assuming......
assuming that the sketch is reading the chip properly.....
assuming the ground connected to the LTC only and not shared......
assuming there are pots on the inputs, not shown.....
assuming the reference voltage is perfect......
can you try a test ?
connect the UNO and LTC ground together. [ not shown????]
use the 3.3v from the uno as power for the LTC
/* check the data sheet . I see 300 microamps as full load.
3.3v regulator can deliver no more than 50mA */
use the 3.3v as reference for the LTC
use the 3.3v as the power for the pots.
with that, your zero voltage is zero at all places
your maximum is max in all places
show the pots on your diagram.
in this way, you have a common ground
and your power is the same everywhere.
not shown is your pot(s) and their power draw. total power from the 3.3v is 50mA for all pots and all devices and power for the LTC
assuming that after this test, you get things much closer to what you expect, then start to change some things.
I would offer that you should evaluate your complete circuit and needed power supply and then work back from there. where are you getting power and how many devices will you use ?