LIS3DSH datasheet(17/53 Pages) STMICROELECTRONICS

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LIS3DSH

Mechanical and electrical specifications

Doc ID 022405 Rev 1

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3.5

Terminology

3.5.1

Sensitivity

Sensitivity describes the gain of the sensor and can be determined e.g. by applying 1

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.

3.5.2

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 in X axis and 0 g in 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 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”. The Zero-

g level tolerance (TyOff) describes the standard

deviation of the range of Zero-

g levels of a population of sensors.

3.6

Functionality

3.6.1

Self-test

Self-test allows to check the sensor functionality 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. 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 in

Table 3, then the sensor is working properly and the parameters of the interface

chip are within the defined specifications.

3.7

Sensing element

A proprietary process is used to create a surface micro-machined accelerometer. The

technology allows to carry out suspended silicon structures which are attached to the

substrate in a few points called anchors and are free to move in the direction of the sensed

acceleration. To be compatible with the traditional packaging techniques, a cap is placed on

top of the sensing element to avoid blocking the moving parts during the moulding phase of

the plastic encapsulation.

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