I want to use a shunt reference voltage to generate 1.225V, as demanded in the AD7705's datasheet. However, to calculate the required impedance before the shunt I need to know the maximum current drawn by the load (in this case the AD7705). This is not given in the datasheet, though. If I assume it to be much smaller than the minimum current drawn by the shunt (45µA) and neglect it I get a value of:
R = ( V_min - V_ref ) / ( I_ref,min + I_load,max )
= (3.3V - 1.225V) / 0.000045A
= 46111 Ohm
However, with a resistor like this I don't get the desired 1.225V so what would be a better assumption or where can I get the exact value?