Impedance of HDMI differential pair tracks

Hello,

HDMI specifies for the differential TMDS signal pairs a differential impedance of 100 ohms and a single ended impedance of 50 ohms.

What is not clear to me: when each of the tracks of a differential pair has single-ended 50 ohms on its own, and I put them close together (like e.g. indicated in the HDMI layout guidelines), then obviously the differential impedance is below 100 ohms always. You can verify this in impedance calculators like  https://www.everythingrf.com/rf-calculators/differential-microstrip-impedance-calculator  
How can I reach the HDMI specification? Or what is my misunderstanding?

Thanks in advance and regards

Tobias

  • Hello Tobias,

    The differential impedance of the diff-pairs has to be 100Ohm.

    When the two traces of the diff-pair are close enough as usual, they have some coupling. In this case, the single ended impedance is automatically higher than 50Ohm. Nevertheless your design goal should be 100Ohm differential impedance.

    Only two not coupled lines have a differential impedance of 100Ohm and a single ended impedance of 50Ohm.

    Helmut