4

Surface Mount Micromachined Accelerometer

MOTOROLA

MMA1210D

PRINCIPLE OF OPERATION

The Motorola accelerometer is a surface-microma-

chined integrated-circuit accelerometer.

The device consists of a surface micromachined ca-

pacitive sensing cell (g-cell) and a CMOS signal condi-

tioning 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 semi-

conductor 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 2).

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 dis-

tance 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 ca-

pacitor'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 acceler-

ation data from the difference between the two capacitors.

The ASIC also signal conditions and filters (switched ca-

pacitor) the signal, providing a high level output voltage

that is ratiometric and proportional to acceleration.

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. Be-

cause the filter is realized using switched capacitor tech-

niques, 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 ap-

plied across the self-test plate and the moveable plate.

the center plate to deflect. The resultant deflection is

measured by the accelerometer's control ASIC and a

proportional output voltage results. This procedure as-

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

sections of the accelerometer are functioning.

Ratiometricity

Ratiometricity simply means that the output offset volt-

age and sensitivity will scale linearly with applied supply

voltage. That is, as you increase supply voltage the sen-

sitivity and offset increase linearly; as supply voltage de-

creases, 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 cancella-

tion of supply induced errors in the analog to digital con-

version process.

Status

Motorola accelerometers include fault detection cir-

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

the fault latch, OR'd with self-test, and is set high when-

ever 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.

Acceleration

Figure 2. Transducer

Physical Model

Figure 3. Equivalent

Circuit Model

Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com