ADXL355 getting the Offset


What is the best approach to measure the offset for an accelerometer? If I understand correctly, offset is the deviation from 0g in an axis. In that case it might be very difficult to calibrate without a special equipment (or I am missing something?). For example, having the device on a very "flat" surface still can give larger offset than those on the data sheet.

How does the data sheet values were obtained? Perfectly aligning one axis to 1g and checking the other 2 axis?

Thanks for the clarification,