3–312
Motorola Sensor Device Data
For More Information On This Product,
Go to: www.freescale.com
Prepared by: Eric Jacobsen and Jeff Baum
Sensor Design and Applications Group, Motorola Phoenix, AZ
INTRODUCTION
For remote sensing and noisy environment applications, a
frequency modulated (FM) or pulse width modulated (PWM)
output is more desirable than an analog voltage. FM and PWM
outputs inherently have better noise immunity for these types
of applications. Generally, FM outputs are more widely
accepted than PWM outputs, because PWM outputs are
restricted to a fixed frequency. However, obtaining a stable FM
output is difficult to achieve without expensive, complex
circuitry.
With either an FM or PWM output, a microcontroller can be
used to detect edge transitions to translate the time–domain
signal into a digital representation of the analog voltage signal.
In conventional voltage–to–frequency (V/F) conversions, a
voltage–controlled oscillator (VCO) may be used in
conjunction with a microcontroller. This use of two time bases,
one analog and one digital, can create additional
inaccuracies. With either FM or PWM outputs, the
microcontroller is only concerned with detecting edge
transitions. If a programmable frequency, stable PWM output
could be obtained with simple, inexpensive circuitry, a PWM
output would be a cost–effective solution for noisy
environment/remote sensing applications while incorporating
the advantages of frequency outputs.
The Pulse Width Modulated Output Pressure Sensor
design (Figure 1) utilizes simple, inexpensive circuitry to
create an output waveform with a duty cycle that is linear to the
applied pressure. Combining this circuitry with a single digital
time base to create and measure the PWM signal, results in
a stable, accurate output. Two additional advantages of this
design are 1) an A/D converter is not required, and 2) since the
PWM output calibration is controlled entirely by software,
circuit–to–circuit variations due to component tolerances can
be nullified.
The PWM Output Sensor system consists of a Motorola
MPX5000 series pressure sensor, a ramp generator
(transistor switch, constant current source, and capacitor), a
comparator, and an MC68HC05P9 microcontroller. These
subsystems are explained in detail below.
Figure 1. PWM Output Pressure Sensor Schematic
PWMOutput
to Mcro
+5.0 V
Pulse Train
fromMcro
Comparator Stage
Ramp Generator
Pressure Sensor
U1
LM311D
C2
1.0
μ
F
R5
22.1 k
R4
4.75 k
Q1
MMBT3904LT1
C1
3.3
μ
F
U2
MDC4010CT1
R1
10 k
R3
4.75 k
X1
MPX5100DP
R2
10 k
SEMICONDUCTOR APPLICATION NOTE
REV 1
F
Freescale Semiconductor, Inc.
n
.