MPXM2102AS datasheet(458/670 Pages) MOTOROLA

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Motorola Sensor Device Data

www.motorola.com/semiconductors

AN1518

Using a Pulse Width Modulated Output with

Semiconductor Pressure Sensors

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

used

conjunction with a microcontroller. This use of two time bases,

one

analog

and

one

digital,

can

create

additional

inaccuracies.

With

either

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

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

PWM Output

to Micro

+ 5.0 V

Pulse Train

from Micro

Comparator Stage

Ramp Generator

Pressure Sensor

LM311D

1.0

µF

22.1 k

Ω

4.75 k

Ω

MMBT3904LT1

3.3

µF

MDC4010CT1

10 k

Ω

4.75 k

Ω

MPX5100DP

10 k

Ω

MOTOROLA

SEMICONDUCTOR APPLICATION NOTE

REV 1

Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com

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