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LIS3L02AQ3
3.1 Terminology
Sensitivity describes the gain of the sensor and can be determined by applying 1g 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, note the output value, rotate the sensor by 180 degrees (point to the sky) and note
the output value again thus applying ±1g acceleration to the sensor. Subtracting the larger output value
from the smaller one and dividing the result by 2 will give the actual sensitivity of the sensor. This value
changes very little over temperature (see sensitivity change vs. temperature) and also very little over time.
The Sensitivity Tolerance describes the range of Sensitivities of a large population of sensors.
Zero-g level describes the actual output signal if there is no acceleration present. A sensor in a steady
state on a horizontal surface will measure 0g in X axis and 0g in Y axis whereas the Z axis will measure
+1g. The output is ideally for a 3.3V powered sensor Vdd/2 = 1650mV. A deviation from ideal 0-g level
(1650mV in this case) is called Zero-g offset. Offset of precise MEMS sensors is to some extend a result
of stress to the 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 of an individual sensor is very stable over lifetime.
The Zero-g level tolerance describes the range of zero-g levels of a population of sensors.
Self Test allows to test the mechanical and electrical part of the sensor. By applying a digital signal to the
ST input pin an internal reference is switched to a certain area of the sensor and creates a defined deflec-
tion of the moveable structure. The sensor will generate a defined signal and the interface chip will perform
the signal conditioning. If the output signal changes with the specified amplitude than the sensor is working
properly and the parameters of the interface chip are within the defined specifications.
Output impedance describes the resistor inside the output stage of each channel. This resistor is part of
a filter consisting of an external capacitor of at least 320pF and the internal resistor. Due to the high resis-
tor level only small, inexpensive external capacitors are needed to generate low corner frequencies. When
interfacing with an ADC it is important to use high input impedance input circuitries to avoid measurement
errors. Note that the minimum load capacitance forms a corner frequency beyond the resonance frequen-
cy of the sensor. For a flat frequency response a corner frequency well below the resonance frequency is
recommended. In general the smallest possible bandwidth for an particular application should be chosen
to get the best results.
4 Functionality
The LIS3L02AQ3 is a high performance, low-power, analog output 3-Axis linear accelerometer packaged in a
QFN package. The complete device includes a sensing element and an IC interface able to take the information
from the sensing element and to provide an analog signal to the external world.
4.1 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.
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 sense capacitor.
At steady state the nominal value of the capacitors are few pF and when an acceleration is applied the maximum
variation of the capacitive load is up to 100fF.
4.2 IC Interface
In order to increase robustness and immunity against external disturbances the complete signal processing
chain uses a fully differential structure. The final stage converts the differential signal into a single-ended one to
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