There is a significant advantage to using a separate reference - especially when you are using 10 bit (or more) A/Ds. I use an LM4120 - 4.096V. The fact that the output is an exact power of 2 makes things nice, also. You get exactly 4mV/count with a 10 bit converter.

If you decide to use the '4120, make certain you load the output with at least .2uF as in the data sheet.