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

  • From: <colin_warwick@xxxxxxxxxxxx>
  • To: <malli.1729@xxxxxxxxx>
  • Date: Thu, 18 Sep 2014 14:43:24 +0000

Hi Mallikarjun,

This posting might give a bit of insight about the characteristic impedance of 
a t-line:

http://signal-integrity.blogs.keysight.com/2009/geometric-form-factor-for-transmission-lines/

It only covers the ideal, lossless case, but it gives some physical intuition 
about the origin. The short answer to "What drives these characteristic 
impedance values? " is:

a) the vacuum impedance (~377 ohms)

b) multiplied by the reciprocal of the square root of the relative dielectric 
constant 

c) multiplied by a unitless form factor, typically of order 1, that is a 
function of ratios of cross-sectional geometry metrics e.g. height over width.

The characteristic impedance of the line is independent of its source and load 
impedances. If there is impedance mismatch, you'll get deleterious reflections 
and ISI.

Best regards,

-- Colin
____________________
Colin Warwick
Product Manager for Signal Integrity, Power Integrity, and EMI/EMC tools for 
High-Speed Digital Design
Keysight EEsof EDA 
Phone +1 978 681 2406
colin_warwick@xxxxxxxxxxxx 
Blog updates: http://signal-integrity.blogs.keysight.com/newsletter/ 




-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On 
Behalf Of Mallikarjun K
Sent: Thursday, September 18, 2014 12:37 AM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] impedance determination of a perticular interface.....

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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