[SI-LIST] impedance determination of a perticular interface.....

  • From: Mallikarjun K <malli.1729@xxxxxxxxx>
  • To: "si-list@xxxxxxxxxxxxx" <si-list@xxxxxxxxxxxxx>
  • Date: Thu, 18 Sep 2014 10:07:14 +0530

Hi,
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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