Will the AD5420 performance meet the specifications under the following conditions?
External Reference 5V
( Can the External Reference be more than the digital supply? )
Looking at the abs max ratings of the part, there are no dependencies to both supplies. I don't see any problem with the external reference being higher than the DVCC pin.
Looking at the DAC architecture, the reference will be used to dictate the amount of current on each leg of the ladder while the DVCC will be controlling the states of the switches.
Hope this answers your question.