2–88
Motorola Sensor Device Data
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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
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
1 bit count.
–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.
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Freescale Semiconductor, Inc.
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