H3LIS331DL datasheet(14/38 Pages) STMICROELECTRONICS

14 / 38 page

Mechanical and electrical specifications

H3LIS331DL

14/38

DocID023111 Rev 2

2.5

Terminology

2.5.1

Sensitivity

Sensitivity describes the gain of the 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 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.

2.5.2

Zero-g level

The 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 measures 0 g for the X-axis and 0 g for the Y-axis whereas the Z-axis measures 1 g.

The output is ideally in the middle of the dynamic range of the sensor (content of OUT

registers 00h, data expressed as two’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 can slightly change after mounting the sensor onto a printed

circuit board or exposing it to extensive mechanical stress. Offset changes little over

temperature, refer to “Zero-g level change vs. temperature” (see TCOff in Table 3). The

zero-g level tolerance (TyOff) describes the standard deviation of the range of zero-g levels

of a population of sensors.

2.5.3

Sleep-to-wakeup

The “sleep-to-wakeup” function, in conjunction with low-power mode, allows to further

reduce the system power consumption and develop new smart applications.

The H3LIS331DL may be set in a low-power operating mode, characterized by lower date

rate refreshments. In this way the device, even if sleeping, continues to sense acceleration

and generate interrupt requests.

When the “sleep-to-wakeup” function is activated, the H3LIS331DL 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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