Incorrect readings from ADC

[James donnelly]

Hello, I am trying to create a circuit using my Raspberry Pi and an ADC to read a piezo disk detecting vibrations.

This it my current circuit. The 2.46V and 5.09V are from 9V batteries using LM317 voltage regulators. Voltages confirmed with a multimeter.
The Raspberry Pi uses the SPI interface and should return results between 0-1023 but more specifically should return 494 (because that's the 2.46V)
The 2.46V is used as a bias so I can read the negative voltages from the piezo.

Because I can't seem to get the correct readings I haven't yet added the piezo.

I get erratic results ranging from 786 to 0 fluctuating quickly. Can anyone explain why?

 

[Harald Kapp]

The 2.46V source needs to be connected with "-" to Vss. Also Vss needs to be connected to the Raspberry's GND, too.

You generaly need a common reference voltage, typically called GND, ground, 0V or similar. Here it is Vss.