“Temperature measurement is an essential function, and RTDs are a common and widely used sensor in this application, even though the correct use may seem complicated. However, it provides high accuracy and repeatability over a wide temperature range when the proper circuitry is used for excitation and detection. As with any high-performance sensor, device characteristics must be understood to obtain optimum performance. As mentioned above, using ICs with different levels of functional integration, users can build RTD-based systems that eliminate surprises and deliver superior performance.
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By Bill Schweber
From healthcare, instrumentation, HVAC and automotive applications to the Internet of Things (IoT), temperature is the most widely used sensing parameter in the real world. Understanding temperature with the right balance of accuracy, precision, and repeatability is critical for many applications. A widely used temperature sensor is a resistance temperature detector (RTD), which is a precision metal element, usually made of pure or near-pure platinum. Platinum-based sensors have fully detailed, repeatable, and characterized resistance-temperature transfer functions, so RTDs are widely used in scientific and instrumentation applications.
However, to realize the full performance potential of this deceptively simple two-terminal sensor, the designer must understand the various excitation methods as well as resistance measurement methods in order to determine temperature. Additionally, many applications require multiple RTDs, so the connection method and associated circuitry must also match the application.
What designers need are RTD-specific components to handle and overcome the inherent characteristics of RTDs. This article shows how to use ICs from Texas Instruments, Maxim Integrated and Analog Devices, and evaluation boards from Microchip Technology to simplify their applications.
How RTD Sensors Work
Somewhat similar to thermistors, RTDs work in a seemingly simple way, but they are not. RTDs are platinum wires or thin films, sometimes with the addition of other precious metals such as rhodium, that have a known nominal resistance and that resistance changes positively with temperature as a function of temperature (i.e. positive temperature coefficient or PTC). RTDs can be manufactured with many different nominal resistance values, the most common being Pt100 and Pt1000 (sometimes written PT100 and PT1000), with nominal resistances of 100 Ω and 1000 Ω respectively at 0⁰C.
Common methods of constructing such sensors include wrapping a platinum wire around a glass or ceramic skeleton, or fabricating with platinum thin films (Figure 1). Due to the wide application and interchangeability requirements of platinum temperature sensors, the international standard DIN EN 60751 (2008) defines the electrical characteristics of platinum temperature sensors in detail. The standard contains resistance-temperature tables, tolerances, curves and temperature ranges.
Figure 1: These RTDs use (left to right) thin-film, glass, and ceramic fabrication techniques. (Image credit: WIKA Alexander Wiegand SE & Co. KG)
Standard platinum RTDs have an operating temperature range of -200⁰C to +800⁰C. Key attributes of RTDs include high stability, repeatability, and accuracy, provided they are properly excited by a current or voltage source, the resistance is measured as the voltage across the two terminals using a suitable analog front end (AFE) circuit, where the voltage reading Linearized for highest accuracy.
The resistance of RTDs varies considerably with temperature, making RTDs more suitable for high-precision measurements. For a standard Pt100 device, the resistance changes from about 25 Ω at -200⁰C to about +375 Ω at +800⁰C. Between 0°C and +100°C, the average slope is called alpha (α) or temperature coefficient, and its value depends on the impurities and their content in platinum. The two most widely used alpha values are 0.00385055 and 0.00392.
There are thousands of specific models of RTDs from many sources. An example is Vishay Beyschlag’s PTS060301B100RP100, a 100 Ω platinum RTD with a basic accuracy of ±0.3% and a temperature coefficient of ±3850 ppm/°C in a 0603 SMT package. The sensors are 100 Ω, 500 Ω and 1000 Ω PTS series lead-free SMT RTDs available in 0603, 0805 and 1206 packages respectively. These devices are fabricated using a uniform platinum film deposited on an advanced ceramic substrate and tuned to achieve the correct temperature coefficient and stability. The sensor element is covered by a protective coating that provides electrical, mechanical and weather protection and meets all relevant IEC and DIN performance and compliance standards. The small size of the 100 Ω device in the 0603 package results in a very fast response under natural ventilation conditions, reaching within 90% of the final resistance value in less than 2 seconds.
