Sensors

4

Freescale Semiconductor

MMA7368L

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

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

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. To use this feature to verify the 0g-Detect function,

the accelerometer should be held upside down so that the

z-axis experiences -1g. 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.

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 1.5g 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