© Freescale Semiconductor, Inc., 2006. All rights reserved.

AN1979

Rev 3, 11/2006

Freescale Semiconductor

Application Note

Altimeter and Barometer System

by: Michelle Clifford and Fernando Gonzalez

Sensor Products Division, Tempe, AZ

INTRODUCTION

With smaller packages and lower costs, pressure sensors

can be designed into more consumer applications. This

document describes a reference design for a digital barometer

and altimeter using the MPXM2102A pressure sensor in the

low cost MPAK package, a quad op-amp, and the

MC68HC908QT4 microcontroller. This system continuously

monitors the barometric pressure and compares it to previous

pressure readings to update altitude and weather predictions.

This reference design enables the user to evaluate a

Freescale Semiconductor, Inc. pressure sensor for barometer,

personal weather station and altimeter applications. This

reference design also allows customers to evaluate barometer

pressure readings obtainable from the MPXM2102A sensor

for watches or GPS systems with this feature. In addition,

many systems require barometric pressure data to correct

system response errors. This application note describes the

reliability and accuracy that our sensors can provide in this

system.

SYSTEM DESIGN

Pressure Sensor

The barometer/altimeter system requires a pressure

sensor that has a pressure range of 64 kPa to 105 kPa.

Freescale Semiconductor, Inc. has a broad portfolio of silicon

piezo-resistive pressure sensors. They provide a very

accurate and linear voltage output directly proportional to the

applied pressure. By evaluating the application design and

cost, the right pressure sensor can be selected from our

portfolio.

Figure 1. Pressure Sensor

There are three types of pressure measurements: gauge,

absolute, and differential. Since this reference design

measures changes in ambient pressure, we need a known

pressure reference. Therefore, an absolute pressure sensor

was selected. Freescale offers three levels of integration: un-

compensated, compensated, and integrated. Since there can

be large temperature changes from one elevation to another

the sensor for this reference design needs to be offset

calibrated and temperature compensated. Therefore a

compensated sensor was selected requiring external

amplification circuitry. However, integrated solutions such as

the MPXM5100A, can also be considered, thereby eliminating

the need for the external amplification circuitry.

Knowing the range of pressure, the type of pressure

measurement, and the level of integration required for this

application, the MPXM2102A sensor was selected. The

sensor has both temperature compensation and calibration

circuitry on the silicon and is capable of producing a linear

output voltage in the range of 0 to 100 kPa, but can be pushed

further up to 105 kPa with linear results. The characteristics of

this sensor are described in greater detail in Table 2. A 5-volt

supply was used throughout the circuit to power the

components. Since the MPXM2102A is ratio metric, meaning

the output voltage changes linearly with the supply voltage,

the sensor will have a full scale span of 20 mV instead of the

specified 40 mV at a 10 V supply. The calculation of the full

scale span is shown below:

(5.0 V/ 10 V) x 40 mV = 20 mV

One of the most important decisions for a pressure

application is the packaging. Freescale has a large offering of

pressure packaging options. To minimize the space of a final

application, the MPAK package was selected. A non-ported

MPAK is the ideal pressure sensor package for hand held

GPS units or altimeter watches due to its small size. However,

a ported MPAK package can also be selected, allowing a tube

to be attached to the port for testing and demonstration

purposes.

Figure 2. MPXM2102A Case 1320A-02