Did you consider using Analog Devices AD 7730 ? No hassle, includes
excitation circuit, input amp, AD converter, programmable digital filter,
serial interface to microprocessor..... and only for about U$ 10 in 1000
quantities..... you can't build it with discrete parts for that money....
I have some (free) samples I got when I was in Europe last month, now I'm
working on a prototype for weight scale applications - but I think I'm going
to use an Atmel AT89C52 micro (sorry, no PIC), together with a serial eeprom
for storing calibration parameters, a MAX232 chip for serial communication
with a PC, and a LCD display....

Albert A. Smulders
InSAD - Encarnaci—n, Paraguay
insad@itacom.com.py