Piezoelectric pressure sensors
You will have to use the ADC to measure the Piezo Sensor, the diodes and op-amps are there to protect the ADC, some piezo elements can produce a swing of upto 90V with a simple knock on the table. The Diodes clamp the voltage into the operable range of the ADC. The op-amp is to amplify the signal, if you see the characteristic load curve from the datasheet you can see that it is not a linear relation and for lower application of loads the sensor give output in the range of a few millivolts. 


You need to clamp the voltage swing so as not to exceed the voltage input of the ADC.
If you want to measure the negative swing also the op-amp can be used to provide a DC offset.
The op-amp can also amplify the signal if your stacks give an output that is too small to measure for the ADC.