How to determine the value of the termination resistor? Is the termination resistor used to reduce the reflection of the signals and also have to match with the transmission impedance? How am I going to determine the transmission impedance in order to put appropriate value of termination resistor in series with the load?
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1 Answer
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I'll assume you're talking about a trace on a pcb so you're looking at either a strip line or a microstrip. The basic impedance is a function of the trace width, the distance of the trace from it's reference plane, and the type of dielectric material between the trace (FR4 for example).
Try one of these calculators http://www.eeweb.com/toolbox/microstrip-impedance
If you are trying to match your driver to a line you'll want to find out the output impedance of the driver. Then that plus your termination resistor should match your line impedance.
Your best bet is to try to consult the datasheet, or ask your vendor. Otherwise if you have access to sim models you should be able to extract. If you happened to have a tdr lying around you could measure it :)
You could also find by connecting a known resistor to gnd and driving the driver high. If you can measure the current used you can use Ohms law to calc the source impedance. You can that in spice too.
Good luck
answered Oct 10, 2012 at 2:35