RTD Linearization
The RTD is fairly linear, but still has a monotonic curve deviation. For applications requiring a degree of accuracy or a few degrees, linearization of the RTD transfer function may not be necessary due to the small deviation (Figure 2). For example, between -20⁰C and +120⁰C, the difference is less than ±0.4⁰C.
Figure 2: Pt100 RTD resistance versus temperature, showing a straight-line approximation from 0°C to +100°C. (Image source: Maxim Integrated)
However, RTDs are often used in precision applications that require an accuracy of one-tenth of a degree or better, thus requiring linearization. Linearization can be achieved by calculations or look-up tables in software. For highly accurate linearization, the Callendar-Van Dusen formula can be used:
where T = temperature (°C); R(T) = resistance at T; R0 = resistance at T = 0°C; A, B, and C are RTD specific constants.
For α = 0.00385055, the DIN RTD standard defines the Callendar-Van Dusen coefficient values A, B and C as:
A = 3.90830 x 10-3,
B = -5.77500 x 10-7, and
From -200°C to 0°C, C = -4.18301 x 10-12; from 0°C to +850°C, C = 0 (This has the advantage of reducing the polynomial to a simpler second-order equation.)
RTD connection
As passive two-terminal resistors, RTD interface excitation and sensing circuits are simple in principle, and the excitation source can be either voltage or current. In its most basic form of voltage source, the RTD leads are connected to the excitation source in series with a known stable resistor (RREF), usually of the same nominal value as the RTD (Figure 3). This forms a standard voltage divider circuit. The voltage across the RTD and the series resistor is measured, and a simple voltage divider calculation is used to derive the RTD resistance. Accuracy can be improved by measuring the voltage across a known resistor as well as the voltage across the RTD.
Figure 3: This simplified RTD signal conditioning circuit uses the RTD in series with a known reference resistor (RREF) and a current source; RTD resistance is calculated by measuring the voltage across the RTD and the voltage across the reference resistor. (Image source: Maxim Integrated)
This configuration, while simple, has many potential sources of inaccuracy, including supply voltage variation, reference resistor temperature coefficient, impedance (IR) drop across the connecting leads, and even the temperature coefficient of the copper connecting leads (approximately +0.4%/ °C). To partially overcome these error sources, RTDs are typically used in a ratiometric Wheatstone bridge configuration.
However, the bridge and voltage excitation methods still have drawbacks. Ratiometric structures, such as bridges, inherently have well-known nonlinear relationships that are independent of the nonlinearity of any bridge element. Therefore, this relationship must be taken into account in the calculation to correct the nonlinearity of the RTD element, which complicates the algorithm and increases the processing load.
For these and other reasons, RTDs are almost always used with current sources. This allows full control of the excitation situation and provides the opportunity to more directly compensate for voltage drops and temperature-dependent changes in the connecting leads. Depending on the application and the distance between the RTD and the AFE, designers can use a two-wire, three-wire, four-wire, or four-wire connection with return (Figure 4).
Figure 4: The interconnection between RTD and AFE can use two, three, or four wires; the latter can be a paired four-wire connection, or a separate loop for two wires. (Image credit: Texas Instruments)
Two-wire connections are the easiest, smallest, and lowest cost. However, it is only suitable for obtaining accurate results if the wires connecting the Pt100 RTD to the AFE circuit have very low resistance, below a few milliohms (mΩ). In this case, the wire resistance is insignificant compared to the RTD resistance. Typically, this limits the distance to about 25 centimeters (cm), but it also depends on the wire gauge of these wires. These wires tend to be thin due to physical installation configurations and limitations. Of course, calculations can be used to correct for the voltage drop. However, this adds complexity, especially when the lead resistance is affected by temperature.
For distances longer to within about 30 meters (m), the three-wire method can be used. In this configuration, the circuit monitors one side of the current loop through the Kelvin connection, measures the voltage drop in the loop resistance, and then compensates for this drop. This method assumes that the voltage drop in the non-Kelvin leads is the same as the voltage drop on the Kelvin lead side.
The four-wire method uses full Kelvin sensing to monitor both sides of the RTD current loop. This method precisely removes the effects of lead resistance, regardless of the difference between the two current source wires. It can be used at a distance of hundreds of meters, but the material and wire volume have the greatest impact.
