I don't see any specifics in the datasheet for the AD7708 regarding crystal selection, other than frequency. Are there guidelines regarding tolerance, stability, and load capacitance? Thanks.
For the AD7708 it is important to make sure to follow manufacturers guidelines to ensure no overtones are present as this will mean that the PLL can lock onto the incorrect frequency. Other than this there are no specific guidelines for the AD7708 regarding tolerance, stability and load capacitance. It should be noted however that all of these will affect the output data rate and consequently any 50/60Hz rejection.
In order to help you select a crystal I can tell you that for our evaluation boards we use Farnell Order Code. 221-533 - IQD FREQUENCY PRODUCTS - LF A103C - CRYSTAL, 32.768000KHZ, IQD FREQUENCY PRODUCTS, Order Code: 9712887, Manufacturer Part No:LF A103C. This part works with no extra resistors or capacitors as the capacitance of the XTAL pins is in fact enough. However, it should be noted that this crystal is only rated to operate -10 to 60 degrees centigrade.
Please let me know if you have further questions.