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Table of Contents

Application Note

RTD Alternative Measurement Methods in Real-Time
Control Applications

ABSTRACTThe purpose of this document is to familiarize the reader with the resistance temperature detector (RTD) asa method of temperature measurement in real-time control applications, as well to introduce possible sourcesof error and how to correct them. There is an extensive portfolio of semiconductor sensing technologies thatallow for new capabilities in efficiency, performance, and low-latency response in real-time control systems.
Therefore, alternative temperature measurement methods and their corresponding advantages are discussed inthis document.

Table of Contents
1 RTD Introduction.....................................................................................................................................................................2
1.1 Common Wiring Configurations......................................................................................................................................... 2
1.2 RTD Tolerances and Accuracy...........................................................................................................................................4
1.3 Error Sources of RTD Systems.......................................................................................................................................... 42 RTD Alternatives.....................................................................................................................................................................6
2.1 TMP116 and TMP117.........................................................................................................................................................6
2.2 TMP1826............................................................................................................................................................................7
2.3 TMP6x................................................................................................................................................................................8
3 Conclusion.............................................................................................................................................................................. 94 References.............................................................................................................................................................................. 9
5 Revision History......................................................................................................................................................................9

List of Figures
Figure 1-1. Two-Wire RTD Configuration.....................................................................................................................................2
Figure 1-2. Three-Wire RTD Configuration..................................................................................................................................3
Figure 1-3. Four-Wire RTD Configuration....................................................................................................................................3
Figure 1-4. Accuracy Classes for RTDs.......................................................................................................................................4
Figure 1-5. Example of RTD Error Minimization Circuitry ........................................................................................................... 5
Figure 2-1. Comparing the Accuracy of an RTD to the TMP116................................................................................................. 6
Figure 2-2. Accuracy Chart for TMP117 and RTD....................................................................................................................... 7
Figure 2-3. TMP1826 Bus Powered Application..........................................................................................................................7
Figure 2-4. TMP1826 Supply Powered Application..................................................................................................................... 7
Figure 2-5. TMP1826 Temperature Error vs Temperature...........................................................................................................8
Figure 2-6. TMP61 Current Biasing Circuit..................................................................................................................................8
Figure 2-7. TMP61 Voltage Biasing Circuit..................................................................................................................................8

List of Tables
Table 2-1. TMP117 Accuracy Across Temperature Range.......................................................................................................... 6
Table 2-2. TMP1826 Accuracy Across Temperature Range........................................................................................................8

Trademarks
SMBus™ is a trademark of Intel Corporation.
All trademarks are the property of their respective owners.

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1 RTD Introduction
A resistance temperature detector is a passive circuit element whose resistance increases as temperature
increases. They are generally constructed using platinum, copper, or nickel, and one major advantage of RTDs
is that they support a wide span of temperature, ranging from –200°C to +850°C. The accuracy limits of an RTD
are defined by the class, or grade, of the RTD. The characteristics of platinum, copper, or nickel determine thelinear approximation of resistance versus temperature within the 0°C to 100°C temperature range. The platinum
RTD is known for its strong linearity and repeatability characteristic.
DIN/IEC 60751 is considered the worldwide standard for platinum RTDs. For a PT100 RTD, the standard
requires the sensing element to have an electrical resistance of 100.00 Ω at 0°C and a temperature coefficient ofresistance (TCR) of 0.00385 Ω/Ω/°C between 0°C and 100°C.
Equation 1 and Equation 2 define the resistance-to-temperature relation for temperature ranges above andbelow 0°C.
RT = R0 1 + AT + BT2 for T ≥ 0℃

RT = R0 1 + AT + BT2 + CT3 100 − T

with:• A = 3.9083 × 10–3
• B = –5.775 × 10–7
• C = –4.183 × 10–12(1)
for T < 0℃

(2)

1.1 Common Wiring Configurations
RTDs are typically designed with three common configurations: two-wire, three-wire, and four-wire. In the
two-wire configuration, as shown in Figure 1-1 below, the RTD is connected with two wires to either end of
the RTD. In this configuration, the lead wire resistances cannot be separated from the RTD resistance, adding
an error that cannot be separated from the RTD measurement. Two-wire RTDs yield the least accurate RTD
measurements and are used when accuracy is not critical or when lead lengths are short. Two-wire RTDs are
the least expensive RTD configuration.

Figure 1-1. Two-Wire RTD Configuration

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RTD Introduction

In the three-wire configuration, as shown in Figure 1-2, the RTD is connected to a single lead wire on one end
and two lead wires on the opposite end. Using different circuit topologies and measurements, lead resistanceeffects can effectively be cancelled, reducing the error in three-wire RTD measurements. Compensation for leadwire resistance assumes that the lead resistances match.

Figure 1-2. Three-Wire RTD Configuration
In the four-wire configuration, as shown in Figure 1-3, two lead wires are connected to either end of the RTD.
In this configuration, the RTD resistance may be measured with a four-wire resistive measurement with superior
accuracy. The RTD excitation is driven through one lead on either end, while the RTD resistance is measured
with the other lead on either end. In this measurement, the RTD resistance is sensed without error contributed
from the lead wire reacting with the sensor excitation. Four-wire RTDs yield the most accurate measurements,
but are the most expensive RTD configuration.

