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I have seen specifying routing impedance of interfaces from manufacturers like for

  • USB Differential impedance of 90 ohms
  • DDR2 Single ended impedance 50-60 Ohms, Differential impedance - 100-120 Ohms.

If not specified, generally we will try to take it as 50 Ohms.

Based on this interface impedance levels, we will design corresponding stripline/microstrip lines parameters like Height above reference plane and width to give required characterstic impedance.

What drives these characteristic impedance values? Whether the drivers output impedance has any affect on this?

If route a 50Ohms interface with 20 Ohms impedance what are the impacts?

In few papers, I have read that intentionally designers will route with higher/lower (not sure ) impedance for crosstalk reduction. Is this impedance has any relationship with the impedance?

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The impedance is determined by the dielectric (air/FR4/teflon/rogers/etc.) and the geometry of the trace(s) and surrounding ground plane(s). There are calculators available online and in various software packages that can calculate various parameters. Generally you will input the copper thickness, ground plane spacing (board or layer thickness), dielectic constant, desired impedance, etc. and it will tell you the required trace width.

If you connect a 50 ohm driver to a 20 ohm trace you can run in to problems with reflections and intersymbol interference.

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  • \$\begingroup\$ I understand on what factors impedance of a trace wil depends.I want to know How they will determine the impedance number for perticular interface like for USB 90 Ohms why not 40Ohms? Once you know this impedance then only you will design trace for that impedance.... \$\endgroup\$ – user19579 Sep 18 '14 at 4:33
  • \$\begingroup\$ USB is 90 ohms because that is what the standard specifies. The specific impedance is arbitrary. The only reason it is specified as 90 ohms intead of something else is so that all of the connectors, cables, drivers, receivers, etc. will all be compatible with each other even if they are made by different companies. \$\endgroup\$ – alex.forencich Sep 18 '14 at 4:50
  • \$\begingroup\$ Still, they(people who prepared standard) can specify any other number(other than 90) and because it is standard every body will anyway follow that. How that number came..? \$\endgroup\$ – user19579 Sep 18 '14 at 5:48
  • \$\begingroup\$ Dunno. 100 ohms would make more sense as it is far more common. I don't know what the reasoning is for 90 ohms. \$\endgroup\$ – alex.forencich Sep 18 '14 at 5:51

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