LIS3DETR datasheet(15/46 Pages) STMICROELECTRONICS

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LIS3DE

Terminology and functionality

3.1

Terminology

3.1.1

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 also time. The sensitivity

tolerance describes the range of sensitivities of a large population of sensors.

3.1.2

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

measures 0 g on the X-axis and 0 g on 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 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”. The Zero-g level tolerance (TyOff) describes the

standard deviation of the range of Zero-g levels of a population of sensors.

3.2

Functionality

3.2.1

Normal mode, low-power mode

The LIS3DE provides two different operating modes: normal mode and low-power mode.

Table 9 summarizes how to select the operating mode.

3.2.2

Self-test

The self-test allows the sensor functionality to be checked without moving it. The self-test

function is off when the self-test bit (ST) is programmed to ‘0’. When the self-test bit is

programmed to ‘1’, an actuation force is applied to the sensor, simulating a definite input

acceleration. In this case the sensor outputs exhibit a change in their DC levels which are

related to the selected full scale through the device sensitivity.

Table 9. Operating mode selection

CTRL_REG1 [3] (LPen bit)

Operating mode

Low-power mode

Normal mode

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