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1998 Microchip Technology Inc. DS00679A-page 1
M
AN679
INTRODUCTION
Of all of the sensing technologies, temperature sensing
is the most common. This phenomena can be
explained by citing examples in a multitude of applica-
tions where knowing and using the actual or relative
temperature is critical. For instance, other sensors
such as pressure, force, flow, level, and position many
times require temperature monitoring in order to insure
accuracy. As an example, pressure and force are usu-
ally sensed with resistive Wheatstone bridge configura-
tions. The temperature errors of the resistive elements
of these bridges can exceed the actual measurement
range of the sensor, making the pressure sensor’s out-
put fairly useless, unless the temperature of the bridge
is known. Flow and level sensor accuracies are depen-
dent on the density of the liquid or gas.
One variable that affects the accuracy of these sensors
is the temperature of that material. Position is most typ-
ically used in motor control. In these circuits, tempera-
ture affects the efficiency of the motor. Consequently,
the understanding of temperature sensing is needed in
order to fully understand how to accurately sense most
other physical phenomena.
This application note will cover the most popular temper-
ature sensor technologies to a level of detail that will
give the reader insight into how to determine which sen-
sor is most appropriate for the application. This note is
written from the perspective of catering to the complex
issues of the sensing environment and required accu-
racy. Once the sensor is selected, subsequent Micro-
chip application notes can be used to design
appropriate microcontroller interface circuits. These cir-
cuits will offer the complete signal path from the low level
output signals of the sensor, through the analog signal
conditioning stages to the microcontroller. Techniques
such as sensor excitation, sensor signal gain, and digital
linearization are reserved for these further discussions.
SO MANY TEMPERATURE SENSORS
The most popular temperature sensors used today are
the Thermocouple, Resistive Temperature Device
(RTD), Thermistor, and the newest technology, the Inte-
grated Silicon Based Sensors. There are other sensing
technologies, such as Infrared (Pyrometers) and Ther-
mal Pile. These alternatives are beyond the scope of
this application note.
Each of these sensor technologies cater to specific tem-
perature ranges and environmental conditions. The
sensor’s temperature range, ruggedness, and sensitiv-
ity are just a few characteristics that are used to deter-
mine whether or not the device will satisfy the
requirements of the application. No one temperature
sensor is right for all applications. The thermocouple's
wide temperature range is unrivalled as is the excellent
linearity of the RTD and the accuracy of the Thermistor
.
Table 1 summarizes the main characteristics of these
four temperature sensors. This table can be used dur-
ing the first pass of the sensor selection process. Fur-
ther details concerning the construction and charac-
teristics of these sensors are given in the following sec-
tions of this application note
To complement the specifications sited in Table 1, a list
of typical applications for these four temperature sen-
sors are shown in Table 2.
Author: Bonnie Baker
Microchip Technology Inc.
Temperature Sensing Technologies
