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Motorola Sensor Device Data
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Prepared by Ador Reodique, Sensor and Systems Applications Engineering and Warren Schultz, Field Engineering
INTRODUCTION
Motorola Integrated Pressure Sensors (IPS) have trimmed
outputs, built–in temperature compensation and an amplified
single–ended output which make them compatible with
Analog to Digital converters (A/D’s) on low cost micro–control-
lers. Although 8–bit A/D’s are most common, higher resolution
A/D’s are becoming increasingly available. With these higher
resolution A/D’s, the noise that is inherent to piezo–resistive
bridges becomes a design consideration.
The two dominant types of noise in a piezo–resistive inte-
grated pressure sensor are shot (white) noise and 1/f (flicker
noise). Shot noise is the result of non–uniform flow of carriers
across a junction and is independent of temperature. The
second, 1/f, results from crystal defects and also due to wafer
processing. This noise is proportional to the inverse of fre-
quency and is more dominant at lower frequencies
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Noise can also come from external circuits. In a sensor sys-
tem, power supply, grounding and PCB layout is important and
needs special consideration.
The following discussion presents simple techniques for
mitigating these noise signals, and achieving excellent results
with high resolution A/D converters.
EFFECTS OF NOISE IN SENSOR SYSTEM
The transducer bridge produces a very small differential
voltage in the millivolt range. The on–chip differential amplifier
amplifies, level shifts and translates this voltage to a single–
ended output of typically 0.2 volts to 4.7 volts. Although the
transducer has a mechanical response of about 500 Hz, its
noise output extends from 500 Hz to 1 MHz. This noise is
amplified and shows up at the output as depicted in Figure 1.
There is enough noise here to affect 1 count on an 8 bit A/D,
and 4 or 5 counts on a 10 bit A/D. It is therefore important to
consider filtering. Filtering options are discussed as follows.
Figure 1. MPX5006 Raw Output
SEMICONDUCTOR APPLICATION NOTE
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