LSM303DLM Datasheet, PDF (13/38 Pages) STMicroelectronics – Sensor module:3-axis accelerometer and 3-axis magnetometer

LSM303DLM
4
Terminology
Terminology
4.1
Linear acceleration sensitivity
Linear acceleration sensitivity describes the gain of the accelerometer sensor and can be
determined by applying 1 g acceleration to it. As the sensor can measure DC accelerations,
this can be done easily by pointing the selected axis towards the ground, noting the output
value, rotating the sensor 180 degrees (pointing to the sky) and noting the output value
again. By doing so, a ±1 g acceleration is applied to the sensor. Subtracting the larger
output value from the smaller one, and dividing the result by 2, leads to the actual sensitivity
of the sensor. This value changes very little over temperature and over time. The sensitivity
tolerance describes the range of sensitivities of a large number of sensors.
4.2
Zero-g level
Zero-g level Offset (LA_TyOff) describes the deviation of an actual output signal from the
ideal output signal if no linear acceleration is present. A sensor in steady-state on a
horizontal surface measures 0 g on both the X and Y axes, whereas the Z axis measures 1
g. Ideally, the output is in the middle of the dynamic range of the sensor (content of OUT
registers 00h, data expressed as 2’s complement number). A deviation from the ideal value
in this case is called Zero-g offset. Offset is, to some extent, a result of stress to the MEMS
sensor and therefore the offset can slightly change after mounting the sensor onto a printed
circuit board or exposing it to extensive mechanical stress. Offset changes little over
temperature, see “Linear acceleration Zero-g level change vs. temperature” (LA_TCOff) in
Table 3. The Zero-g level tolerance (TyOff) describes the standard deviation of the range of
Zero-g levels of a group of sensors.
4.3
Sleep-to-wakeup
The “sleep-to-wakeup” function, in conjunction with low-power mode, allows further
reduction of system power consumption and the development of new smart applications.
The LSM303DLM may be set to a low-power operating mode, characterized by lower data
rate refreshing. In this way, the device, even if sleeping, continues sensing acceleration and
generating interrupt requests.
When the sleep-to-wakeup function is activated, the LSM303DLM is able to automatically
wake up as soon as the interrupt event has been detected, increasing the output data rate
and bandwidth. With this feature the system may be efficiently switched from low-power
mode to full-performance depending on user-selectable positioning and acceleration events,
therefore ensuring power-saving and flexibility.
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