Finally, the four-wire method with loop allows the designer to choose how to measure losses in the loop. The resistance of the loop connection wire can be measured as a simple resistance, independent of the actual RTD loop, assuming that the two extra leads are identical to the RTD leads. This approach seems to be more of a headache to install and compute than direct Kelvin configuration, but there are practical cases where it is difficult to provide a regular Kelvin connection at the RTD. However, this configuration is not often used in modern installations, as four-wire or even three-wire methods can provide comparable results with proper setup and calibration.
Note that choosing to use a two-, three-, or four-wire connection has nothing to do with the RTD, any connection will work with any RTD as long as there is room and the necessary physical connections can be made. However, in settings with smaller physical dimensions, the mass of the wire harness may introduce thermal drift and other thermal time constants. In general, it is a good practice to keep the thermal mass of the sensing configuration as small as possible relative to the mass being sensed.
Issues related to connecting leads and signal integrity are not limited to basic DC resistance. Noise is often a concern, and although temperature is a relatively slow-changing phenomenon compared to most noisy signals, it can still corrupt the voltage at the AFE if it happens to happen when the voltage on the RTD is being sampled or converted. Signal. In extreme cases, noise can saturate the front end and “blind” it for a few milliseconds (ms) until it comes out of saturation.
For this and other reasons, RTDs with sense leads longer than a meter or so should be balanced with the same impedance to ground (sometimes called longitudinal balance). The reason is that these parallel leads may have common mode voltage (CMV) and noise, but the differential front end of the AFE will keep these out. However, if the leads are unbalanced, the circuit will convert some common-mode signals to unbalanced signals that are not rejected by the differential inputs of the AFE.
Pt100 and Pt1000 RTD selection
Since the most common RTDs have 100 Ω or 1000 Ω resistance at 0°C, choosing between them is an obvious question. As always, we need to make tradeoffs, and there is no single “right” answer, as it depends on the specifics of the application. Note that for Pt100 and Pt1000 RTDs, the linearity of the characteristic curves, the operating temperature range, and the response time are all the same or nearly the same, and their temperature coefficient of resistance is also the same.
The nominal resistance of Pt100 RTDs is low, so as mentioned earlier, only a two-wire configuration can be used for short-range applications, as the lead resistance will be significant relative to the RTD. In contrast, the lead resistance is much smaller relative to the Pt1000 resistance, making the Pt1000 more suitable for longer two-wire applications.
Since a Pt1000 RTD has a higher resistance, less excitation current is required to produce a given voltage across it according to Ohm’s law (V = IR). At 0⁰C, a modest 1 mA current will produce a voltage drop of 1 V, and as the temperature increases, the voltage will increase from that value.
However, since the RTD voltage may exceed the range of the AFE front end at higher temperatures, there may be undesirable consequences of higher voltages. In addition, the current source needs to have a sufficient compliance voltage to excite a fixed value of current through the resistor. For example, a 1 mA current through a 1000 Ω resistor requires a current source with a compliance voltage slightly higher than 1 V, but as the RTD heats up and its resistance increases, the required compliance voltage increases proportionally. Therefore, high resistance RTD current sources may require higher voltage rails to ensure adequate compliance voltage.
Pt1000 requires lower current for a given voltage drop, which brings two benefits. First, less power is required, which can increase battery life. Second, the self-heating of the RTD is reduced, which has a large impact on the accuracy of the readings. Proper engineering practice is to use an excitation current level that minimizes sensor self-heating, consistent with creating enough voltage drop across the RTD to achieve adequate resolution.
This does not mean that the status of Pt100 RTDs is low. In fact, it is widely used in industry for legacy reasons where lead length, low power operation and self-heating are not the main application factors. As a low impedance loop, Pt100 RTD units are also much less sensitive to noise pickup than Pt1000 RTDs, which inherently have ten times higher loop impedance.
Furthermore, in addition to the electrical aspects, there are also mechanical considerations. Pt100 sensors are available in both wire-wound and thin-film structures with different physical properties, while Pt1000 RTDs are generally only available in thin-film devices.
Note that for higher precision applications, additional measures may be required to minimize RTD self-heating errors. One way is to pulse current through the RTD and measure the voltage during the pulse period. The shorter the duty cycle of the pulse, the smaller the self-heating error. However, this approach also requires a slightly more complex interface to properly manage pulse timing and duty cycle, as well as synchronize voltage readings with pulses.
