EMC Question of the Week: June 10, 2024

a differential trace pair above a ground plane with equations for the common-mode and differential mode impedances

For a differential trace pair, terminating each trace in the odd-mode impedance provides a matched termination for both the odd-mode and even-mode propagation provided the coupling between the traces is  

  1. strong
  2. weak
  3. ideal
  4. intrinsic


The best answer is “b.” If the traces are weakly coupled (ZM is large relative to 2Z1), the differential-mode impedance is the sum of the two trace impedances. In this case, matching each trace independently provides a good match for both differential-mode and common-mode propagation. The value of the matching impedance is Z1, the odd-mode impedance. 

On the other hand, if the traces are strongly coupled, the differential-mode impedance is not simply the sum of the two trace characteristic impedances. It is a function of the mutual impedance between the traces as well. In this case, terminating each trace with the odd-mode impedance still matches the differential-mode (odd-mode) propagation. However, those two impedances in parallel are no longer the correct match for the common-mode (even-mode) propagation.  

Have a comment or question regarding this solution? We'd like to hear from you. Email us at This email address is being protected from spambots. You need JavaScript enabled to view it..