[SI-LIST] Re: Terminations Query

Karthik,

You are describing two different methods of termination, source (or 
series) and parallel.

The purpose of the termination is to avoid reflections. Ideally, if you 
terminate using one method, you don't need the other. Parallel 
terminations generally require two or more components, while series 
terminations only require one. But series terminations allow a single 
reflection from the receiver end, which has other issues.

If you feel you can't depend on one method to adequately prevent 
reflections, you could use both series and parallel terminations. This 
would be very rare, likely, I suppose, for high-sensitivity D-A 
converter inputs, not digital ccts. It would result in the amplitude of 
the signal at the receiver being about half, due to the voltage divider 
created by the source impedance plus the series termination and the 
parallel termination. You would need to account for the reduced 
amplitude by driving the signal at double the voltage.

Alan

Karthik Raj wrote On 05/31/06 06:12 AM,:

>Hi,
>I have a basic query on the parallel termination techniques that are widely
>used.
>
>When a transmission line is terminated (lets assume a 50ohm parallel
>termination) we tend to match the Parallel termination resistance with the
>Trace Impedence. Instead shouldn't we match the Termination resistance to
>Source impedence of the driver + Trace Impedence, since they both are in
>series?
>
>Please forgive me if i am sounding stupid.
>
>Thanks,
>
>Karthik
>
>
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-- 
Alan Hilton-Nickel
Signal Integrity Engineer
Sun Microsystems Inc.
Netra Systems and Networking
Newark, CA

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