1. Sort out the analogue domain first. Does the output voltage of the 741 indicate correctly with respect to relative to the piezo pressure? Is there any noise on its output?, oscillations? or any other possible cause.
Once you are sure that you have a good voltage to measure then re-check to see what your ADC is doing, it is possible that the 741 is actually picking up radiation from the ADC and causing an analogue to digital to analogue feed back loop and oscillating.
If there is a problem at this stage try to feed the ADC from a variable voltage source (a pot across the supply volts?). Then with only the voltage supply and the ADC you can fault find the ADC circuit.
Frank