AD5791 operate voltage

Hello, 

  I have use AD5541A ( 16 bit DAC ) for a while and have very good use experience with AD5541A.

  now I hope to try their family which is enhance to 18  or 20 bit DAC, that is  AD5791. 

  unfortunately AD5791's positive reference voltage  is recommend to operate with 5.0V ~ ( VDD - 2.5V ) 

  but not 2V ~ VDD such as AD5541A ( 16 bit DAC )..

  is there have another choice can meet my hope ?   thank you!

  My hope are list below:

  1. VREFp : 2V ~ ( VDD - 2.5V ) 

  2. 18 or 20 bit    ( 18 bit may better )

  3. un-buffer voltage output 

  thank you for help!

Ian