ISM303DAC datasheet(27/80 Pages) STMICROELECTRONICS

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ISM303DAC

Functionality

4.1.4

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:

Hout = Hread – HHI

where:



Hread is the generic uncompensated magnetic field data, as read by the sensor;



HHI is the hard-iron distortion field;



Hout 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 ISM303DAC 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.

4.1.5

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.

When the magnetometer self-test is activated, the sensor output level is given by the

algebraic sum of the signals produced by the magnetic field acting on the sensor and by the

forced current.

The self-test procedure is described in the following figure.

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