1. Is such accuracy correct for AD5301BRM?
1. You should indeed get a result of 1 LSB if you are measuring correctly.
Did you notice "note 4" at the bottom of page 2 of the datasheet
"4 Linearity is tested using a reduced code range: AD5301 (Code 7 to 250);".
Since the AD5301 uses its power supply as the reference , using this slightly
restricted range removes the
deadband near to ground and Vdd; we have to do this because the output buffer
can never reach the full 0 and Vdd rails : "rail to rail" really means "almost
rail to rail".
Results will also be worse if you are driving less than 2 Kohms at the output.
The ideal measurement would be between codes 7 and 250 with a clean, properly
decoupled voltage reference used as the power supply. Figure 31 in the
datasheet shows an example of this.