We are using an MMA8452Q and are utilizing the Self Test feature during production test.
The datasheet indicates that when self test is activated, the XYZ axes will be deflected by +45, +64, and +420 LSBs off of the current acceleration acting on the sensor respectively.
Our procedure is as follows:
Set to ACTIVE mode
Set to ST mode
Compute absolute differences in XYZ for ACTIVE and ST mode.
(We are doing quite a bit of averaging along the way to account for any noise that may exist in the system.)
We routinely get values that are 380 and below for the Z axis. The XY are usually pretty marginal, but sometimes are dead-on.
Each accel is different.
Can you please give me some insight into why the Z axis is not even close to the +420 specified in the datasheet?
(Note that we do set the accel range to +/-4G. The Active bit in CTRL_REG1 is cleared before we set ST. The accel is still during the test).