why is USB impedance 90ohms

[techzee]

Hi all,

Sorry if am re-posting the same query once again.

1. When the differential impedance of the trace be 100E why does a USB signal has impedance of 90E defined.
The other high speed interfaces it self runs on a 100E impedance like PCIE etc then y a usb to have 90E impedance.

2. I read in one of the thread that the user group of usb found 90E to be easily achieved for a twisted pair cable. why not 100E??

do share your insight on this.

 

[tpetar]

This document is good to read, you will find even more data about USB :

High Speed USB Platform Design Guidelines
http://www.usb.org/developers/docs/hs_usb_pdg_r1_0.pdf

http://www.interfacebus.com/USB-Interface-Description.html

The USB bus is a [Differential] Bi-directional serial interface cable bus. Differential NRZI data is transmitted Isochronous or Asynchronous between devices. Data is transferred at three different rates over a maximum cable length of 4 meters ~ over 4 wires, 2 of which carry data on a balanced twisted pair.

USB may operate at any speed from 10kbps to 400Mbps in one of three speed modes. A Slow-Speed mode of 10kbps to 100kbps is used for devices such as a USB keyboard or USB mouse. Full-Speed mode is used by most devices and allows a transfer rate of 500kbps to 10Mbps. High-Speed mode [defined by USB 2.0] allows rates of up to 480Mbps, with a speed range of 25Mbps to 400Mbps. Transmission at the High-Speed mode requires the addition of 45 ohm termination resistors between each data line and ground. Operation at Full-Speed mode is 2.8 volts [High] to 0.3 volts [Low]. Operation at High-Speed mode is at 400mV +/-10% [High] to 0V +/- 10mV {Low].

Cable impedance for both modes is 90 ohms +/- 15% (differential).