What I really would like to find is a formula for
calculating R2R
ladder values so that I could get a 0V-5V output from an 8-bit
parallel port. Constructing it is easy, once one knows what
resistors to pick.
The output impedance of an R2R ladder is R so you chose R to suit the
stage the ladder will drive. You won't get 0V-5V without some sort of
amplifier on the output, even using CMOS gates to drive the ladder.
For a more monotonic output 2R should really be 2R-Rout where Rout is
the output impedance of the driving gate.
Lee.