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

enter image description here

It mentions that the transmission line will have a differential impedance of 100ohms between them. In case of Single ended it would be 50 ohms between the signal and the ground.

Can someone help me to understand how does a single ended 50 ohm impedance would look like 100ohm differential impedance with a diagram?

  • \$\begingroup\$ 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 \$\endgroup\$ – Lorenzo Marcantonio Mar 23 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.

  • \$\begingroup\$ 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 \$\endgroup\$ – Newbie Mar 23 at 12:52

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.