PT100信号调理电路
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2003 Microchip Technology Inc. DS00687B-page 1
M
AN687
INTRODUCTION
The most widely measured phenomena in the process
control environment is temperature. Common ele-
ments, such as Resistance Temperature Detectors
(RTDs), thermistors, thermocouples or diodes are used
to sense absolute temperatures, as well as changes in
temperature. For an overview and comparison of these
sensors, refer to Microchip’s AN679, “Temperature-
Sensing Technologies”, DS00679.
Of these technologies, the platinum RTD temperature-
sensing element is the most accurate and stable over
time and temperature. RTD element technologies are
constantly improving, further enhancing the quality of
the temperature measurement (see Figure 1). Typi-
cally, a data acquisition system conditions the analog
signal from the RTD sensor, making the analog
translation of the temperature usable in the digital
domain.
This application note focuses on circuit solutions that
use platinum RTDs in their design. Initially, the RTD
temperature-sensing element will be compared to the
negative temperature coefficient (NTC) thermistor,
which is also a resistive temperature-sensing element.
In this forum, the linearity of the RTD will be presented
along with calibration formulas that can be used to
improve the off-the-shelf linearity of the element. For
additional information concerning the thermistor tem-
perature sensor, refer to Microchip’s AN685, “Ther-
mistors in Single Supply Temperature Sensing
Circuits”, DS00685. Finally, the signal-conditioning
path for the RTD system will be covered with
application circuits from sensor to microcontroller.
FIGURE 1: Unlike thermistors, RTD
temperature-sensing elements require current
excitation.
RTD OVERVIEW
The acronym “RTD” is derived from the term “Resis-
tance Temperature Detector”. The most stable, linear
and repeatable RTD is made of platinum metal. The
temperature coefficient of the RTD element is positive.
This is in contrast to the NTC thermistor, which has a
negative temperature coefficient, as is shown graphi-
cally in Figure 2. An approximation of the platinum RTD
resistance change over temperature can be calculated
by using the constant 0.00385Ω/Ω/°C. This constant is
easily used to calculate the absolute resistance of the
RTD at temperature.
EQUATION
Author: Bonnie C. Baker
Microchip Technology Inc.
Precision Current Source <1 mA
V
OUT
RTD, most popular element
is made using platinum,
typically 100Ω @ 0°C
RTD T() RTD
0
TRTD
0
× 0.00385ΩΩ⁄°C⁄×+=
where:
RTD(T) is the resistance value of the RTD element at
temperature (Celsius),
RTD
0
is the specified resistance of the RTD element
at 0°C and,
T is the temperature environment that the RTD is
placed (Celsius).
Precision Temperature-Sensing With RTD Circuits
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