Specifications

BMA180
Preliminary data sheet
Bosch Sensortec
Rev. 1.0 Page 47 / - proprietary information - 06 March 2009
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Specifications within this document are preliminary and subject to change without notice. Document is not intended for publication.
7.10.3 Offset_finetuning
7.10.3.1 General explanation
The sensor is providing a possibility to regulate offsets down to very small values. This can be
done for each axis separately. Remaining offsets are within a certain range below/above 0g
(also for z-axis), the value of which are depending on the fine-tuning mode (see below).
7.10.3.2 Configuration parameters and settings for offset fine-tuning
Offset fine-tuning method is controlled via a 2-bit register, called offset_finetuning, the regulation
procedure itself is enabled by 3 bits called en_offset_*.
The definition of the offset_finetuning register is the following:
offset_finetuning<1:0> Offset regulation
00
no action
01 (default)
fine calibration
10
coarse calibration
11
full calibration
Setting these two bits enables the offset regulation and disables automatically the low interrupt
function (low_int = ‘0’). This is due to optimized register structure with a limited amount of free
bits.
The offset cancellation function has two sub-functions:
• Coarse calibration
: This one is built-in to correct up to ±1g, in addition to the standard
offset correction of the sensor (see section before). E. g. using this calibration method
could eliminate the 1g offset of the z-axis in applications where optimum full scale
measurements are necessary – e. g. high accurate tilt measurements with high
resolution in 1 g mode, where all 3 acceleration signals are used. Coarse calibration is
done via DAC inside the sensor, thus final remaining offset is same as after standard
calibration (see table 2).
• Fine calibration
: the fine calibration is done in the digital part of the sensor and allows
reaching an offset correction with a step size of 1 LSB
adc
and a range of ±64 LSB
adc
, that
means 7 bits per channel, called fine_offset_* bits (“2’s complement” coded). The
following table defines the correspondence between the fine_offset_* bits and the low
interrupt bits (in offset-tuning mode the fine_offset_* bits are stored in the below
mentioned low_* bits).