What is the differential impedance of a USB interface? What is the reason to have such impedance?

 

The differential impedance of USB2.0 lines is 90 ohms +/- 15%. 

The differential impedance of super speed USB 3.0 is 90 ohms +/- 7 ohms. 

The difference between the pin out of USB2.0 and USB3.0 pin out can be seen from below image:

As per differential trace requirements:

• The distance between the D+ and D- traces (S)
• Trace width of the traces (W)
• The dielectric constant of the substrate material (Er)
• Thickness of the trace (T)
• Dielectric height (H)

Depending on the above parameters, it is easy to achieve 90 ohms than 100 ohms. Whether it be coupling between the traces, effect of noise to be same on the traces, dielectric material properties, width and thickness of trace all change to different values if we try to achieve 100 ohms. Ultimately, all these signals need to terminate to connector which makes routing them 100 ohms more critical. If we have worked on any impedance calculator tool, we shall get to know these critical points.