AN2335 datasheet(6/40 Pages) STMICROELECTRONICS

6 / 40 page

Theory of operation

AN2335

6/40

Theory of operation

The LIS302DL is an ultracompact, low-power, digital output 3-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 a signal to the

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

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 pico Farad and when an

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

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

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

by analog-to-digital converters.

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

device particularly suitable for direct interfacing with a microcontroller.

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

usage of the Data-Ready signal (RDY) which indicates when a new set of measured

acceleration data is available thus simplifying data synchronization in digital system

employing the device itself.

The LIS302DL features also 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 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.

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

AN2335 Datasheet(PDF) 6 Page - STMicroelectronics

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