Routing trace characterstic impedance determination based on interface


I have seen specifying routing impedance of interfaces from manufacturers like

  • 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?

Best Answer

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.