Hello Juan,
I have used this circuit
(http://members.aol.com/cpu8052/adc89C2051.gif) to measure
0-5VDC. If your voltage levels are higher just half the
values with two high value resistors. The documentation
for this circuit can be found at this web site.
http://www.atmel.com/dyn/resou...OC0524.PDF
It is easy to add a software clock and serial transmission at 2400 baud software.
Good luck,
Charles Bannister