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USB 3.0 and USB 2.0, pci signals, Why its always recommended to route at 85E impedance and not 100E impedance?

Below are the signals to be routed at 85E

D− (USB 2.0 differential pair) D+ (USB 2.0 differential pair)

StdA_SSRX− (SuperSpeed receiver differential pair) StdA_SSRX+ (SuperSpeed receiver differential pair)

StdA_SSTX− (SuperSpeed transmitter differential pair) StdA_SSTX+ (SuperSpeed transmitter differential pair)

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    \$\begingroup\$ It's more often specified as 90 ohms and not 85 ohms. 90 ohms is the spec. It's a standard by which cable manufacturers and circuit designers should aim to comply with. \$\endgroup\$
    – Andy aka
    Feb 3 at 13:21
  • \$\begingroup\$ Double Check the tolerances of traces and connectors usb.org/sites/default/files/… There are many reasons \$\endgroup\$ Feb 3 at 13:23
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The standards define the electrical impedance that must be used to be compliant with the standard and to allow best interoperability between devices.

Only PCI-E defines 85 ohms. USB is 90 ohms. Many other interfaces do use 100 ohms.

85 ohms is used because it allows for less losses on longer traces and it can be more easily made with multi-layer PCBs with smaller height between layers that 100 ohms.

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