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Differential vs Single Ended Impedance

Beko , 04-27-2020, 03:27 AM
Hi All,

I have a simple beginners question and you help would be appreciated.

When designing for controlled impedance, I read in a lot of places that the diff pair impedance must be 100 ohms (for HDMI) and single ended should be 50 ohms. Question is are these the same thing or do we have to satisfy both requirement? In other words, if I simply satisfy the 100 ohms diff pair impedance for an HDMI data pair, does it mean I automatically satisfy the 50 ohm rule?

When I use the impedance profile calculator in Altium and set S50 and D100 profiles, if I choose the same track width for both, only one can be satisfied.

Thank you in advance for your help.


robertferanec , 04-28-2020, 03:29 AM
Yes, very often if you have differencial pair, it still has requirements SE50 (Single ended impedance 50 OHM) for the single line - but together with the other signal they need to create for example DIFF100. Some peripherals may require lower single ended impedance.

Simply to say, the single ended impedance will "specify" the width of the track in differential pair.

PS: Do not forget, in many designs differential pairs are simply two single pins which are just running the same but opposite signal.

Here you can see some examples (page184):

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