AN1646 Freescale Semiconductor / Motorola, AN1646 Datasheet - Page 2

• Manufacturer Part Number: AN1646
• Description: AN1646 Noise Considerations for Integrated Pressure Sensors
• Manufacturer: Freescale Semiconductor / Motorola

approaches are effective, low pass filtering with hardware,
and low pass filtering with software. When filtering with hard-
which has a successive approximation A/D converter.
Successive approximation A/D’s are generally compatible
with the DC source impedance of the filter in Figure 2. Results
are shown in Figure 4.
a single pole RC filter. Please consult your A/D converter tech-
ing is a form of low pass filtering in software. A rolling average
of 8 to 64 samples will clean up most of the noise. A 10 sample
average reduces the noise to about 2.5 mV peak to peak and
a 64 sample average reduces the noise to about 1 mV peak
to peak (see Figures 5 and 6).
nents. However, it does require RAM for data storage, extra
computation cycles and code. In applications where the mi-
crocontroller is resource limited or pressure is changing rela-
tively rapidly, averaging alone may not be the best solution. In
these situations, a combination of RC filtering and a limited
For mitigating the effects of this sensor noise, two general
This filter has been tested with an HC05 microcontroller
Some A/D’s will not work well with the source impedance of
Averaging is also effective for filtering sensor noise. Averag-
This method is simple and requires no external compo-
2
NOISE FILTERING TECHNIQUES
AND CONSIDERATIONS
Figure 3. Use a Rail–to–Rail Buffer to Reduce Output Impedance of RC Filter
Figure 2. Integrated Pressure Sensor with RC LP Filter to Filter Out Noise
Freescale Semiconductor, Inc.
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ware, a low–pass RC filter with a cutoff frequency of 650 Hz
is recommended. A 750 ohm resistor and a 0.33 F capacitor
have been determined to give the best results (see Figure 2)
since the 750 ohm series impedance is low enough for most
A/D converters.
nical data sheet if input impedance is a concern. In applica-
tions where the A/D converter is sensitive to high source
impedance, a buffer should be used. The integrated pressure
sensor has a rail–to–rail output swing, which dictates that a
rail–to–rail operational amplifier (op amp) should be used to
avoid saturating the buffer. A rail–to–rail input and output op
amp works well for this purpose (see Figure 3).
number of samples gives the best results. For example, a roll-
ing average of 4 samples combined with the RC filter in Figure
2 results in a noise output on the order of 1 mV peak to peak.
fectiveness of averaging tends to fall off as the number of sam-
ples is increased. In other words, the signal–to–noise (S/N)
ratio goes up more slowly than the number of samples. To be
more precise, the S/N ratio improves as the square root of the
number of samples is increased. For example, increasing the
number of samples from 10, in Figure 5, to 64, in Figure 6, re-
duced noise by a factor of 2.5.
Another important consideration is that the incremental ef-
Motorola Sensor Device Data