LIS2L02AS4

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4

Functionality

The LIS2L02AS4 is a high performance, low-power, analog output two axes linear accelerometer pack-

aged in a SO24 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 pack-

aging 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 re-

sponse 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 process-

ing chain uses a fully differential structure. The final stage converts the differential signal into a single-end-

ed one to be compatible with the external world.

The signals of the sensing element are multiplexed and fed into a low-noise capacitive charge amplifier

that implements a Correlated Double Sampling (CDS) at its output to cancel the offset and the 1/f noise.

The output signal is de-multiplexed and transferred to two different S&Hs, one for each channel and made

available to the outside.

The low noise input amplifier operates at 200 kHz while the two S&Hs operate at a sampling frequency of

66 kHz. This allows a large oversampling ratio, which leads to in-band noise reduction and to an accurate

output waveform.

All the analog parameters (zero-g level, sensitivity and self-test) are ratiometric to the supply voltage. In-

creasing or decreasing the supply voltage, the sensitivity and the offset will increase or decrease almost

linearly. The self test voltage change varies cubically with the supply voltage.

4.3 Factory calibration

The IC interface is factory calibrated for Sensitivity (So) and Zero-g Level (Voff). The trimming values are

stored inside the device by a non volatile structure. Any time the device is turned on, the trimming param-

eters are downloaded into the registers to be employed during the normal operation. This allows the user

to employ the device without further calibration.