# Output Impedance of a Digital Output

In High-Speed Digital Design, Dr. Johnson illustrates an example of calculating the output impedance of an IC (page 48.). He uses VOL of 0.15 V and VOH of 4.32 V (@ Vcc = 4.5 V) with an Io of 4 mA to compute a typical low state output resistance of 37 Ohm and 45 Ohm for the high state.

In both calculations the current was 4 mA. My question is, is this result only valid for a current draw of 4 mA?

How would I utilize this calcuation in the real world? For example, I'm using NC7WZ16 buffer but plan on putting a series/source terminator at the output. But before I decide on the value of the source resistor I need to know the output impedence of the chip. The datasheet lists this at a Vcc of 3V:

VOL (typ.) = 0.16V @ Io = 16mA. This gives as Rout as 10 Ohm. But, again, is this result only valid for Io of 16 mA? In a typical design today the inputs are very impedance and so I believe that the current draw will be minimal.

Similarly, for the high-state, the impedance is ((3-2.75)/16mA) = 15 Ohm. So does the output impedance change with current draw or does it stay the same and I can utilize the above results?