AIS3624DQ Datasheet, PDF (15/41 Pages) STMicroelectronics – ultra-low-power digital output 3-axis accelerometer

AIS3624DQ
Mechanical and electrical specifications
2.5
2.5.1
2.5.2
2.5.3
2.5.4
Terminology
Sensitivity
Sensitivity describes the gain of the sensor and can be determined, for example, by
applying 1 g acceleration to it. As the sensor can measure DC accelerations this can be
done easily by pointing the axis of interest towards the center of the Earth, noting the output
value, rotating the sensor by 180 degrees (pointing to the sky) and noting the output value
again. By doing so, ±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 time. The sensitivity tolerance
describes the range of sensitivities of a large population of sensors.
Zero-g level
Zero-g level offset (TyOff) describes the deviation of an actual output signal from the ideal
output signal if no acceleration is present. A sensor in a steady state on a horizontal surface
will measure 0 g for the X-axis and 0 g for the Y-axis whereas the Z-axis will measure 1 g.
The output is ideally 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 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 “Zero-g level change vs. temperature”.
Self-test
Self-test allows checking the sensor functionality without moving it. The self-test function is
off when the self-test bit (ST) of CTRL_REG4 (control register 4) is programmed to ‘0‘.
When the self-test bit of CTRL_REG4 is programmed to ‘1’, an actuation force is applied to
the sensor, simulating a definite input acceleration. In this case the sensor outputs will
exhibit a change in their DC levels which are related to the selected full scale through the
device sensitivity. When self-test is activated, the device output level is given by the
algebraic sum of the signals produced by the acceleration acting on the sensor and by the
electrostatic test-force. If the output signals change within the amplitude specified inside
Table 3, then the sensor is working properly and the parameters of the interface chip are
within the defined specifications.
Sleep-to-wake
The “sleep-to-wakeup” function, in conjunction with low-power mode, allows to further
reduce the system power consumption and develop new smart applications.
AIS3624DQ may be set in a low-power operating mode, characterized by lower data rate
updates. In this way the device, even if sleeping, continues to sense acceleration and
generate interrupt requests.
When the “sleep-to-wake” function is activated, AIS3624DQ 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, thus
ensuring power saving and flexibility.
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