AN1979 Datasheet, PDF (1/10 Pages) Freescale Semiconductor, Inc – Altimeter and Barometer System

Freescale Semiconductor
Application Note
AN1979
Rev 3, 11/2006
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.
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:
(V_{S actual}/V_{S spec}) x V_{OUT full-scale spec} = V_{OUT full-scale}
(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 1. Pressure Sensor
There are three types of pressure measurements: gauge,
absolute, and differential. Since this reference design
Figure 2. MPXM2102A Case 1320A-02
© Freescale Semiconductor, Inc., 2006. All rights reserved.