AM3X Quickstart Guide

http://www.freetronics.com/pages/am3x-quickstart-guide#.US8x_TBTCf8

Understanding The Output Values

From the above:

"The outputs have a voltage proportional to the acceleration currently being experienced. Because each axis can display both positive and negative values, the outputs are offset to half the operational voltage of the accelerometer chip. The chip runs at 3.3V, so any axis that is experiencing 0 acceleration should read approximately 1.65V.

... In 1.5g mode (the default) the outputs vary by 800mV/g, which means that an output experiencing +1g acceleration will show:

1.65V + (1 x 0.8V) = 2.45V

Similarly, when experiencing -1g acceleration it will show:

1.65V + (-1 x 0.8V) = 0.85V

If you have the module sitting horizontal and still, both the X and Y axes should be experiencing 0g acceleration so they will output approximately 1.65V. However, the Z (vertical) axis will be experiencing +1g due to the effect of gravity, so it will output approximately 2.45V.

Likewise, if you tilt the module vertically so that the X axis is vertical you will see 1.65V (0g) on the Y and Z axes, and 2.45V (+1g) on the X axis."

Test

Here's it working in the flesh