DIGITAL ISOLATORS

Hi,

I would like to use ADuM26xN in my design.

1.Can VDD1 be >=VDD2? or VDD2be >=VDD1 ?

2.If VDD1 is +1.8V then can V1a be +3.3V or it should be +1.8v signal only?

3.If VDD1 is +3.3V then can V1a be +1.8V or it should be +3.3V signal only?

i.e VDD1 side of the signals should be of same voltage level as VDD1 or can they be less than or greater than VDD1?