ISM303DAC datasheet(24/80 Pages) STMICROELECTRONICS

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Functionality

ISM303DAC

24/80

DocID030988 Rev 2

4.1.2

Magnetometer offset cancellation

The ISM303DAC is based on AMR technology: a set pulse is needed to set an initial

operating condition.

Offset cancellation is the result of performing a set and reset pulse in the magnetic sensor

and it can be enabled to remove the intrinsic sensor offset.

The offset cancellation technique is defined as follows:

where Hn and Hn-1 are two consecutive magnetic field measurements, one after a set pulse,

the other after a reset pulse.

Considering a magnetic offset (Hoff), the two magnetic field measurements are:



Set: Hn = H + Hoff



Reset: Hn-1 = –H + Hoff

The offset is cancelled according to the offset cancellation technique:

If the device is operating in Continous mode, the offset cancellation is enabled by setting the

OFF_CANC bit to 1 in CFG_REG_B_M (61h). In this case, set/reset pulses are

continuously performed; a set pulse is applied to one measurement, a reset pulse is applied

to the next measurement. If the offset cancellation is disabled (OFF_CANC = 0) and

Continuous mode is selected, the set pulse frequency can be configured by setting the

Set_FREQ bit in CFG_REG_B_M (61h). If Set_FREQ is set to 0, the set pulse is released

every 63 ODR, otherwise if Set_FREQ is set to 1, the set pulse is released only at power-on

from Idle mode (a set of the magnetic sensor is performed anyway, even if the offset

cancellation is disabled).

If the device is operating in Single mode, in order to enable the offset cancellation, both

OFF_CANC and OFF_CANC_ONE_SHOT bits must be set to 1 in CFG_REG_B_M (61h).

Enabling these bits, the impulse polarity is inverted between a single read and the next one.

While offset cancellation is automatically managed by the device in Continuous mode, if this

feature is enabled in Single mode, the user has to remove the offset manually using the

formula below:

Offset cancellation using single reads is effective only if the reads are close in time, thus

ensuring the offset does not drift between two consecutive reads.

Hout

Hn Hn1

–

---------------------------

Hout

Hn Hn1

–

---------------------------

2H Hoff

Hoff

–



--------------------------------------------------

Hout

Hn Hn1

–

---------------------------

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