MPXM2102AS datasheet(435/670 Pages) MOTOROLA

435 / 670 page

AN1326

3–289

Motorola Sensor Device Data

www.motorola.com/semiconductors

INTRODUCTION

Figure 1 shows the overall system architecture chosen for

this application. This system serves as a building block, from

which more advanced systems can be developed. Enhanced

accuracy, resolution, and additional features can be

integrated in a more complex design.

There are some preliminary concerns regarding the

measurement of barometric pressure which directly affect the

design considerations for this system. Barometric pressure

refers to the air pressure existing at any point within the earth’s

atmosphere. This pressure can be measured as an absolute

pressure, (with reference to absolute vacuum) or can be

referenced to some other value or scale. The meteorology and

avionics industries traditionally measure the absolute

pressure, and then reference it to a sea level pressure value.

This complicated process is used in generating maps of

weather systems. The atmospheric pressure at any altitude

varies due to changing weather conditions over time.

Therefore, it can be difficult to determine the significance of a

particular

pressure

measurement

without

additional

information. However, once the pressure at a particular

location and elevation is determined, the pressure can be

calculated at any other altitude. Mathematically, atmospheric

pressure is exponentially related to altitude. This particular

system is designed to track variations in barometric pressure

once it is calibrated to a known pressure reference at a given

altitude.

For simplification, the standard atmospheric pressure at

sea level is assumed to be 29.9 in–Hg. “Standard” barometric

pressure is measured at particular altitude at the average

weather conditions for that altitude over time. The system

described in this text is specified to accurately measure

barometric pressure variations up to altitudes of 15,000 ft. This

altitude corresponds to a standard pressure of approximately

15.0 in–Hg. As a result of changing weather conditions, the

standard pressure at a given altitude can fluctuate

approximately

±1 in–Hg. in either direction. Table 1 indicates

standard barometric pressures at several altitudes of interest.

MC68HC11E9

MICRO–

CONTROLLER

4–DIGIT LCD

& MC145453

DISPLAY DRIVER

SIGNAL

COND.

AMPLIFIER

MPX2100AP

PRESSURE

SENSOR

DATA

CLOCK SYNCH

Figure 2. Barometer System Block Diagram

Table 1. Altitude versus Pressure Data

Altitude (Ft.)

Pressure (in–Hg)

29.92

500

29.38

1,000

28.85

6,000

23.97

10,000

20.57

15,000

16.86

SYSTEM OVERVIEW

In order to measure and display the correct barometric

pressure, this system must perform several tasks. The

measurement strategy is outlined below in Figure 2. First,

pressure is applied to the sensor. This produces a proportional

differential output voltage in the millivolt range. This signal

must then be amplified and level–shifted to a single–ended,

microcontroller (MCU) compatible level (0.5 – 4.5 V) by a

signal conditioning circuit. The MCU will then sample the

voltage at the analog–to–digital converter (A/D) channel input,

convert the digital measurement value to inches of mercury,

and then display the correct pressure via the LCD interface.

This process is repeated continuously.

There

are

several

significant

performance features

implemented into this system design. First, the system will

digitally display barometric pressure in inches of mercury, with

a resolution of approximately one–tenth of an inch of mercury.

In order to allow for operation over a wide altitude range (0 –

15,000 ft.), the system is designed to display barometric

pressures ranging from 30.5 in–Hg. to a minimum of 15.0

in–Hg. The display will read “lo” if the pressure measured is

below 30.5 in–Hg. These pressures allow for the system to

operate with the desired resolution in the range from sea–level

to approximately 15,000 ft. An overview of these features is

shown in Table 2.

Table 2. System Features Overview

Display Units

in–Hg

Resolution

0.1 in–Hg.

System Range

15.0 – 30.5 in–Hg.

Altitude Range

0 – 15,000 ft.

DESIGN OVERVIEW

The following sections are included to detail the system

design. The overall system will be described by considering

the subsystems depicted in the system block diagram, Figure

2. The design of each subsystem and its function in the overall

system will be presented.

Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com

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