Thanks for the reply everyone.

I am not very concerned about using extra hardware as much as I am about the accurate readings from the sensor. If I use separate external A/D chip for each sensor would this allow me the freedom to select the suitable Vref for the various sensors or will I still be restricted to choosing one Vref?

Lets say I use an external A/D Chip this means that I can use different Vref for each sensor. Now what Vref do I choose for the PIC18F6520. Do I simply use Vref = 5V? This is where I am getting confused. Can you please help me. any suggestion is greatly appreciated. Thanks

-Srig