LIS331AL

Functionality

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3

Functionality

The LIS331AL is a nano low-power, analog output three-axis linear accelerometer packaged

in a LGA 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.

3.1

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 in fF range.

3.2

IC Interface

The complete signal processing uses a fully differential structure, while the final stage

converts the differential signal into a single-ended one to be compatible with the external

world.

The first stage is a low-noise capacitive amplifier that implements a Correlated Double

Sampling (CDS) at its output to cancel the offset and the 1/f noise. The produced signal is

then sent to three different S&Hs, one for each channel, and made available to the outside.

All the analog parameters (output offset voltage and sensitivity) are ratiometric to the

voltage supply. Increasing or decreasing the voltage supply, the sensitivity and the offset will

increase or decrease linearly. The feature provides the cancellation of the error related to

the voltage supply along an analog to digital conversion chain.

3.3

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 parameters are downloaded into the registers to be

employed during the normal operation. This allows the user to employ the device without

further calibration.