HI,
I must assume that your "ref voltage" miss all capacitor, especially a bigger electrolytic!? :-(
You must know pls; these sort of "Uref" is not a reference source!...
You need for a good ref. low impedances & an attenuator is not lo impedance source!
It will be better if you buffer it w. an OpAmp, but a real usable Uref is only from a dedicated Ref-voltage IC to become; ev. you can take a zener & buffer/amplify it, but it will be never so good as a reference element...
Your ADC can not sense a difference between variable Uref + added signal as you can not tell too what is the water in your river from the Nr 1 or Nr 5 smaller rivers their are flows in that...!
K.