LIS332AX

Functionality

Doc ID 16932 Rev 1

9/14

5

Functionality

The LIS332AX is a 3-axis ultracompact low-power, analog output linear accelerometer

packaged in an LGA package. The complete device includes a sensing element and an IC

interface capable of taking information from the sensing element providing an analog signal

to the external world.

5.1

Sensing element

A proprietary process is used to create a surface micro-machined accelerometer. The

technology allows the creation of suspended silicon structures which are attached to the

substrate at several points called “anchors” and are free to move in the direction of the

sensed acceleration. To be compatible with traditional packaging techniques, a cap is

placed on top of the sensing element to prevent blocking of the moving parts during the

moulding phase of plastic encapsulation.

When an acceleration is applied to the sensor, the proof mass shifts 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 a few pF, and when an acceleration

is applied the maximum variation of the capacitive load is in the fF range.

5.2

IC interface

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

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

applications.

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 signal produced 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 absolute with respect to

the voltage supply. Increasing or decreasing the voltage supply will not cause a change in

the sensitivity and the offset. The feature allows the coupling of the sensor with an ADC,

having a fixed voltage reference independent from Vdd.

5.3

Factory calibration

The IC interface is factory-calibrated for sensitivity (So) and Zero-

g level (Voff). The trimming

values are stored in the device in a non-volatile structure. Any time the device is turned on,

the trimming parameters are downloaded to the registers to be employed during normal

operation. This allows the user to use the device without further calibration.