CAN bus layout design

I am trying to design a CAN bus node. The CAN bus shall be split-terminated with 120Ohm, 60Ohm for each line.

Therefore i tried using this paper to calculate a characteristic impedance of 60Ohm for a coplanar strip attaching the CAN-Transceiver to a Twisted Pair cable, the CAN bus medium. But somehow i cant obtain reasonable values for this setup. I reach 60Ohm only with dimensions far too small or too large for manufacturing.

What is the best approach for attaching a CAN-Transceiver to a Twisted Pair cable regarding a PCB layout?

Edit: I guess I'll go by trial and error then. But how would i solve this theoretically? With USB3.0 devices i would run into the same issue without the option of scrificing some of the signal.

Answer

You are getting confused about the impedance. The type of CAN you are apparently using is implemented as twisted pair with roughly 120 Ω impedance. That is why there is a 120 Ω resistor on each end. That means the bus looks like 60 Ω to a driver, but the transmission line itself is still 120 Ω. Since drivers drive in the middle of the cable somewhere, they are essentially driving two separate transmission lines, one in each direction.

Anyway, as others have said, don't worry about it. Put the CAN transceiver chip as close as you reasonably can to the CAN bus connector or where the bus lines are soldered to the board, and it won't matter.

Consider the wavelength. The maximum CAN bit rate is 1 MHz. Let's say to get reasonably square edges you want up to the 10th harmonic, so 10 MHz. The speed of light is 300 Mm/s, so 30 m at 10 MHz. Let's say that the speed of propagation on the transmission line is half the speed of light, so 15 m. Even if all this is off by a order of magnitude (or you wanted to carry up to the 100th harmonic), that would still be 1.5 m wavelength. 1 inch would be a long distance between connector and CAN transceiver chip, but even that is only 1.7% of a wavelength.

Put another way, you have a lumped system unless you really go out of your way to do something silly. Don't worry about it.