MPXM2102AS datasheet(100/670 Pages) MOTOROLA

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Motorola Sensor Device Data

www.motorola.com/semiconductors

Figure 4. Aliasing Comparison

(a)

(b)

(c)

SAMPLING

FREQUENCY

INJECTED SIGNAL

FREQUENCY

1.0E+0

1.0E–1

1.0E–3

1.0E–2

1.0E–4

1.0E–5

1.0E–6

41.0

41.2

41.4

41.6

41.8

42.0

FREQUENCY (kHz)

Vout

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

INJECTED SIGNAL

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

Points to note:

• Under clean dc bias, VoutandVCC,Figures3aand3bhave

a signal component at the sampling rate. This is due to

switched capacitor currents coupling through finite power

supply source impedances and PCB paracitics.

• The low frequency output spectrum, Figure 3c, displays the

internal lowpass filter characteristics. (The filter and sam-

pling characteristics are sometimes useful in system de-

bugging.)

• When an ac component is superimposed onto VCC near

the sampling frequency, as shown in Figure 4b, the output

will contain the original signal plus a mirrored signal about

the sampling frequency, shown in Figure 4a. Signals on the

VCC line will appear at the output due to the ratiometric

characteristic of the accelerometer and will be one half the

amplitude.

• As a result of sampling, the output waveform of Figure 4c

is produced where the injected high frequency signal has

now produced a signal in the passband.

• Harmonics of the aliased signal in the pass band are also

shown in Figure 4c.

• Aliased signals in the passband will be amplified versions

of the injected signals. This is due to the signal conditioning

circuitry in the accelerometer that includes gain.

ALIASING AVOIDANCE KEYS

• Use a linear regulated power source when feasible. Linear

regulators have excellent power supply rejection offering a

stable dc source.

• If using a switching power supply, ensure that the switching

frequency is not close to the accelerometer sampling fre-

quency or its harmonics. Noting that the accelerometer will

gain the aliasing signal, it is desirable to keep frequencies

at least 4 kHz away from the sampling frequency and its

harmonics. 4 kHz is one decade from the –3 dB frequency,

therefore any signals will be sufficiently attenuated by the

internal 4–pole lowpass filter.

• Proper bias decoupling will aid in noise reduction from oth-

er sources. With dense surface mount PCB assemblies, it

is often difficult to place and route decoupling components.

However, the accelerometer is not like a typical logic de-

vice. A little extra effort on decoupling goes a long way.

• Good PCB layout practices should always be followed.

Proper system grounding is essential. Parasitic capaci-

tance and inductance could prove to be troublesome, par-

ticularly during EMC testing. Signal harmonics and

sub–harmonics play a significant role in introducing aliased

signals. Clean layouts minimize the effects of parasitics

and thus signal harmonics and sub–harmonics.

Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com

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