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Using series or parallel termination resistor aims to improve signal integrity by reducing reflections caused by discontinuities in the characteristic impedance. This much is clear.

What is not clear is, why would one need to use AC termination (also called RC termination) or the Thevenin termination (one resistor tied to Vcc and other to GND). It should always be possible to match impedance of transmission line among transmitter, receiver and PCB track by merely using series or a parallel termination resistor. So why would one need these more complex techniques at all?

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If you use series resistor on transmitter side, it will match the output to the transmission line.

On the receiver side, you could simply use a parallel termination, for example to ground, but it will make a DC load and voltage divider with any source impedance, so e.g. a 5V CMOS output will either be divided by resistances to e.g. 2.5V if resistances are equal, or without source resistance, it is a DC load.

If you use AC termination with RC network, it does not cause DC loading, only signal transitions are terminated.

With Thevenin termination, you terminate with a voltage divider, or a single resistor to a suitable DC voltage. This means that transmitter can drive the signal below and above the DC threshold so both logic high and low just have less amplitude, but it's far better than just single resistor to 0V.

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  • \$\begingroup\$ is reducing power dissipation the only real benefit of this? This means that if the device is not battery operated, this does not matter anymore. \$\endgroup\$
    – gyuunyuu
    Commented Jun 26, 2023 at 15:50
  • \$\begingroup\$ Depends what you need. If you are making a supercomputer with billions of terminations that consume power and convert it to heat, you also have a problem of larger electricity bill, higher temperature, moving the heat away from supercomputer and cool it down, and larger cooling system costs more and also needs more electricity. And you likely need a bigger buildig to handle the things needed for the computer, and also larger power infrastructure such as transformer or larger electricity wires leased from power company requiring larger monthly costs even if the computer is shut down. \$\endgroup\$
    – Justme
    Commented Jun 26, 2023 at 15:58

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