[SI-LIST] Reflection on differential termination

  • From: shekhar sharma <sinnovate88@xxxxxxxxxxx>
  • To: si-list@xxxxxxxxxxxxx
  • Date: Thu, 17 Aug 2006 11:42:55 +0100 (BST)

Hello all,
                  I have a differential driver terminated by 100-ohm at the far 
end. This is a differential termination between two lines A and B. Termination 
resistance is matching with the differential characteristic impedance of the 
trance.
  Initially signal value on A is 300mV and signal value on B is 600mV. This 
time common mode voltage is 450mV. Now A is rising from 300mV to 600mV with 
rise time of 1ns. B falls from 600mV to 300mV with 1ns fall time but skew 
between A and B is 500ps, means B starts falling 500ps after A starts rising. 
For the 500ps time A reaches from 300mV to 450mV. During this period there will 
be change in common mode values of 150mV.
  I have seen observed that this 150mV rise of A (B remains constant), will 
cause 75mV reflection on A and 75mV rising of signal at B. I am not able to 
explain this effect that why the half of delta Vos (change in common mode) is 
going to B and half is reflected back to the source on trace A.
   
  Thanks in advance,
   
  Sekhar
                                
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