XMMA1000P XMMA2000W

5

Motorola Sensor Device Data

PRINCIPLE OF OPERATION

The Motorola accelerometer is a surface–micromachined

integrated–circuit accelerometer.

The device consists of a surface micromachined capaci-

tive 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 semi-

conductor materials (polysilicon) using semiconductor pro-

cesses (masking and etching). It consists of two stationary

plates with a moveable plate in–between. The center plate

can be deflected from its rest position by subjecting the sys-

tem to an acceleration (Figure 1).

When the center plate deflects, the distance from it to one

fixed plate will increase by the same amount that the dis-

tance to the other plate decreases. The change in distance is

a measure of acceleration.

The G–Cell plates form two back–to–back capacitors

(Figure 2). As the center plate moves with acceleration, the

distance between the plates changes and each capacitor’s

value will change, (C = A

ε/D). Where A is the area of the

plate,

ε is the dielectric constant, and D is the distance

between the plates.

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 ratio-

metric and proportional to acceleration.

Acceleration

Figure 1.

Figure 2.

SPECIAL FEATURES

Filtering

The Motorola accelerometers contain an onboard 4–pole

switched capacitor filter. A Bessel implementation is used

because it provides a maximally flat delay response (linear

phase) thus preserving pulse shape integrity. Because the fil-

ter is realized using switched capacitor techniques, there is

no requirement for external passive components (resistors

and capacitors) to set the cut–off frequency.

Noise Calculation

The noise for the Motorola accelerometer is specified as

an rms value which is a statistical value of a gaussian noise

source. To convert the rms values to a peak to peak value at

a particular confidence level refer to Table 1. A sample cal-

culation at a 99.9% confidence level is shown.

Table 1.

Nominal Peak to Peak Value

% Confidence Level

2.0

rms

68%

3.0

rms

87%

4.0

rms

95.40%

5.0

rms

98.80%

6.0

rms

99.73%

6.6

rms

99.90%

Noise rms = 3.5mVrms

Noise peak to peak at a 99.9% confidence level:

3.5mVrms* 6.6 = 23.1mVpp

Self–Test

XMMA sensors provide 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 automotive airbag

systems where system integrity must be ensured over the life

of the vehicle. A fourth “plate’’ is used in the g–cell as a self–

test plate. This plate is fixed and is located under an ex-

tended portion of the center (moveable) plate. When the user

applies a logic high input to the self–test pin, a calibrated po-

tential is applied across the self–test plate and the moveable

plate. The resulting electrostatic force (Fe = 1/2 AV2/d2)

causes the center plate to deflect. The resultant deflection, is

measured by the accelerometer’s control ASIC and a propor-

tional output voltage results. This procedure assures that

both the mechanical (g–cell) and electronic sections of the

sensor are functioning.

Ratiometricity

The XMMA1000P and XMMA2000W are designed to be

“ratiometric’’. That is, their transfer function will be propor-

tional to the applied supply voltage. This feature allows easy

interfacing to common microcontrollers that use ratiometric

A/D converters for system cost benefits.

In operation, a ratiometric sensor’s gain or “sensitivity’’ will

change 1:1 with applied supply voltage and the zero signal

output will be at midsupply. (2.5 V for a 5 V VDD and 2.625 V

for a 5.25 VDD).

Minimum G Range Calculation

To calculate the minimum g range values of an accelerom-

eter several factors have to be taken into consideration.

These considerations include, the supply voltage, the

device’s sensitivity, offset voltage and output rail. A sample

calculation for the minimum g range is shown below.

To complete the calculation the rail and offset voltages

must be subtracted from the supply voltage, then divided by

the supply voltage multiplied by the device’s worst case

(highest) sensitivity.

V

V

+

0.44V

V

+ 50.93 * 34.72

V

DD

g

Using the standard five volt power supply, the minimum g

range is calculated to be:

50.926

* 34.722

5.00

+ 43.98 [ 44g