What is the characteristic of piezo transducer?
Piezo-electric transducers approximate, to a first order analysis, a capacitor.

Assuming you have hooked up the device like this:







simulate this circuit &ndash; Schematic created using CircuitLab

As a transducer of this type has no current flow in a static situation, then with no vibration at the transducer, the voltage read in the ADC is indeed the voltage at the positive side of the transducer.

The reason you even see other readings when you tap the device lies in the input circuitry of the ADC you are using:



On the first measurement (no tapping) the input sampling capacitor will charge up  to Vpos; when you tap the transducer (assuming the channel is connected for sampling), a small ac current will flow from Vpos and the sampling capacitor in the ADC, changing the voltage on the sampling capacitor, so you read a value other than full scale.

Note that the sensitivity of the input will depend on the sampling frequency (which I do not know); the sampling capacitor is in circuit for 1.5 clock cycles.

Tap it often enough and the sampling capacitor will completely discharge (where you will read 0 from the ADC result).

To properly read it, you should really hook it up like this:







simulate this circuit

The value of resistor here is not necessarily accurate; there are only small currents induced in most of these devices.