FreeLists / si-list / [SI-LIST] Re: Source impedance?

[Mike Jenkins <jenkins@xxxxxxxx>]

David,

You are certainly correct...as long as you stay in that "perfect 
world".  As D.C. Sessions pointed out, there are always mismatches 
and parasitic elements in this (unfortunately imperfect) world.  
Depending on the speed (i.e., rise/fall times), just getting off 
the chip and package of the driver can provide some mismatches.  
Also, any receiver has some load capacitance, causing a glitch 
to be reflected back.  If your driver is unterminated, that 
glitch is then bounced right back to the receiver.

Regards,
Mike

David Kaiser wrote:
> 
> Theory question. Assume perfect world with no parasitics.
> 
> If a 50 Ohm trace on a board is terminated with a 50 Ohm resistor at the end
> of the trace, is there any advantage in terms of SI
> to driving the trace with a source that has 50 Ohm output impedance compared
> to a source with 10 or 20 Ohms of output impedance?
> 
> It seems to me that the only thing the series source impedance would do to a
> 50 Ohm line that is terminated with a 50 Ohm resistive load would be to
> divide the amplitude in half without improving the signal integrity.
> Correct?
> 
> David
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