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MMA7450L

±2g/±4g/±8g Three Axis Low-g Digital Output Accelerometer

Freescale Semiconductor 

PRINCIPLE OF OPERATION

The Freescale accelerometer is a surface-micromachined

integrated-circuit accelerometer. The device consists of a

surface micromachined capacitive sensing cell (g-cell) and a

signal conditioning ASIC contained in a single package. The

sensing element is sealed hermetically at the wafer level

using a bulk micromachined cap wafer. The g-cell is a

mechanical structure formed from semiconductor materials

(polysilicon) using semiconductor processes (masking and

etching). It can be modeled as a set of beams attached to a

movable central mass that move between fixed beams. The

movable beams can be deflected from their rest position by

subjecting the system to an acceleration (Figure 3).

As the beams attached to the central mass move, the

distance from them to the fixed beams on one side will

increase by the same amount that the distance to the fixed

beams on the other side decreases. The change in distance

is a measure of acceleration. The g-cell beams form two

back-to-back capacitors (Figure 3). As the center beam

moves with acceleration, the distance between the beams

changes and each capacitor's value will change, (C = Aε/D).

Where A is the area of the beam, ε is the dielectric constant,

and D is the distance between the beams.

The ASIC uses switched capacitor techniques to measure

the g-cell capacitors and extract the acceleration data from

the difference between the two capacitors. The ASIC also

signal conditions and filters (switched capacitor) the signal,

providing a high level digital output voltage that is ratiometric

and proportional to acceleration.

Acceleration

g-Select

The g-Select feature enables the selection between 3

sensitivities for measurement. Depending on the values in

the Mode control register ($16), the MMA7450L’s internal

gain will be changed allowing it to function with a 2g, 4g or 8g

measurement sensitivity. This feature is ideal when a product

has applications requiring two or more sensitivities for

optimum performance and for enabling multiple functions.

The sensitivity can be changed during the operation by

modifying the two GLVL bits located in the mode control

register.

$16: Mode control register (Read/Write)

D7 D6 D5

D4

D3

D2

D1

D0

Bit

-- DRPD SPI3W STON GLVL[1] GLVL[0] MODE[1] MODE[0] Function

0

0

0

0

0

0

0

0 Default

GLVL [1:0]

00: 8G is selected for measurement range.

01: 2G is selected for measurement range.

10: 4G is selected for measurement range.

Table 4. g-Select Description

g-Select

00

01

10

g-Range

8g

2g

4g

Sensitivity

16 LSB/g

64 LSB/g

32 LSB/g

Table 5. Mode Descriptions

MODE [0:1]

Function

00

Standby Mode

01

Measurement Mode

10

Level Detection Mode

11

Pulse Detection Mode

Figure 3. Simplified Transducer Physical Model

FEATURES

Self Test

The sensor provides a self test feature that allows the

verification of the mechanical and electrical integrity of the

accelerometer at any time before or after installation. This

feature is critical in applications such as hard disk drive

protection where system integrity must be ensured over the

life of the product. Customers can use self test to verify the

solderability to confirm that the part was mounted to the PCB

correctly. To use this feature to verify the 0g-Detect function,

the accelerometer should be held upside down so that the

z axis experiences -1g. When the self test function is initiated

through the mode control register, accessing the “self test”

bit, an electrostatic force is applied to each axis to cause it to

deflect. The z-axis is trimmed to deflect 1g. This procedure

assures that both the mechanical (g-cell) and electronic

sections of the accelerometer are functioning.

Sensors

Freescale Semiconductor

Standby Mode

This digital output 3-axis accelerometer provides a

standby mode that is ideal for battery operated products.

When standby mode is active, the device outputs are turned

off, providing significant reduction of operating current. When

the device is in standby mode the current will be reduced to

5 µA typical. In standby mode the device can read and write

to the registers with the I2C/SPI available, but no new

measurements can be taken in this mode as all current

consuming parts are off. The mode of the device is controlled

through the mode control register by accessing the two mode

bits as shown in Table 5.

Measurement Mode

During measurement mode, continuous measurements on

all three axes enabled. The g-range for 2g, 4g, or 8g are

selectable with 8-bit data and the g-range of 8g is selectable

with 10-bit data. The sample rate during measurement mode

is 125 Hz. Therefore, when a conversion is complete

(signaled by the DRDY flag), the next measurement will be

ready 8 ms later.

MMA7450L

5

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