Sensors

4

Freescale Semiconductor

MMA2260D

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 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

acceleration (Figure 3).

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 beams form two back-to-back capacitors (). As

the central mass moves with acceleration, the distance

between the beams change and each capacitor's value will

change, (C = NA

ε/D). Where A is the area of the facing side

of the beam, e is the dielectric constant, D is the distance

between the beams, and N is the number of beams.

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.

SPECIAL FEATURES

Filtering

Freescale accelerometers contain an onboard 2-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 that allows 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 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

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.

Acceleration

Figure 3. Transducer

Physical Model

Figure 4. Equivalent

Circuit Model