[SI-LIST] Re: Reflection on differential termination

  • From: steve weir <weirsi@xxxxxxxxxx>
  • To: sinnovate88@xxxxxxxxxxx,si-list@xxxxxxxxxxxxx
  • Date: Thu, 17 Aug 2006 05:28:01 -0700

Sekhar, A to reference and B to reference are each transmission 
lines.  When you don't terminate a transmission line, it 
reflects.  The two lines together support a net even mode of 150mV 
that divides equally between the two.

Steve.
At 03:42 AM 8/17/2006, shekhar sharma wrote:
>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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