90 ohm for USB

Discussion in 'General Electronics Chat' started by bals, Jul 14, 2008.

  1. bals

    Thread Starter New Member

    Jul 14, 2008
    1
    0
    Hi all,

    Why are routing USB pair as 90 ohm diff pair.?can anybody explain the reason.


    Thanks
    bal
    http://pcb.mybb3.org/
     
  2. theamber

    Active Member

    Jun 13, 2008
    318
    0
    Your question is compound, so first why the USB standard transmits data differentially:
    Some of the properties to transmit differentially are high noise immunity and long distance capabilities. USB uses it for noise immunity and to filter EMR.
    One good one is that differential transmissions have good immunity to ground offsets, then the receiver will ignores the wires voltages with respect to ground, so small changes in ground potential will not affect the receiver's ability to detect the signal. Also most of the USB devices use low voltage one reason is because of power efficiency and reduce radiation to the surroundings. One of the side effect of using low voltage is the less the voltage the more noise that can get into, so differential transmission helps to reduce these problems because it will give twice the noise immunity of a single-ended system with one voltage reference. They counteract it this way.
    The 90ohms +/- 15% impedance has to do with the internal components, the high frequency signals USB transmit and to match the other USB devices and to reduce noise. Each data line uses a 1.5K and a 15K ohms pull down and up resistors respectively. If you want more information I can look for some pdfs I have about the USB standard.
     
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