[SI-LIST] Re: Why we need to use "Series resistor" at Transmitter ?
- From: "Ingraham, Andrew" <Andrew.Ingraham@xxxxxx>
- To: <si-list@xxxxxxxxxxxxx>
- Date: Wed, 23 Oct 2002 15:08:22 -0400
When using series termination, and IF you want to exactly match the
impedance there, the total source resistance should equal the trace
impedance. If a series resistor is used, its value should be smaller
than the trace impedance because the output impedance of the driver
itself is not zero.
The resistor should be very close to the driver.
But sometimes it is desirable to not match the trace impedance.
Other devices attached to the trace lower its effective impedance, so a
smaller termination resistor might match the loaded line better.
Sometimes the receivers can tolerate overshoot better than undershoot
(reaching a plateau short of the full swing); and sometimes fast, clean
edges are more desirable than slow, rounded edges with plateaus.
Choosing a series resistor that is smaller than ideal, can help drive
your signals cleanly through the input levels with only moderate
overshoot.
When you margin all parameters, sometimes you find that it's better to
have the resistor value a little too small.
Why use series termination? It is only one method of terminating
signals so that they don't bounce around forever. Find a good book and
read about different termination methods and when to use each one.
Regards,
Andy
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