[SI-LIST] Re: Impedance match of a 100Mhz line

  • From: "Andrew Ingraham" <a.ingraham@xxxxxxxx>
  • To: <si-list@xxxxxxxxxxxxx>
  • Date: Mon, 9 Mar 2009 11:42:34 -0400

> Question is :
> They will still be reflection as per the equation (ZL-Z0)/(ZL+Z0)
> Since the input impedance  of digtial pin is  very high  reflection (ZL
> tending to infinity)coefficient at the digital pin is  1. Does that sound
> correct ?


If you need to have little reflection, then you need to add a termination.
(Easiest way to tell is to simulate it.)

Some digital nets work fine with a +1 reflection at the far end but small
reflection coefficient at the sending end.  This is source terminated.
Exactly one reflection, and signal looks good at the far (receiver) end.
This may be by design of the driver's strength, or by adding an external

Some digital nets also work OK despite reflections because the trace length
(delay) is short compared to a risetime.


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