Sensors

4

Freescale Semiconductor

MMA7331LC

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 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 acceleration.

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

ε/D). Where A is the area of the

beam,

ε is the dielectric 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 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.

Figure 3. Simplified Transducer Physical Model

SPECIAL FEATURES

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 hard disk drive

protection where system integrity must be ensured over the

life of the product. Customers can use self test to verify the

solderability to confirm that the part was mounted to the PCB

correctly. When the self test function is initiated, an

electrostatic force is applied to each axis to cause it to deflect.

The X- and Y-axis are deflected slightly while the Z-axis is

trimmed to deflect 1g. This procedure assures that both the

mechanical (g-cell) and electronic sections of the

accelerometer are functioning.

g-Select

The g-Select feature allows for the selection between two

sensitivities. Depending on the logic input placed on pin 10,

the device internal gain will be changed allowing it to function

with a 4g or 9g sensitivity (Table 3). This feature is ideal when

a product has applications requiring two different sensitivities

for optimum performance. The sensitivity can be changed at

anytime during the operation of the product. The g-Select pin

can be left unconnected for applications requiring only a 4g

sensitivity as the device has an internal pull-down to keep it

at that sensitivity (308 mV/g).

Sleep Mode

The 3-axis accelerometer provides a Sleep Mode that is

ideal for battery operated products. When Sleep Mode is

active, the device outputs are turned off, providing significant

reduction of operating current. A low input signal on pin 7

(Sleep Mode) will place the device in this mode and reduce

the current to 3

μA typ. For lower power consumption, it is

recommended to set g-Select to 4g mode. By placing a high

input signal on pin 7, the device will resume to normal mode

of operation.

Filtering

The 3-axis accelerometer contains 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.

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

Table 3. g-Select Pin Description

g-Select

g-Range

Sensitivity

0

4g

308 mV/g

1

9g

83.6 mV/g