the output voltage range of the ad420 is only 4,90V and not 5V!!
The AD420 is primarily used as a precision current output DAC in 4-20mA loop
systems delivering a programmed current into a grounded load. In voltage output
mode the current source is set to swing 0 to 4mA and fed into an internal
trimmed 1.25kohm resistor load. The Vout performance does not meet the
performance of the Iout output. The expected total output error for Vout is 1%
typical, this is with REFIN= REFOUT and includes reference errors.
If you see that the voltage output swing is 0 to 4.90V, this is a total error
of 2%. You may be able to improve on this error by ensuring you have
REFIN=REFOUT, using a higher power supply voltage, reducing the output load.
However a voltage error of 2% is possible at extremes.
If you require a precision voltage output, I recommend using the AD420 in
current output mode (0-20mA) with an external precision resistor.