LSM303AGR Datasheet, PDF (24/68 Pages) STMicroelectronics – ultra-low-power 3D accelerometer and 3D magnetometer

Functionality
LSM303AGR
4.1.4
4.1.5
Magnetometer hard-iron compensation
Hard-iron distortion occurs when a magnetic object is placed near the magnetometer and
appears as a permanent bias in the sensor’s outputs.
The hard-iron correction consists of compensating magnetic data from hard-iron distortion.
The operation is defined as follows:
H_out = H_read – H_HI
where:
 H_read is the generic uncompensated magnetic field data, as read by the sensor;
 H_HI is the hard-iron distortion field;
 H_out is the compensated magnetic data.
The computation of the hard-iron distortion field should be performed by an external
processor. After the computation of the hard iron-distortion field has been performed, the
measured magnetic data can be compensated.
The LSM303AGR offers the possibility of storing hard-iron data inside six dedicated
registers from 45h to 4Ah.
Each register contains eight bits so that the hard-iron data can be expressed as a 16-bit
two’s complement number. The OFFSET_axis_REG_H registers contain the MSBs of the
hard-iron data, while the OFFSET_axis_REG_L registers contain the LSBs.
Hard-iron data have the same format and weight of the magnetic output data. The hard-iron
values stored in dedicated registers are automatically subtracted from the output data.
Magnetometer self-test
The self-test function is available for the magnetic sensor. When the magnetic self-test is
enabled, a current is forced into a coil inside the device. This current will generate a
magnetic field that will produce a variation of the magnetometer output signals. If the output
signals change within the amplitude limits specified in Table 3, then the sensor is working
properly and the parameters of the interface chip are within the defined specifications.
The self-test procedure is described in the following figure.
24/68
DocID027765 Rev 5