MPXM2202GS datasheet(99/670 Pages) MOTOROLA

99 / 670 page

AN1559

2–63

Motorola Sensor Device Data

www.motorola.com/semiconductors

Given the brief example on how aliasing can occur, how

does the accelerometer relate to aliasing? To answer this

question, a brief summary on how the accelerometer works is

in order.

The accelerometer is a two chip acceleration sensing

solution. The first chip is the acceleration transducer, termed

G–Cell, constructed by Micro Electro–Mechanical Systems

(MEMS) technology. The G–Cell is a two capacitor element

where the capacitors are in series and share a common center

plate. The deflection in the center plate changes the capaci-

tance of each capacitor which is measured by the second chip,

termed control chip.

The control chip performs the signal conditioning (amplifica-

tion, filtering, offset level shift) function in the system. This chip

measures the G–Cell output using switched capacitor tech-

niques. By the nature of switched cap techniques, the system

is a sampled data system operating at sampling frequency fs.

The filter is switched capacitor, 4–pole Bessel implementation

with a –3 dB frequency of 400 Hz.

As a sampled data system, the accelerometer is not immune

to signal aliasing. However, given the accelerometer’s internal

filter, aliased signals will only appear in the output passband

when input signals are in the range | n

•fs–fsignal|≤fBW.Where

fs is the sampling rate, fSignal is the input signal frequency, fBW

is the filter bandwidth and n is a positive integer to account for

all harmonics. The graphical representation is shown in Figure

2. The bounds can be extended beyond fBW to ensure an alias

free output.

Figure 2. Input signal frequency range where a signal

will be produced in the output passband.

KEEP OUT ZONE

n*fs – fBW

n*fs

n*fs + fBW

ACCELEROMETER INPUT SIGNALS

The accelerometer is a ratiometric electro–mechanical

transducer. Therefore, the input signals to the device are the

acceleration and the input power source.

The acceleration input is limited in frequency bandwidth by

the geometry of the sensing, packaging, and mounting

structures that define the resonant frequency and response.

This response is in the range of 10 kHz, however, the practical

range is less than 600 Hz for most mechanical systems.

Therefore, aliasing an acceleration signal is unlikely.

The power input signal is ideally dc. However, depending on

the application system architecture, the power supply line can

be riddled with high frequency components. For example, dc

to dc converters can operate with switching frequencies

between 20 kHz and 200 kHz. This range encompasses the

sampling rate of the accelerometer and point to the power

source as the culprit in producing aliased signal.

DEMONSTRATION OF ALIASING

Under zero acceleration conditions a 100 mVrms signal was

injected onto the power supply line of 5.0 Vdc. The frequency

of the injected signal was tuned in to produce an alias in the

accelerometer’s passband. Figures 3 and 4 show the

difference in output when a high frequency signal is not and is

present on the VCC pin of the accelerometer.

Figure 3. Normal Waveforms

(a)

(b)

(c)

1.0E+0

1.0E–1

1.0E–3

1.0E–2

1.0E–4

1.0E–5

1.0E–6

1.0E–7

41.0

41.2

41.4

41.6

41.8

42.0

FREQUENCY (kHz)

Vout

SAMPLING

FREQUENCY

1.0E+0

1.0E–1

1.0E–3

1.0E–2

1.0E–4

1.0E–5

1.0E–6

1.0E–7

41.0

41.2

41.4

41.6

41.8

42.0

FREQUENCY (kHz)

VCC

SAMPLING

FREQUENCY

1.0E+0

1.0E–1

1.0E–3

1.0E–2

1.0E–4

1.0E–5

1.0E–6

200

400

600

800

1000

FREQUENCY (Hz)

Vout

Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com

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