MMA6231Q

Sensors

4

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 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 a set

of beams attached to a movable central mass that moves

between fixed beams. The movable beams can be deflected

from their rest position by subjecting the system to an

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 plates form two back-to-back capacitors

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 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.

SPECIAL FEATURES

Filtering

These Freescale accelerometers contain an onboard

single-pole switched capacitor filter. 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 allowing the

verification of the mechanical and electrical integrity of the

accelerometer at any time before or after installation. A fourth

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

high input to the self-test pin is applied, a calibrated potential

is applied across the self-test plate and the moveable plate.

center plate to deflect. The resultant deflection is measured

by the accelerometer's ASIC and a proportional output

voltage results. This procedure assures both the mechanical

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

functioning.

Freescale accelerometers include fault detection circuitry

and a fault latch. Parity of the EEPROM bits becomes odd in

number.

Self-test is disabled when EEPROM parity error occurs.

Ratiometricity

Ratiometricity simply means the output offset voltage and

sensitivity will scale linearly with applied supply voltage. That

is, as supply voltage is increased, 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.

Acceleration

Figure 3. Transducer

Physical Model

Figure 4. Equivalent

Circuit Model