I have heard transmission line termination but today I came across this "Digital termination" but couldn't find out why is it used and what effect does it have on digital signals?
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1\$\begingroup\$ Where did you come across the term? \$\endgroup\$– Nick AlexeevCommented Mar 12, 2014 at 1:22
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\$\begingroup\$ I was doing a JTAG boundary scan for one of the random board and the JTAG tool says that there is one resistor on the board that is used as a digital termination. To me that resistor is nothing but it is simply connected to one of the IO pin on FPGA to the GND which seems like that resistor is acting like a pulldown resistor but don't know why that JTAG tool calls it "Digital Termination". \$\endgroup\$– dr3patelCommented Mar 12, 2014 at 2:10
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1 Answer
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Digital termination is the same thing as transmission line termination. It's just a nuance that indicates that the goal is to reduce reflections of fast edges rather than match a specific impedance per se, although in reality, they're the same thing.
There are a number of ways to terminate a digital line. If a digital driver has lower impedance than the line impedance, you'll often see a series resistor to make up the difference. On the other hand, digital receivers usually have an input impedance that's higher than the line impedance, so a shunt resistor (AKA "parallel termination") is used.
When using parallel termination, the other end of the resistor is attached to a voltage source. This can be ground, Vcc, or some other voltage in between (in which case, two resistors are used as a voltage divider to create the equivalent Thévenin voltage and resistance).
What you have is an example of parallel termination to ground.
answered Mar 12, 2014 at 5:05