LIS2DS12 datasheet(18/68 Pages) STMICROELECTRONICS

18 / 68 page

Mechanical and electrical specifications

LIS2DS12

18/68

DocID027753 Rev 4

2.6

Terminology

2.6.1

Zero-g offset

Zero-g offset 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 on the X-axis and 0 g on 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 two’s complement number). A deviation from 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 offset change vs. temperature”.

2.6.2

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.

2.6.3

Self-test

The self-test allows checking the sensor functionality without moving it. The self-test

function is off when the self-test bits (ST) are programmed to ‘00’. When the self-test bits are

changed, 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 the 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.

2.7

Sensing element

A proprietary process is used to create a surface micromachined accelerometer. The

technology allows processing 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. In order 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 molding

phase of the plastic encapsulation. When an acceleration is applied to the sensor the proof

mass displaces from its nominal position, causing an imbalance in the capacitive half-

bridge. This imbalance is measured using charge integration in response to a voltage pulse

applied to the capacitor.

At steady-state the nominal value of the capacitors are a few pF and when an acceleration

is applied, the maximum variation of the capacitive load is in the fF range.

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