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Texas Instruments Temperature sensing fundamentals Application notes
Temperature sensing fundamentals
Introduction to Temperature Sensing
µA
In embedded systems, there is a constant need for
higher performance and more features in a smaller
form factor. This requires system designers to monitor
the overall temperature to ensure safety and protect
the systems. The trend of sensor data-logging further
drives the need for temperature measurement to not
only measure system or environmental conditions, but
to compensate for temperature-sensitive components
and maintain accuracy of the total system.
E
V
BE
B
Thermal Design Considerations
Considerations for efficient thermal monitoring and
protection include:
i. Accuracy: Sensor accuracy represents how close
the temperature is to the true value. Applications
should consider factors such as linearity and
acquisition circuits across the operating
temperature range.
ii. Size: While the size of the sensor makes an impact
on the design, analyzing the overall circuit can yield
a more optimized design.
iii. Sensor Placement: Package and placement can
impact the response time and conduction path.
Both are critical for effective thermal design.
Table 1. Temperature Sensing Technology
Comparison
IC Sensor
Thermistor
RTD
Thermo
couple
Range
–55°C to
+200°C
–100°C to
+500°C
–240°C to
600°C
–260°C to
+2300°C
Accuracy
Good /
Best
Calibration
dependant
Best
Better
Footprint /
Size
Smallest
Small
Moderate
Large
Complexity
Easy
Moderate
Complex
Complex
Linearity
Best
Low
Best
Better
Topology
Point-topoint, Multidrop, Daisy
Chain
Point-topoint
Point-topoint
Point-topoint
Low to
Moderate
Low to
Moderate
Expensive
Expensive
Price
IC Sensors
IC temperature sensors rely on the predictive
temperature dependence of a Silicon bandgap. The
precision current sources the internal forward biased
p-n junction with the resulting ∆VBE that corresponds to
the device temperature.
C
GND
Figure 1. Temperature Dependence of Silicon
Bandgap
'VBE
§I ·
KT
u In¨ C1 ¸
q
© IC2 ¹
(1)
Given the predictable behavior, ICs offer high linearity
and accuracy across a wide temperature range (up to
±0.1°C). IC sensors can integrate system functionality
that offers a small footprint and low power
consumption. These sensors do operate in a limited
temperature range and offer fewer packaging options
that can measure off-board temperature when
compared to thermistors.
These sensors are typically fully-integrated, and
monolithic sensors and accuracy are designed for the
entire system instead of for one element.
Thermistors
Thermistors are passive components that change
resistance with temperature. Thermistors fall into two
categories, negative temperature coefficient (NTC) and
positive temperature coefficient (PTC).
While thermistors offer a variety of package options for
onboard and off-board temperature sensing, typical
implementation requires more system components.
NTC thermistor are non-linear and often bear
increased calibration cost and software overhead. An
exception to this are Silicon-based PTC thermistors.
The true system accuracy for Thermistors are often
difficult to determine. NTC system error contributors
include NTC tolerance, bias resistor (Tolerance,
Temperature Drift), ADC (quantization error),
linearization error, reference voltage (Accuracy,
Temperature Drift).
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Temperature sensing fundamentals
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1
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The temperature of the cold junction must be known to
derive the hot junction temperature. Here, accuracy is
limited by the fact that there are two systems that have
separate tolerances and capabilities interacting with
each other.
V
CC
Thermistor
RT
Amp
ADC
Isothermal Block
Type J
Iron
Figure 2. Typical Thermistor Implementation
Copper
Th
Tc
Constantan
Resistive Temperature Detectors (RTD)
Copper
RTDs are temperature sensors made of pure material,
typically in platinum, nickel, or copper, with a highly
predictable resistance-temperature relationship.
Cold Junction Compensation
Temperature Sensor
ISOURCE
Figure 4. Thermocouple With CJC Temperature
Sensor
RL
RL
+
PGA
û ADC
±
RL
RL
-
TMP
V+
VMEAS
+
RBIAS
Thermocouples do not require external excitation, and
hence are not impacted by self-heating issues. They
can also support extreme temperatures (>2000°C).
While they are rugged and inexpensive,
thermocouples do require an additional temperature
sensor for cold-junction-compensation (CJC). They
tend to be non-linear and are highly sensitive to
parasitic junctions where the thermocouple is attached
to the board.
Finally, digitizing a thermocouple would be susceptible
to previously discussed ADC errors.
Device Recommendations
Figure 3. Complex 4-Wire RTD Circuit
Platinum RTDs can be highly accurate and very linear
across a very wide temperature range up to 600°C.
Implementation with these analog sensors involve
complex circuitry and design challenges. Ultimately,
the accurate systems involve complex error analysis
due to a higher number of contributing components
that also impact the overall system size. RTDs also
require calibration during manufacturing followed by an
annual calibration process in the field.
Contributors to the RTD system error include RTD
Tolerance, self-heating, ADC (quantization error) and
references used in the system.
Thermocouples
Table 2. Related Documentation
Thermocouples are made of two dissimilar electrical
conductors that form electrical junctions at different
temperatures. A thermocouple produces a
temperature-dependent voltage as a result of the
thermoelectric seebeck effect. This voltage translates
to the difference of temperature between the hot
junction (Th) and the cold junction (Tc).
2
For 40 years, Texas Instruments has manufactured
several IC-based temperature sensors, including:
• Digital temperature sensors:
– Highest accuracy temperature sensors
– Lowest power with the smallest footprint
– LM75 / TMP75 temperature sensors
– Multi-channel remote diode temperature
sensors
• High-accuracy analog temperature sensors
• Linear thermistors
• Temperature switches or thermostats that offer
integrated hysteresis for enhanced noise immunity
COLLATERAL
DESCRIPTION
Application Report Accuracy
Layout Considerations for Accurately
Measuring Ambient Temperature
Application Report Temperature
Calibration
Methods to Calibrate Temperature
Monitoring Systems
Temperature sensing fundamentals
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