I wanted to know what is the maximum and minimum ripple required for the voltage pins AVDD1 and DVDD. We are trying to give 2.5V from the switch mode power supply but the AVDD1 pin is connected to the modulator.
The power supply you suggested will probably give you an output voltage ripple in the 15mV-20mV range. I am not sure this is low noise enough to power the AD7764. Unfortunately we have not tested it with this level of noise on the power supply and the best suggestion I can give you is that you should try to power it with a lower noise switcher design.
Here is an example using the ADP2119 (a 2A solution) that can achieve <1mVpp or <200uVrms noise and no high frequency spurs higher than 3uVpp this is equivalent to LDO performance - see attached file, low noise board available on request at SimPowerTool@analog.com
How much current does you system really needs, the parts you specified are 3A and 1.5A parts but I am interested to know if you need that much current or if you selected those parts because of other criteria. You indicated the LDO is used for +5V power supply, but that LDO is rated for 5.5V input and has 470mV drop-out at full load. Which means if you are using it at full load it is running in drop-out and therefore it is not going to have any PSRR or low noise performance.
If you need less current then you may want to consider the ADP125 with <230mV drop out at 500mA.
I may be able to help further with more details about your input output and current requirements.
Thanks for your inquiry