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Application Brief

Application Brief

RTD Replacement in High Accuracy Sensing andCompensation Systems Using Digital TemperatureSensors

Introduction
A resistance temperature detector (RTD) is apassive component whose resistance changes withtemperature. RTDs are made using metals—suchas platinum, copper, or nickel—and support awide temperature range (approximately –200°C to+850°C).
The accuracy of an RTD is defined by its class. The
IEC/EN60751 standard defines four RTD classes—
Class C, B, A, and AA—where Class C is the leastaccurate and Class AA is the most accurate.
Most RTD applications use a current source to excite
the RTD element and create a voltage difference
across the RTD, as shown in Figure 1. This voltage
is proportional to the resistance of the RTD and theexcitation current. The voltage potential is amplified,converted to a digital output by an ADC, and is thenfed into to a MCU where a lookup table is used toconvert the digital output to temperature.

Solid state heat meters are becoming more popular
for billing heat energy for residential and industrial
users. Heat meters are essentially flow meterswith an integrated precise differential temperature
measurement subsystem to record the difference intemperature between the inlet and outlet flow. Figure
2 shows a block diagram of a heat meter systemusing RTDs.
Heat meters use the differential temperature
measurement subsystem to measure temperature
every few seconds. This measurement tells the user
if heat is being dissipated or absorbed in the system.
The system’s ability to quickly wake from power-off
mode, sample the RTD temperature, and return topower-off mode extends battery life and minimizes
energy consumption.

Figure 2. Heat and Cold Meter With RTD Block
DiagramFigure 1. Simplified RTD Circuit
RTDs in Heat Meters and CJC Systems
RTDs are one of the most widely used temperaturesensing components in industrial applications wherehigh accuracy and longevity are desired. Heat
Meters and Cold Junction Compensation (CJC) in
Thermocouples are a couple of examples where
RTDs are used; however, these systems can be
optimized to reduce complexity and cost by replacing
RTDs with temperature-sensing ICs, like the TMP117
digital temperature sensor.

SNOAA06C – JULY 2018 – REVISED JUNE 2022
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Thermocouples are temperature-sensing devices thatcover a large temperature range and are made byjoining wires of two different metals. The output
voltage is approximately proportional to the differencein temperature between the hot junction and cold
junction. Remember that because the thermocouple
measures a differential temperature, the temperature
at the cold junction must be known to determine
the temperature at the hot junction—this process isknown as cold junction compensation (CJC).
PT100 RTDs are popularly used to measure thetemperature of the cold junction due to their highaccuracy. Figure 3 shows a block diagram of a cold
junction compensated thermocouple system using aRTD reference.

RTD Replacement in High Accuracy Sensing and Compensation Systems
Using Digital Temperature Sensors
Copyright © 2022 Texas Instruments Incorporated

1Application Brief

www.ti.comof operation minimizes power losses and the highaccuracy of these devices allows for increased
efficiency in real-time control systems. The accuracy
and response time of the TMP117 and TMP1826
can improve efficiency of the power system byallowing operation closer to thermal limits and limiting
unnecessary current throttling.
Figure 3. Cold Junction Compensation With RTD
Block Diagram
Replacing RTD With TMP117 and TMP1826 Digital
Temp Sensors
The TMP117 is a digital temperature sensor designed
for low-power, high-accuracy applications. The device
provides a 16-bit temperature result with a resolutionof 0.0078°C and a factory-calibrated performance of±0.2°C across –55°C to +150°C which exceeds theaccuracy of a Class A RTD. The TMP1826 is ahigh-accuracy, single-wire compatible digital output
temperature sensor. The typical accuracy of thisdevice is ±0.1°C across the range of –20°C to +85°C.
In bus-powered mode TMP1826 only requires a single
connection and a ground return, which helps tosimplify design and reduce cost by limiting the numberof wires in the system.
These devices provide additional simplicity over an
RTD, eliminating the need for calibration, external
circuitry, matched traces, and Kelvin connections.
Figure 3 depicts the results of an oil bath experimentconducted on the TMP116, which is pin-compatible
to the higher accuracy TMP117. The graph showsthat even TMP116 can meet the accuracy of aClass AA RTD. These high accuracy solutions canenable smaller and more reliable real-time controlsystems, as fast response times will enable fastererror correction when it matters most.

Figure 4. TMP116 Target Board Meets Class AA
Limits Without Calibration
The TMP117 and TMP1826 feature a shutdown
mode where the device aborts the currently running
conversion and enters a low-power shutdown
mode. Both devices can perform quick temperature
conversions using the one-shot conversion mode.
After completing a one-shot conversion, the device
returns to low-power shutdown mode. This mode
2The TMP117 is comparable in accuracy to the ClassAA thin-film RTD, used in Heat Meter applications,and consumes a fraction of the power of a PT100
RTD, typically used in CJC applications. Figure 5 andFigure 6 show block diagrams of Heat Meter and CJC
systems using the TMP117 to replace the RTDs. 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.
Overall, the high-accuracy TMP117 and TMP1826
temperature sensors, fast conversion, and extremely
low-power shutdown mode eliminate the need for
multiple narrow-tolerance discrete components andintegrated devices, which can save PCB space,complexity, and cost in Heat Meter and CJC systems.

Figure 5. Heat and Cold Meter With TMP117 Block
DiagramFigure 6. Cold Junction Compensation WithTMP117 Block Diagram
Table 1. Recommended Documentation
COLLATERALDESCRIPTION

Application Report

Replacing Resistance Temperature Detectors Withthe TMP116 Temp Sensor

TI Design

High-Precision Temperature Measurement for Heatand Cold Meters Reference Design

TI Design

Thermocouple Analog Front End (AFE) Using RTDand Internal Temperature Sensor of ADS1220 forCold Junction Compensation (CJC)

RTD Replacement in High Accuracy Sensing and Compensation SystemsUsing Digital Temperature Sensors
Copyright © 2022 Texas Instruments Incorporated

SNOAA06C – JULY 2018 – REVISED JUNE 2022
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