FreeLists / si-list / [SI-LIST] Re: Differential pair impedance

["Abe Riazi" <ARIAZI@xxxxxxxxxxx>]

RE: [SI-LIST] Re: Differential pair impedancePat Diao Wrote:

  In a differential pair, compared against Differential Impedance, most times 
we refer Single-ended Impedance as the Characteristic Impedance of one of the 
single lines.  Now we have the following definition of Single-Ended Impedance.  
Any relationship between these two, or just another case of mis-used terms.

  Pat 

  >   Sometimes, it is also desirable to calculate the single ended  impedance 
  >    ( Z_se )  which is obtainable via: 
  > 
  >   Z_se = (2 * Z_even * Z_odd) / (Z_odd + Z_even) 

  Pat Diao, Ph.D. 
  Manager - Package Thermal, Electrical 
  and Mechanical Engineering and Development 
  ASAT Inc. 
  Fremont, CA 

  Hi:

  By single-ended impedance (Z_se), I was also relating  to the case when one 
of the two coupled line is driven and the second one is grounded.

  As a numerical example for Z_se and differential mode impedance Z_diff , let 
us consider the following resutl from a XFX generated Transmission Line 
Parameters (.tlp) file:



  Configuration Name: STRIPLINE Conductors: 2
  Conductor index: 0   name: $$GND$$
  Conductor index: 1   name: A
  Conductor index: 2   name: B

   i         j         Lij              Cij        Ze           Zo          Se  
   So      Fwdx   Rvsx
  from  to       (nh/in)       (pf/in)   (ohms)   (ohms)      (ns/ft)(ns/ft)   
(s/s)       (v/v)
  
-----------------------------------------------------------------------------------------------------------------------
    1    1     12.509        2.693     69.60       -            2.16      -     
   -     -
    1    2       2.534        0.546     83.70     55.50       2.16      2.16   
-1.0E-4  0.203
    2    2     12.503        2.694     69.57       -            2.16       -    
      -     -
  ;



  Here we note that the odd mode impedacne Z_odd is  Zo = 55.50     and  the 
even mode impedance Z_even = Ze = 83.70 Ohms.

  Subsequently, 

   Z_diff = 2 * Z_odd = 111.0 Ohms


   Z_se = (2 * Z_even * Z_odd) / (Z_odd + Z_even)  =   66.74 Ohms

  Thus, the calculated Z_se is smaller than impedance of each line when 
uncoplued (e.g. 69.6 Ohms as given by .tlp file )

   For those further interested in the basis for the Z_diff and Z_se formulas, 
an insightful source is Ron K. Poon, " Computer Circuits Electrical Design", 
Pages 204 - 211.

  Best Regards,

  Abe Riazi

  ServerWorks


   








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