# Transmission line, cable impedance

I am currently doing a design where I basically have two different types of (digital) signals. The first signal group is fully differential (P/N pair). The other group are single ended signals referenced to ground.

I would like to transfer both these signals via Twisted Pair cabling to another board. The cable producer specifies that the twisted pair cable has a (differential) impedance of 100Ohms, which is what if perfectly fine for the differential signals.

For the single ended signals, lets assume I have an output driver impedance of 50 Ohms and an input impedance of 50 Ohms as well so what I would basically want is a cable impedance of 50 Ohms.

How is this issue addressed in the real world? Is this an issue at all? Is the differential impedance of 100 Ohms "the same" as a single ended impedance around 50 Ohms?

What I understand is that a typical coaxial cable has an impedance of around 50 Ohms, but what about single ended signals on a twisted pair cable?

• How fast are these signals? Do you expect that the track+cable will be more than 1/8 of the wavelength of the highest frequency component? Feb 14, 2016 at 13:18
• You should look abot charactericstics impedance. There is no single ended circuit, if the wires are not equidistant like in a cable then it has no exactly defined characteristics impedance, because it is varying, it is assumed that it will not be used for high data transfer. Feb 14, 2016 at 14:20
• Are they digital signals? If so, it might be worth looking at single->differential converter ICs. There are a great many tiny package ICs which will do LVCMOS/LVTTL/etc. to LVDS/PCML/etc. and vice versa. Feb 14, 2016 at 15:32
• @tomnexus: They probably won't but this is also more a general question than one for this specific problem. I've come across this a few times already in the past and usually I didn't have to worry but I think it's important to understand the reasons. Also, I think that it's not the speed of the signal that matters but rise times. Feb 14, 2016 at 18:29
• @TomCarpenter: They are, but this is more a general question than one for this specific problem. We're using differential line receivers and transmitters but there the problem is a different one. Feb 14, 2016 at 18:32

You can get the effect of a 50$\Omega$ impedance line by putting two 100$\Omega$ lines in parallel, if you have sufficient number of pairs to do that.
The reason differential lines tend to be 100$\Omega$ is they expect the user to use an anti-phase pair of 50$\Omega$ drivers and receivers for them. Being anti-phase, the driver impedances are effectively in series, to match the line, and the same for the receivers.