Figure 1-3. Four-Wire RTD Configuration

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1.2 RTD Tolerances and Accuracy
There are four tolerance classes specified in DIN/IEC751:
Class

ToleranceAA± (0.1 + 0.0017·|T|)A± (0.15 + 0.002·|T|)B± (0.3 + 0.005·|T|)C± (1.2 + 0.005·|T|)

These tolerance classes also represent the interchangeability of a detector. Should a detector become damaged,
good interchangeability assures that the replacement sensor delivers the same readings under the same
conditions as the predecessor. Another important criterion for selecting a temperature sensor is the long-term
stability. Great stability produces little output signal drift over time, thus reducing the frequency of costly
calibrations. Depending on the application requirement, today’s RTDs can provide long-term drifts from as little
as 0.003°C/year up to 0.01 and 0.05°C/year.
RTDs are considered to be amongst the most accurate temperature sensors available. In addition to highaccuracy, they offer excellent stability, repeatability, and high immunity to electrical noise. However, for eachclass, the accuracy varies as the temperature changes. Figure 1-4 shows the accuracy of different classes of
RTDs.

Figure 1-4. Accuracy Classes for RTDs

1.3 Error Sources of RTD Systems
To convert the change in resistance of an RTD into a sensible output signal, a current source that drives aconstant current through the sensing element is commonly used, thus creating a temperature dependent voltage
across the RTD. This method creates two sources of measurement errors.
First, the current through the RTD causes a certain amount of self-heat that adds to the sensing elements
temperature, thus falsifying the actual measurement reading. Therefore, TI recommends currents in the range of
500 μA to 1 mA maximum to minimize the impact of self-heating.
The second error source is the voltage drop across long measurement leads as is expected in PT100
applications. The voltage division between the lead resistance and the RTD can significantly reduce themeasured output voltage at the signal amplifier input, yielding a false temperature reading. To minimize theimpact of lead resistance, the leads must either be short when using a 2-wire RTD, or the RTD itself must
accommodate lead-compensation wires, as provided in 3-wire and 4-wire RTD designs.

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RTD Introduction

1.3.1 Error Minimization Circuitry
The circuitry in Figure 1-5 is designed to take advantage of the four-wire RTD, which is the most accurate
configuration to use. The two wires (W1 and W4) are the force leads and connect the RTD to the constantcurrent source. The other two wires (W2 and W3) are the sense leads and connect voltage across the RTD to
the amplifier. This arrangement separates the constant-current source driving the RTD from the measurement
circuit. The voltage drop in wires W1 and W4 is removed from the measurement of the voltage across the RTD.

Figure 1-5. Example of RTD Error Minimization Circuitry

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2 RTD Alternatives
Semiconductor temperature sensors are manufactured using technology that allows these devices to be
produced efficiently and inexpensively. As a result, these devices have properties designed to easily interface
with many other types of semiconductor devices, such as amplifiers, power regulators, buffer output amplifiers,
and microcontrollers.
The fast thermal response time of these sensors enables fast error correction in real-time digital control loops,
which can help maximize control speed for higher productivity and safety. This also allows for faster adaptation
to control motor position and velocity with a low-latency response. The high accuracy of these sensors helps
reduce maintenance costs and system downtime, as systems become more reliable and control is more efficient.
The ability to operate systems closer to the thermal limits increases system efficiency and allows for immediate
failure detection and reaction.

2.1 TMP116 and TMP117
The TMP116 has significantly better accuracy than the Class B RTD. In addition, when compared to the ClassA RTD, the TMP116 accuracy is better over most of the –55°C to +125°C operating temperature range. This
improvement in accuracy is in addition to lower cost and simplified designs when compared with RTDs.

Figure 2-1. Comparing the Accuracy of an RTD to the TMP116
The TMP117 is another semiconductor temperature sensor that can replace RTDs. It is a high-precision digitaltemperature sensor which provides a 16-bit result with a resolution of 0.0078°C and an accuracy of up to±0.1°C across the –20°C to +50°C temperature range with no calibration. The TMP117 is I2C- and SMBus™
interface-compatible, has a programmable alert function, and can allow up to four devices on a single bus. Table
2-1 shows the overall accuracy of the TMP117 across its operating range.
Table 2-1. TMP117 Accuracy Across Temperature
Range
Temperature Range

Accuracy–20°C to +50°C±0.1°C–40°C to +100°C±0.2°C–55°C to +150°C±0.3°C

Figure 2-2 shows the accuracy of the TMP117 versus an RTD across the operating temperature range of –55°C
to +150°C. It is evident looking at Figure 2-2 that the TMP117 with no calibration has the same or better
accuracy as an RTD Class-AA sensor. Note that this is the raw accuracy of the two devices and that the final
system layout has a minor effect on the TMP117 and a major effect on the accuracy of an RTD sensor due to anumber of parameters such as the choice of ADC, layout of signal traces, and component tolerances.

