Hello, In the end the digital domain controls the switches implemented in the resistor-based DAC core. As such those switches (MOSFETs) must be biased appropriately from the digital domain to allow for proper analog functionality. If the reference voltage is higher than the digital supply voltage then the appropriate VGS cannot be realized to actually control the switches. Therefore, if this condition is violated the analog output voltage may be inconsistent with the expected DAC transfer function. This condition must be met for this device. For more modern devices you will be more likely to see just IOVDD and Analog supplies versus an additional DVDD to avoid inconveniences like this, where internal LDOs are implemented to derive these type of supply needs. You may consider looking at the DAC81408 .
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