I have tried to understand how to match my 50 ohm pcb trace, which is carrying a high-speed data signal, with a digital chip's input pin.
The problem is that I don't understand why the calculations explained in this note from Texas Instruments are correct or even usefully.
On page 9 the characteristic impedance of the pin is calculated, and it seems that this should be used to match my series termination with the transmission line using the standard Zseries = Zo - Zout. They state that this formula defines both input and output impedance of the pin.
I assume that the transmission is longer than the raise/fall time of the output driver for my questions, and that there is no termination at the receiver, so a wave of double voltage is reflected, which I think is normal for series termination.
Now for the questions:
If the drivers MOSFET is ON, this would cancel the effect of C_COMP in the IBIS model, so does the reflected wave returning form the receiver really see the calculated impedance (+ the series termination) ?
If the pin is configured for input, I don't see that the characteristic impedance has any use. Say that we have calculated the pins characteristic impedance to be 50 ohm, it would be perfectly matched the line, but when configured for input, the wave has no where to go but back towards the driver, since the L-C circuit is reactive ?
But what am I missing...