MMA1260D_04 データシートの表示（PDF） - Motorola => Freescale

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MMA1260D_04

Low G Micromachined Accelerometer

Motorola => Freescale 

PRINCIPLE OF OPERATION

The Freescale Semiconductor accelerometer is a surface--

micromachined integrated--circuit accelerometer.

The device consists of a surface micromachined capaci-

tive sensing cell (g--cell) and a CMOS signal conditioning

ASIC contained in a single integrated circuit package. The

sensing element is sealed hermetically at the wafer level us-

ing a bulk micromachined “cap’’ wafer.

The g--cell is a mechanical structure formed from semicon-

ductor materials (polysilicon) using semiconductor pro-

cesses (masking and etching). It can be modeled as two

stationary plates with a moveable plate in--between. The

center plate can be deflected from its rest position by sub-

jecting the system to an acceleration (Figure 2).

When the center plate deflects, the distance from it to one

fixed plate will increase by the same amount that the dis-

tance to the other plate decreases. The change in distance is

a measure of acceleration.

The g--cell plates form two back--to--back capacitors (Fig-

ure 3). As the center plate moves with acceleration, the dis-

tance between the plates changes and each capacitor’s

value will change, (C = Aε/D). Where A is the area of the

plate, ε is the dielectric constant, and D is the distance be-

tween the plates.

The CMOS 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 output voltage that is ratio-

metric and proportional to acceleration.

Acceleration

SPECIAL FEATURES

Filtering

The Freescale Semiconductor accelerometers contain an

onboard 2--pole switched capacitor filter. A Bessel imple-

mentation is used because it provides a maximally flat delay

response (linear phase) thus preserving pulse shape integri-

ty. Because the filter is realized using switched capacitor

techniques, there is no requirement for external passive

components (resistors and capacitors) to set the cut--off fre-

quency.

Self--Test

The sensor provides a self--test feature that allows the ver-

ification of the mechanical and electrical integrity of the ac-

celerometer at any time before or after installation. A fourth

“plate’’ is used in the g--cell as a self--test plate. When the

user applies a logic high input to the self--test pin, a cali-

brated potential is applied across the self--test plate and

the moveable plate. The resulting electrostatic force

(Fe = 1/2 AV2/d2) causes the center plate to deflect. The re-

sultant deflection is measured by the accelerometer’s control

ASIC and a proportional output voltage results. This proce-

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

sections of the accelerometer are functioning.

Status

Freescale Semiconductor accelerometers include fault

detection circuitry and a fault latch. The Status pin is an out-

put from the fault latch, OR’d with self--test, and is set high

whenever the following event occurs:

• Parity of the EPROM bits becomes odd in number.

The fault latch can be reset by a rising edge on the self--

test input pin, unless one (or more) of the fault conditions

continues to exist.

Figure 2. Transducer

Physical Model

Figure 3. Equivalent

Circuit Model

MMA1260D

4

Sensor Device Data

Freescale Semiconductor

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