Sensors

8

Freescale Semiconductor

MMA7455L

*The bandwidth for detecting interrupts in level and pulse is 600Hz which is changed from measurement mode.

PRINCIPLE OF OPERATION

The Freescale accelerometer is a surface-micromachined integrated-circuit accelerometer. The device consists of a surface mi-

cromachined capacitive sensing cell (g-cell) and a signal conditioning ASIC contained in a single 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 move 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 accel-

eration. The g-cell beams form two back-to-back capacitors (Figure 3). As the center beam moves with acceleration, the distance

between the beams changes and each capacitor's value will change, (C = A

electric constant, and D is the distance between the beams.

The ASIC uses switched capacitor techniques to measure the g-cell capacitors and extract the acceleration data from the differ-

ence between the two capacitors. The ASIC also signal conditions and filters (switched capacitor) the signal, providing a digital

output that is proportional to acceleration.

Figure 3. Simplified Transducer Physical Model

Table 4. Function Parameters for Detection

Characteristic

Symbol

Min

Typ

Max

Unit

Level Detection

Detection Threshold Range

0

—

FS

g

Pulse Detection

Pulse detection range (Adjustable range)

Time step for pulse detection

Threshold range for pulses

Detection levels for threshold

Latency timer (Adjustable range)

Time Window (Adjustable range)

Bandwidth for detecting interrupt*

Time step for latency timer and time window

0.5

—

0

—

1

1

—

—

—

0.5

—

127

—

—

600

1

127

—

FS

—

150

250

—

—

ms

ms

g

Counts

ms

ms

Hz

ms

Acceleration