[SI-LIST] Re: Impedance match of a 100Mhz line
- From: "Bharathi" <bharathi@xxxxxxxxxxxx>
- To: "'Andrew Ingraham'" <a.ingraham@xxxxxxxx>
- Date: Tue, 10 Mar 2009 08:48:19 +0530
Andy,
Thanks. By placing a terminating resistor on the far-end would'nt that load
the driver (5V source, 5V/50Ohms =>100ma)
Regards
Bharathi
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Andrew Ingraham
Sent: Monday, March 09, 2009 9:13 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Impedance match of a 100Mhz line
> 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 ?
Yes.
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
resistor.
Some digital nets also work OK despite reflections because the trace length
(delay) is short compared to a risetime.
Andy
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