i2c with a dac

[Acetronics2]

Did you had a look at table 4-4 page 10 from the TC Datasheet ???
Seems that the 6 lower data bits must be 0, and 16 bits to be sent.
so your example gives the 2 to 16 lower values ( less than 4% of full output ... )
that at a very high speed , as there is no PAUSE between each FOR-NEXT ... and then jumps and stops to the max value.
output seems not to be clearly visible ... even on a good scope !

Alain