I liked to use a AD780 as a 2,56V reference voltage source and would like to calculate the resistor value(s) i would need to achieve my goal. Unfortunately the datasheet only says use a 25k poti as Rpot for +- 4% adjustment. Sadly it does not say how to calculate values for other reference voltages near the default 2,5V and there is no internal circuit shown so that I could have a try on my own. There are no idealized values for the Resistors so it is not very easy to start calculation.
I know that the trim pin is not designed for this application but I think it should be possible.
Does anyone know how to calculate the value which values for the essential resistors are nominal or with which value as a resistor between trim and gnd i could calculate? An experimental value decision could be possible but would not be the optimum (because each device could spread a bit when i use the circuit several times, it would be a good point to start).
Datasheet: http://www.analog.com/static/imported-files/data_sheets/AD780.pdf page 6
Thanks in advance :-)