The title says the question.... I don't quite understand the need for a single-end impedance if I'm routing a differential pair. What I mean is, if I need to route a differential pair on a circuit board for a high speed signal (like USB), do I need to worry about the single-end impedance?
Let's take USB 2 as an example. In the USB 2 spec, there is a 90 Ohm differential requirement, but I don't see a single end requirement. Do I even need to worry about a single-end impedance for each track in the pair, or does the receiver only see the differential?
I ask because, in a previous question I asked on split termination with differential pairs, it seems like only the single-end impedance matters for termination, not the differential. The differential impedance requirement sets some spacing. But obviously, I can route two 45 Ohm traces as a 90 Ohm differential without very close spacing, or I could route two 50 Ohm traces as a 90 Ohm pair as long as the spacing is close enough. The USB 2 spec seems to imply that the single-ended impedance doesn't matter. Is this correct?