedance of a Common Mode Filter/Choke

  • Common mode impedance is measured by connecting your analyzer to the filter/choke coil as shown in Fig. 1.


Fig. 1: Connection Diagram for Measuring Common Mode Impedance

  • Differential mode impedance is measured by connecting the filter/choke coil as shown in Fig. 2.


Fig. 2: Connection Diagram for Measuring Differential Mode Impedance

How to Select a Common Mode Filter/Choke
Select parts that have a cutoff frequency three to five times greater than the transmission signal frequency and exhibit high impedance at the frequency at which noise is desired to be suppressed.
In addition, please remember that rated currents differ by product in DC lines. Be sure to check the rated current.

Click here for Digi-Key’s full selection of common mode filters/chokes.

Content and images provided courtesy of TDK’s FAQs:

2 Likes