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RTD Alternatives

Figure 2-2. Accuracy Chart for TMP117 and RTD
The TMP117 is comparable in accuracy to the Class AA thin-film RTD and consumes a fraction of the powerof a PT100 RTD. The systems using the TMP117 require less components, such as delta-sigma ADCs,
programmable gain amplifiers, and RC filters, than the systems using RTD elements.

2.2 TMP1826
The TMP1826 is a high-accuracy, single-wire compatible digital output temperature sensor. The typical accuracyof this device is ±0.1°C across the temperature range of –20°C to +85°C. Figure 2-3 shows the bus-poweredmode where the TMP1826 only requires a single connection and a ground return, which helps to simplify design
and reduce cost by limiting the number of wires in the system. Alternatively, the VDD pin gives users the option
to use a dedicated power supply for certain applications that may require it. This is seen in Figure 2-4. The
TMP1826 also has integrated 8-kV IEC-61000-4-2 ESD protection on the data pin, which removes the need forany external protection components.
1.7 V to 5.5 V

VDD

1.1 kΩMCU

GPIO

SDQ TMP1826

Temperature
SourceGND
Figure 2-3. TMP1826 Bus Powered Application
1.7 V to 5.5 V
0.1 µF
5 kΩ

MCU

GPIO

VDD
SDQ TMP1826

Temperature
SourceGND
Figure 2-4. TMP1826 Supply Powered Application

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Figure 2-5 shows the accuracy of the TMP1826 across temperature. The data sheet minimum and maximum
temperature error specifications are identified by the red lines on the graph, while the typical performance of
the TMP1826 is shown in black. This device can provide another high-accuracy alternative to complicated RTD
circuits.
0.6

Temperature Error (C)

0.4

0.2

0

-0.2

-0.4

-0.6
-40 -30 -20 -10

0

10

20

30

40

50

60

70

80

90 100 110 120 130

Temperature (C)

Figure 2-5. TMP1826 Temperature Error vs Temperature
Table 2-2. TMP1826 Accuracy Across Temperature
Range
Temperature Range

Accuracy–20°C to +85°C±0.3°C–40°C to +125°C±0.5°C

2.3 TMP6x
Most RTD applications use a current source to excite the RTD element and create a voltage difference.
The TMP61 is a linear silicon-based thermistor that has consistent sensitivity across temperature. Using the
TMP61 in a current biasing circuit such as Figure 2-6 can offer a method of temperature monitoring to
replace the expense and complexity of RTDs. The TMP61 has a positive temperature slope and a temperature
measurement range of –40°C to 125°C. The TMP61 can also be used in a voltage biasing circuit, seen in Figure2-7. The output voltage that corresponds to the measured temperature, VTEMP , is measured across RBIAS.

Figure 2-6. TMP61 Current Biasing Circuit

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Figure 2-7. TMP61 Voltage Biasing Circuit

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Conclusion

Note
For additional design resources TI provides the Thermistor Design Tool, which offers resistanceversus temperature table computation, alternative methods for deriving temperature, and example
C-code.

3 Conclusion
There are a variety of replacement options for RTD applications. Depending on specific system needs, a
digital temperature sensor such as the TMP116, TMP117, or TMP1826 can increase accuracy while keeping
printed circuit board costs and complexity low. For other systems, the TMP61 silicon-based linear thermistor
can be easily integrated using a current or voltage biasing source from an already-existing RTD design.
The high accuracy and fast response time of these semiconductor temperature sensors are key performancedifferentiators that allow these devices to replace expensive RTD systems in real-time control applications. In
applications where milliseconds can make or break system stability, TI enables the sensing, processing, control
and communication necessary to implement and optimize real-time control.

4 References
•
•
•
•
•
•
•
•

Texas Instruments, Replacing Resistance Temperature Detectors with the TMP116 Temp Sensor applicationnote
Texas Instruments, Analog linearization of resistance temperature detectors analog design journal
Texas Instruments, A Basic Guide to RTD Measurements application note
Texas Instruments, Thermistor Design Tool
Texas Instruments, Measuring an RTD Sensor with the TDC1000 and TDC7200 for Ultrasonic Sensing
application note
Texas Instruments, RTD Class-AA Replacement With High-Accuracy Digital Temperature Sensors in Field
Transmitters application note
Texas Instruments, RTD Replacement in High Accuracy Sensing and Compensation Systems Using Digital
Temperature Sensors application brief
Texas Instruments, Semiconductor Temperature Sensors Challenge Precision RTDs and Thermistors in
Building Automation application note

5 Revision History
NOTE: Page numbers for previous revisions may differ from page numbers in the current version.
Changes from Revision * (May 2021) to Revision A (June 2022)
Page• Expanded the Abstract ...................................................................................................................................... 1• Updated the RTD Alternatives section, including expanding the discussion on the TMP116 and TMP117, andadding sections for the TMP1826 and TMP6x....................................................................................................6

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