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3–368
Motorola Sensor Device Data
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by: Eric Jacobsen
Systems and Applications Engineer
Sensor Products Division
Motorola, Inc.
A digital output is more desirable than an analog output in
noisy environments (e.g. automotive, washing machines,
etc.) and remote sensing applications (building controls,
industrial applications, etc.) because a digital signal inherently
has better noise immunity compared to analog signals.
Additional applications requiring a sensor with a digital output
include microcontroller–based systems that have no A/D in
the system or that have no A/D channels available for the
sensing function. For these applications, there is no other
option but a digital output to further process the signal.
Via a design example this paper shows how to easily
convert an analog voltage output sensor to a digital output
sensor. For the design example, each of the required circuit
components is discussed in detail. While the design is
applicable to analog voltage output sensors (differential or
single–ended output) in general, the design example and
following
semiconductor pressure sensors.
The digital output sensor in Figure 1. consists of the
following:
Motorola MPX2000 series pressure sensor
A two op amp gain stage to amplify the sensor’s signal
An integrator (i.e. a low pass filter consisting of one resistor
and one capacitor)
An LM311 comparator
An MC68HC05P9 microcontroller with which only two pins
are used: the output compare timer channel (TCMP) and
one general I/O pin (the input capture timer channel, TCAP,
can be used in place of the general I/O pin). Since only two
of the MC68HC05P9’s pins are used, the remaining pins
are available for other system functions.
discussions
will
pertain
specifically
to
Figure 1. The Digital Output Sensor Schematic
AMPLIFIER
COMPARATOR
INTEGRATOR
TO MC68HC05P9’s
GENERAL I/O PIN OR
TCAP PIN
PRESSURE SENSOR
+
–
+
–
FROMMC68HC05P9’s
TCMP PIN
+
–
R6
+5 V
R5
C1
R4
R2
U1
MC33272
U1
MC33272
+5 V
+5 V
U2
LM311
R3
R+SHIFT1
R+SHIFT2
R1
+5 V
X1
MPX2000
SERIES
2
4
3
1
+5 V
RH
(OPTIONAL)
SEMICONDUCTOR APPLICATION NOTE
F
Freescale Semiconductor, Inc.
n
.