MMA7260Q

Sensors

2-4

Freescale Semiconductor

PRINCIPLE OF OPERATION

The Freescale accelerometer is a surface-micromachined

integrated-circuit accelerometer.

The device consists of two surface micromachined

capacitive sensing cells (g-cell) and a signal conditioning

ASIC contained in a single integrated circuit package. The

sensing elements are 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

g-Select

The g-Select feature allows for the selection among 4

sensitivities present in the device. Depending on the logic

input placed on pins 1 and 2, the device internal gain will be

changed allowing it to function with a 1.5g, 2g, 4g, or 6g

sensitivity (Table 3). This feature is ideal when a product has

applications requiring different sensitivities for optimum

performance. The sensitivity can be changed at anytime

during the operation of the product. The g-Select1 and g-

Select2 pins can be left unconnected for applications

requiring only a 1.5g sensitivity as the device has an internal

pulldown to keep it at that sensitivity (800mV/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 12

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

the current to 3uA typ. For lower power consumption, it is

recommended to set g-Select1 and g-Select2 to 1.5g mode.

By placing a high input signal on pin 12, the device will

resume to normal mode of operation.

Filtering

The 3 axis accelerometer contains onboard single-pole

switched capacitor filters. 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 Descriptions

g-Select2

g-Select1

g-Range

Sensitivity

0

0

1.5g

800mV/g

0

1

2g

600mV/g

1

0

4g

300mV/g

1

1

6g

200mV/g