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MMA1260KEGR2

Low G Micromachined Accelerometer

NXP Semiconductors. 

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 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

semiconductor materials (polysilicon) using semiconductor

processes (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

subjecting the system to an acceleration (Figure 3).

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

fixed plate will increase by the same amount that the distance

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 4). As the center plate moves with acceleration, the

distance 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

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

ratiometric and proportional to acceleration.

Acceleration

SPECIAL FEATURES

Filtering

The Freescale accelerometers contain an onboard 2-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

filter 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 electrostatic

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

resultant deflection is measured by the accelerometer'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.

Status

Freescale 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 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 3. Transducer

Physical Model

Figure 4. Equivalent

Circuit Model

MMA1260KEG

4

Sensors

Freescale Semiconductor

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