VIO range is 1.8V to Vdd, but Timing Specification is guaranteed with 2.3V to 5.5V or VDD+0.3 V. Please refer to Page5.
Can you guaranteed with 1.8V to Vdd? If not, please let me know about the below specification with VIO=1.8V.
At the time of release for this product, 1.8 volt logic levels were not an industry standard and were therefore not guaranteed. It has been proven through evaluation that the AD7688 is functional at a VIO level below 2.3 volts. However, Analog Devices does not provide a set of guaranteed timing specifications for between 1.8 and 2.3 volts. The ADI portfolio contains products that have quite similar user interfaces as the AD7688 and can be used as a guideline for developing user interface timing. The AD7699 is a good reference for 1.8 volt user interface timing as this product is specified all the way down to 1.8 volts.
For the AD7699, the timing parameters you mention are specified as follows:
tSCK = 30 nS
tDSDO = 28 nS
tEN = 25 nS
tDSDOSDI = 32 nS (tDSDOSDI is not a specification associated with the AD7699. The AD7699 contains an SDO disable spec that is very similar to tDSDOSDI. 32 nS should be a good starting point).
Again, these numbers provided are just a guideline. Any VIO operation between 1.8 volts and 2.3 volts is guaranteed to be functional, but the final timing parameters on which you settle will need to be determined through analysis of your system.