used in the ladder.
The input impedance of the R-2R DAC is R, that's inherent in the design. See
figure 13 in datasheet working from right to left.
2R || 2R = R
R+R|| 2R = R etc etc.
So R of ladder is equal to input resistance which varies from a min of 5K to a
max of 9K with a typical of 7K.
Please note your simulation may need to do a Monte Carlo analysis/simulation
with R_tolerance/temperature in mind.