Hi,
It would help if you stated which micro, and which ADC. I know that PICs generally have ESD protected inputs, and have directly connected them with Microchips ADCs and Linear Tech's ADCs.
If I understand correctly, you are interfacing an external ADC? In that case, I would look at your micro's data sheet under input specs. Hope this is of some help. I was doing something similar, and ended up just connecting the ADC to the micro directly. No problem.
Regards,
Robert