IIS2DH datasheet(16/49 Pages) STMICROELECTRONICS

16 / 49 page

Mechanical and electrical specifications

IIS2DH

16/49

DocID027668 Rev 2

2.6

Terminology and functionality

Terminology

2.6.1

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.

2.6.2

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 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 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 on

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

over temperature, see Table 3 “Zero-g level change vs. temperature” (TCOff). The zero-g

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

population of sensors.

Functionality

2.6.3

High-resolution, normal mode, low-power mode

The IIS2DH provides three different operating modes: high-resolution mode, normal mode

and low-power mode.

The table below summarizes how to select the different operating modes.

Table 9. Operating mode selection

Operating mode

CTRL_REG1[3]

(LPen bit)

CTRL_REG4[3]

(HR bit)

BW [Hz]

Turn-on

time [ms]

So @ ±2 g

[mg/digit]

Low-power mode

(8-bit data output)

ODR/2

Normal mode

(10-bit data output)

ODR/2

1.6

High-resolution mode

(12-bit data output)

ODR/9

7/ODR

Not allowed

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