This is my first try at laying out USB 2.0 high-speed traces. I'm using a 2 layer board.
It took me forever to get all the info together. Zdiff = 2 * Zodd and Zsingle = Zeven / 2
With my calculations, I should get 92 ohm diff impedance and 75 ohm single ended impedance.
This is using a 26 mil trace and 6 mil space. The traces are just shy of 12mm long.
The difference between the two traces is 0.03mm.
From all the app notes I read, it is acceptable if the traces change size but only very close to the ends.
How does my layout look?
Looks about right. On 2layer the microstrip will be so far from its reference plan (you must put ground under it, obviously) that the traces will be thick. It's even worse for 50ohm, you can see those traces from space.
Don't worry about it too much, USB2 could work over wirewrap. I've seen USB3 hubs on 2layer with slots cut all over the reference paths and it still worked....
Don't worry about it too much. Look at any development kit with USB - they all just use standard trace with and it works fine. And for such short distances, it matters even less.
Thanks guys, I'll let you know how it works after I have it made.
Only 11mm between connector and chip? Then F the impedance rule, it's not worth it. Just make sure you are still following differential pair rule.
This. Also, it looks like you can shove the chip about 6mm over, making it even less important.
Tim