AN2960 datasheet(6/41 Pages) STMICROELECTRONICS

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Theory of operation

AN2960

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Doc ID 15557 Rev 1

Theory of operation

The LIS331DL is an ultra compact, low-power, digital output 3-axis 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 and providing a signal to

external applications through an I2C/SPI serial interface (Figure 1).

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 a few 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 avoid the blockage 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.

In steady state, the nominal value of the capacitors are a few pico farads, and when an

acceleration is applied the maximum variation of the capacitive load is of a few femto farads.

The complete measurement chain is composed of a low-noise capacitive amplifier which

converts the capacitive unbalancing of the MEMS sensor into an analog voltage, and by

analog-to-digital converters.

The acceleration data may be accessed through an I2C/SPI interface, making the device

particularly suitable for direct interfacing with a microcontroller.

Data synchronization in a digital system employing the device is made simpler through the

use of the DataReady signal (RDY), which indicates when a new set of measured

acceleration data is available, thus simplifying data synchronization in the digital system

employing the device.

The LIS331DL also features two independent, fully-programmable interrupt sources which

can be programmed to generate an interrupt signal when a programmable acceleration

threshold is exceeded along one of the three axes, or to detect a free-fall event.

The IC interface is factory calibrated for sensitivity (So) and zero-g level (Off).

The trimming values are stored in the device by a non-volatile structure. Any time the device

is turned on, the trimming parameters are loaded into the registers to be employed during

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

Figure 1.

Device block diagram

CHARGE

AMPLIFIER

MUX

Y+

Z+

Y-

Z-

X+

X-

I2C

SPI

SCL/SPC

SDA/SDO/SDI

SDO

CONTROL LOGIC

INTERRUPT GEN.

INT 1

CLOCK

TRIMMING

CIRCUITS

REFERENCE

SELF TEST

CONTROL LOGIC

A/D

CONVERTER

INT 2

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