AN929 Datasheet, PDF (10/22 Pages) Microchip Technology – Temperature Measurement Circuits for Embedded Applications

AN929
Thermocouples
Thermocouples are the most common sensor used in
high-temperature measurements. A thermoelectric
electromagnetic-force (emf) or voltage results when
two dissimilar metals are joined together that produce
an output voltage that is proportional to temperature.
Figure 16 shows a block diagram of a typical thermo-
couple system. The thermocouple probes are typically
located remotely from the amplifier circuit and are
connected to the amplifier via the thermocouple wires
that enter the enclosure through a connector. The cold
junction occurs at the point where the copper wires of
the PCB meet the Alumel and Chromel connector pins.
The cold junction will be formed at the inside wall of the
enclosure if the connector uses Alumel and Chromel
pins. The temperature of the cold junction or “isother-
mal block” is estimated by a sensor that is located as
close as possible to the connector on the PCB.
Measurement Site
(T_MEAS)
Type K _{Chromel Wire}
Alumel Wire
T_MEAS = T_HOT - T_COLD
“Isothermal
Block”
+
V_S
-
CJC Circuit
(T_COLD)
Copper Wire
Copper Wire
PCB
Thermocouple
Amplifier (T_HOT)
Enclosure
FIGURE 16:
Thermocouple Basics – Typical Measurement System.
RTDs
RTDs are the standard sensor chosen for precision
sensing applications because of their excellent repeat-
ability and stability characteristics. RTDs are based on
the principle that the resistance of a metal changes with
temperature. A RTD can be characterized against
temperature to obtain a table of temperature correction
coefficients. The correction can be added to the
measured temperature to obtain an accuracy greater
than 0.05°C. RTDs are available in two basic designs:
wire wound and thin film. Wire wound RTDs are built by
winding the sensing wire around a core to form a coil,
while thin film RTDs are manufactured by depositing a
very thin layer of platinum on a ceramic substrate.
Thermistors
The main advantages of thermistors are that they are
inexpensive and available in a wide variety of
packages. Thermistors are built with semiconductor
materials and can have either a positive (PTC) or a
negative (NTC) temperature coefficient. However, the
NTC devices are typically used for temperature
sensing. The main negative feature of thermistors is
that the change in resistance with temperature is very
non-linear at temperatures less than 0°C and greater
than 70°C.
Silicon Integrated Circuits
Silicon IC sensors provide an accurate temperature
measurement for a steady-state or relatively constant
temperature. However, their thermal response time to
a rapid change in temperature is poor. Silicon sensors
provide a non-contact temperature measurement.
Thus, the location of the sensor is important. These
sensors measure temperature by monitoring the volt-
age of a diode located on the IC die, as shown in
Figure 17. The substrate of the die is typically
grounded and connected to the PCB’s ground plane via
a bonding wire and the lead of the package. The
ground pin of the IC provides a low-impedance thermal
path between the die and the PCB, allowing the sensor
to effectively monitor the temperature of the PCB
board. The thermal path between the top of the pack-
age to the ambient air and between the bottom of the
package and the PCB is not as efficient because the
plastic IC housing package functions as a thermal insu-
lator. Silicon sensors provide a measurement of the
temperature of the PCB’s ground plane and the ambi-
ent air temperature has only a small effect on the mea-
surement.
DS00929A-page 10
 2004 Microchip Technology Inc.