I see the same question coming quite often.... when doing acceleration data integration, it is very important to calibrate the sensor .
calibration is the task that will define :
- what is the voltage output of the sensor at rest (physically). Your output is certainly not truly zero volt, so that dc ofset must be retrieved before you do the integration. otherwise , a dc offset will create a linear drift in the velocity output , which itself create an even faster drift in displacement output (parabolic drift).
- what is the sensivity of the sensor : this should be defined by the supllier specification, but it's always good to double check. A simple method is to put the senor once top side upward (record output value for +1g acceleration) , once top side downward (record output value for -1g acceleration) . Compute sensivity = difference of the two values /2