Mechanical and electrical specifications

LIS331DLF

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Doc ID 15101 Rev 4

2.5

Terminology

2.5.1

Sensitivity

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

2.5.2

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

will measure 0

g in X axis and 0 g in 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 2’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 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.

2.5.3

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) of CTRL_REG4 (control register 4) is programmed to ‘0‘.

When the self-test bit of CTRL_REG4 is programmed to ‘1‘ 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 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 inside

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

within the defined specifications.

2.5.4

Sleep to wake-up

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

reduce the system power consumption and develop new smart applications.

LIS331DLF may be set in a low-power operating mode, characterized by lower date rates

refreshments. In this way the device, even if sleeping, keep on sensing acceleration and

generating interrupt requests.

When the “sleep to wake-up” function is activated, LIS331DLF 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, thus

ensuring power saving and flexibility.