BMA400 calibration

[cmorris]

Hello, 

we are utilizing the BMA400 in one of our new product designs, and are communicating via SPI to a MSP430 processor. 

We calibrate the BMA400 to remove any DC offset according to the design guidelines posted here: (https://community.bosch-sensortec.com/t5/Knowledge-base/BMA400-accelerometer-design-guide/ta-p/7397).

I notice that the Y and Z axes reads just shy of 1g when aligned with gravity; I read about  +/-0.97g on both channels. However, the X  axis outputs the +/- 1g as expected. Our full scale rangle is +/-2g. Since the resolution is 12bit, the bit range is +/- 2048. I should see a count of +/-1024 when aligned with gravity, but I see closer to +/-1000 on both the T and Z axes. Again, this is after calibrating out the DC offset. 

Do we need to account for this rescaling in software, or is there something I am missing with the sensor? There's no mention of this in the calibration procedure, so I'm confused as to why I don't see a count of +/- 1024 when the sensor is aligned with gravity. 

Thanks!

[BSTRobin]

Hi cmorris,

As your full scale rangle is +/-2g, BMA400 range should be set to 4g range. You should see +/-512 data when aligned with gravity.
See it from BMA400 data sheet:

[cmorris]

I believe you are incorrect about this. It says multiple times in the datasheet that the ranges are +/-2g,+/-4,+/-8,+/-16,

[BSTRobin]

Hi cmorris,

You can do the test:
Set BMA400 range to 4g, place sensor still. Z axis LSB data should be about 512.