I have a problem with understanding the AD527x datasheet in the point, where the relation between code written to the chip and its actual resistance is explained: Table 1 states, that the wiper resistance is between 35 and 70 Ω. Later, in the paragraph PROGRAMMING THE VARIABLE RESISTOR, there is a formula for calculation the chip's resistance, this formula omits the influance of the wiper's resistance to the overall resistance. The next paragraph states, that in the zero-scale condition, a finite total wiper resistance of 120 Ω is present – this is significantly higher value, than 35 to 70 Ω (as stated in Table 1).
Moreover: the formula for the resistance calculation is as follows Rwa(D) = D/256×Rwa; the highest possible value of D is 255, i.e. the maximum resistance value of the chip is (say 100 kΩ nominally) is 255/256×100=99.6 kΩ?
Could you please provide me clear instructions on how to calculate the resistance?