The AD5791 and AD5790 DACs appear to be very similar devices, yet their respective data sheets have different equations for the output voltage.
The AD5791 equation seems to be a bit of an anomaly as most other ADI DACs do not include the minus 1 term in the denominator.
Is there a mistake in one or the other data sheet?
From AD5791 data sheet:
From AD5790 data sheet: