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?