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The last pop quiz posted on our web site, was a real test of how well you understand differential impedance and odd mode impedance. If you watched our last free webinar, the answer to this pop quiz would have been obvious.

By definition, the odd mode impedance of a line that is part of a differential pair is always ½ x the differential impedance. If the differential impedance is 100 Ohms, the odd mode impedance of either line is 50 Ohms.

In this question, the single-ended impedance is designed as 50 Ohms. How do you have a single-ended impedance and odd mode impedance of 50 Ohms?  It can only be if there is no coupling between the two lines that make up the pair.

As the coupling increases, the single-ended impedance won’t change, but the odd mode impedance will decrease. The only correct answer to this question is, uncoupled.

It is interesting that only 42% of you got it correct! This means 58% got it wrong. This way of specifying differential impedance: a single-ended impedance and a differential impedance, is a common way of specifying line impedance. Now you know, it’s really specifying uncoupled lines, with a spacing about 3x the line width.

To really understand differential impedance, check out the last webinar I did on “Stack up Design for Differential Pairs”. It’s free!.