Using an OP-AMP to obtain absolute value
To get the proper output for both positive and negative inputs, you need a full wave precision rectifier. Here is a workable schematic: 



You don't need R4 and the resistor values can be increased for relatively low frequency. This particular circuit uses an OPA2211 but other op-amp types can be substituted. 

The dual op-amp requires a dual supply (+/-) for this to work properly. Something like +/-10V should work well. You can divide down the output to stay within range of your ADC.