MPXM2102AS datasheet(57/670 Pages) MOTOROLA

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MMA1220D

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Motorola Sensor Device Data

www.motorola.com/semiconductors

PRINCIPLE OF OPERATION

The Motorola 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

using 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

(Figure 3). As the center plate moves with acceleration, the

distance between the plates changes and each capacitor’s

value will change, (C = A

ε/D). Where A is the area of the

plate,

ε is the dielectric constant, and D is the distance

between 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

Figure 2. Transducer

Physical Model

Figure 3. Equivalent

Circuit Model

SPECIAL FEATURES

Filtering

The Motorola accelerometers contain an onboard 4–pole

switched capacitor filter. A Bessel implementation is used

because it provides a maximally flat delay response (linear

phase) thus preserving pulse shape integrity. Because the fil-

ter is realized using switched capacitor techniques, there is

no requirement for external passive components (resistors

and capacitors) to set the cut–off frequency.

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 automotive airbag

systems where system integrity must be ensured over the life

of the vehicle. 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 calibrated potential is applied across the

self–test plate and the moveable plate. The resulting elec-

trostatic force (Fe = 1/2 AV2/d2) causes the center plate to

deflect. The resultant deflection is measured by the accel-

erometer’s control ASIC and a proportional output voltage

results. This procedure assures that both the mechanical

(g–cell) and electronic sections of the accelerometer are

functioning.

Ratiometricity

Ratiometricity simply means that the output offset voltage

and sensitivity will scale linearly with applied supply voltage.

That is, as you increase supply voltage the sensitivity and

offset increase linearly; as supply voltage decreases, offset

and sensitivity decrease linearly. This is a key feature when

interfacing to a microcontroller or an A/D converter because

it provides system level cancellation of supply induced errors

in the analog to digital conversion process.

Status

Motorola accelerometers include fault detection circuitry

and a fault latch. The Status pin is an output from the fault

latch, OR’d with self–test, and is set high whenever one (or

more) of the following events occur:

• Supply voltage falls below the Low Voltage Detect (LVD)

voltage threshold

• Clock oscillator falls below the clock monitor minimum

frequency

• 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.

Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com

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