LIS2L02AL

3 Functionality

CD00068420

9/17

3

Functionality

The LIS2L02AL is a high performance, low-power, analog output 2-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 up to 100fF.

3.2

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 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 system (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. Increasing 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.

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.