I'm designing a board with one ADC AD7997 and one DAC AD5612. I'm going to use REF198 as a voltage reference for AD7997.
Could I use the same REF198 to supply the DAC, that use the supply voltage as the reference?
Yes you could use the DAC as a reference but you should note the sources of error:
Noise information is also available in the datasheet. 120nV/root Hz is the NSD at 10kHz and the 1/f noise is 2uV.