MPXM2202GS datasheet(124/670 Pages) MOTOROLA

124 / 670 page

AN1632

2–88

Motorola Sensor Device Data

www.motorola.com/semiconductors

The total output noise would be the sum of the individual

components:

Total Output Noise = (4.24 + 2.20 + 9.04 + 1.50 + 6.40

+ 1.03) mV

Total Output Noise = 24.41 mV peak–to–peak.

If this output signal were fed into an 8 bit A/D converter,

referenced to 5 V full scale, the worst case error due to power

supply noise would be equivalent to

"1 bit count.

The error that can occur in the output due to aliasing gain

can be avoided very easily. The easiest method is to power the

part with a voltage regulator. Since the voltage regulator pro-

vides a clean, steady supply, the possibility of aliasing is elimi-

nated. If the accelerometer is powered with a switching supply,

a filter should be placed on the power supply output to elimi-

nate the noise of the harmonics. If placing a filter on the switch-

ing supply is not feasible, the user must ensure that the

operating frequency of the switching power supply is outside

the frequency ranges of the peaks shown in Figure 3. The plot

shown is a superposition of the response of the internal four

pole Bessel lowpass filter, scaled by the corresponding alias-

ing gain for each harmonic. The Bessel filter has the –3 dB

frequency at 400 Hz and, being of fourth order, has a very

steep roll–off outside the passband, with approximately

–80 dB of attenuation at 4 kHz. If a switching power supply

must be used, its operating frequency should be at least 800

Hz from the accelerometer’s sampling frequency. Any switch-

ing noise present will be aliased to 800 Hz or higher, where the

attenuation will be approximately –24 dB or lower, thus reduc-

ing the power supply induced noise below the part’s noise floor.

CONCLUSION

The MMA1201P accelerometer demonstrates Motorola’s

commitment to continuous product improvement. A new oscil-

lator lowers the noise in the part and enables tighter control of

the –3 dB bandwidth of the internal lowpass filter. The supply

voltage is routed to the transducer and the DC level reference

of the capacitance–to–voltage converter stage through a

newly added filter, thus reducing the part’s susceptibility to

power supply noise. The capacitance–to–voltage converter

stage uses new signal conditioning methods, which virtually

eliminate ratiometric errors. The temperature compensation

for sensitivity is improved, producing a very flat response over

temperature. Overall the part offers much enhanced perfor-

mance and is simpler to use. Equally important, Motorola’s

MMA1201P accelerometer has remained very price competi-

tive, making it ideal for most applications requiring accelera-

tion sensors.

Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com

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