MPXM2102AS datasheet(98/670 Pages) MOTOROLA

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Motorola Sensor Device Data

www.motorola.com/semiconductors

AN1559

Application Considerations for a

Switched Capacitor Accelerometer

By Wayne Chavez

INTRODUCTION

Today’s low cost accelerometers are highly integrated

devices employing features such as signal conditioning,

filtering, offset compensation and self test. Combining this

feature set with economical plastic packaging requires that the

signal conditioning circuitry be as small as possible. One

approach is to implement sampled data system and switched

capacitor techniques as in the Motorola accelerometer.

As in all sampled data systems, precautions should be

taken to avoid signal aliasing errors. This application note

describes the Motorola accelerometer and how signal aliasing

can be introduced and more importantly minimized.

BACKGROUND

What is aliasing? Simply put, aliasing is the effect of

sampling a signal at an insufficient rate, thus creating another

signal at a frequency that is the difference between the original

signal frequency and the sampling rate. A graphical

explanation of aliasing is offered in Figure 1. In this figure, the

upper trace shows a 50 kHz sinusoidal waveform. Note that

when sampled at a 45 kHz rate, denoted by the boxes, a

sinusoidal pattern is formed. Lowpass filtering the sampled

points, to create a continuous signal, produces the 5 kHz

waveform shown in Figure 1 (lower). (The phase shift in the

lower figure is due to the low–pass filter).

Aliased signals, like the one in Figure 1 (lower) are often

unintentionally produced. Signal processing techniques are

well understood and sampling rates are chosen appropriately

(i.e. Nyquist criteria). However, the assumption is that the

signals of interest are well characterized and have a limited

bandwidth. This assumption is not always true, as in the case

of wideband noise.

Figure 1. Aliased Signals

MOTOROLA

SEMICONDUCTOR APPLICATION NOTE

REV 1

Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com

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