# Differential impedance matching

Below is a small portion of a schematic which I have seen:

It mentions that the transmission lines will have a differential impedance of 100 Ω between them. In case of single-ended it would be 50 Ω between signal and ground.

Can someone help me understand how a single-ended 50 Ω impedance would look like a 100 Ω differential impedance, with a diagram?

• not quite. The impedance between the two lines (in differential transmission) is 100 ohm. The impedance between each line and the reference (ground) is 50 ohm (as two independant single ended transmission lines). You'll see them also called even and odd impedance. tek.com/support/faqs/… explains better Mar 23, 2021 at 9:14

I've drawn a schematic that might help:

simulate this circuit – Schematic created using CircuitLab

The single 50 Ohms is the impedance of a single-ended transmission line that has a ground connection.

If you use two of those and operate them differentially !!! then the characteristic impedance can be treated as a 100 Ohms differential impedance. This 100 Ohm has no ground connection.

But there's a ground between R7 and R8!

Well spotted, but if the signal is purely differential then the voltage at that node will always be zero. If the Data+ = + 1 V and the Data- = -1 V then that node between R7 and R8 will be at 0 V anyway, the fact that it is connected to ground makes no difference! There is no current flowing into that grounding point, so nothing changes if we remove it resulting in R9 = 100 Ohms.

Notes: the resistors show the termination resistors that are needed to properly terminate the transmission lines. Assume that the characteristic impedance of the transmission lines is always 50 Ohms in this example.

• Or maybe can I understand like - the signal does not return to the common ground at the node between R7 and R8 because, the signal will always return back to the source instead of a common node
– user220456
Mar 23, 2021 at 12:52
• can you please confirm my above query? So, if I require a 100 ohm termination, I can place 2 50 ohm resistors in parallel termination to ground for the differential signal? (I am planning to use LVDS protocol on the differential lines). Is it Ok?
– user220456
Jun 8, 2021 at 7:34
• I can place 2 50 ohm resistors in parallel termination to ground for the differential signal? That's EXACTLY the schematic above the text "DATA must be a differential signal". Do realize that LVDS signals might have a DC voltage superimposed on them. Then you cannot directly terminate to ground. Jun 8, 2021 at 7:46

This simplify further through visualization, adding images here.

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