IC Simplifies RTD Interface
Like other resistor-based temperature sensing components, RTDs look simple and should be used. After all, it’s a two-terminal resistor with no serious parasitics in the relatively slow realm of temperature sensing. However, as with thermistors and many other basic sensors, we see users of this sensor need to consider a range of issues, including excitation, linearization, calibration, lead compensation, and more; complexity is also added when multiple RTDs are used increase, as is often the case.
To address the issues associated with RTD connections, IC vendors have developed application-specific ICs that simplify the front-end connections on the RTD-facing analog side and the conditioned output, even further including a full processor-compatible digital interface. For example, for basic RTD connections, Texas Instruments’ OPA317IDBVT op amp uses a proprietary auto-calibration technique that simultaneously provides low offset voltage (20 μV typical, 90 μV maximum), close proximity over time and temperature. Zero drift, and near-zero bias current. Therefore, the op amp does not “load” or affect the RTD, but is “invisible” and consistent. The op amp operates from a single-ended or bipolar supply of 1.8 V (±0.9 V) to 5.5 V (±2.75 V) and has a maximum quiescent current of 35 μA, making it ideal for battery-powered applications.
One of the properties of this op amp is that it can be configured to handle signals very close to ground, and the same goes for “cold” RTDs, ie, operating at low current levels, and thus low voltages across them. In contrast, many single-supply op amps experience problems when the input and output signals are near 0 V (near the lower limit of the output swing of a single-supply op amp). While a good single-supply op amp may swing close to single-supply ground, it may not really hit ground. The output of the OPA317IDBVT can be made to swing to ground or slightly lower on a single-voltage supply by adding another resistor and an extra supply that is more negative than the op amp’s negative supply (Figure 5). Adding a pull-down resistor between the output and the additional negative supply reduces the output below what it would otherwise be.
Figure 5: By adding a pull-down resistor (RP) and an additional negative supply, the OPA317IDBVT can handle signals close to ground. (Image credit: Texas Instruments)
More than just an analog interface op amp, Maxim Integrated’s MAX31865 is an easy-to-use resistor-to-digital converter optimized for Pt100 and Pt1000 RTDs (Figure 6). Available in tiny 20-pin TQFN and SOIC packages, the IC can be configured for two-, three-, and four-wire RTD interfaces, while providing an SPI-compatible interface on the processor side.
Figure 6: Maxim Integrated’s MAX31865 RTD-to-digital converter includes an analog interface, digitizer, and SPI outputs for two-, three-, and four-wire RTDs. (Image source: Maxim Integrated)
A single external resistor sets the sensitivity of the RTD used, while a precision 15-bit sigma-delta ADC converts the RTD resistance to reference resistance ratio to digital form with a nominal temperature resolution of 0.03125⁰C over all operating and extreme conditions, Accuracy is 0.5⁰C.
Many temperature measurement applications require the use of multiple RTDs and other temperature sensors to form a complete test setup. For these applications, Analog Devices’ LTC2983 sensor-to-digital high-accuracy digital temperature measurement system IC supports a variety of sensors and options. The device handles up to 20 sensor channels, which can be a mix of two-, three-, and four-wire RTDs, thermocouples, thermistors, and even diodes (Figure 7). The IC can be programmed for a specific type of sensor and excitation required, and then provides built-in standard coefficients for those sensors; it also supports user-specified custom coefficients.
Figure 7: Analog Devices’ LTC2983 has 20 general-purpose inputs that can be mixed as needed between thermocouples, two-, three- or four-wire RTDs, thermistors, and diodes used as temperature sensors. (Image credit: Analog Devices)
The device provides digital results in °C or °F with 0.1°C accuracy and 0.001°C resolution over the SPI interface. It operates from a single 2.85 V to 5.25 V supply and includes excitation current sources and fault detection circuitry for each temperature sensor, as well as cold junction compensation (CJC) for any thermocouple.
Teams looking to create a complete circuit tailored to their RTD data acquisition design but don’t want to “do it all over again” can use the TMPSNS-RTD1 Pt100 RTD Evaluation Board from Microchip Technology. The evaluation board supports two RTDs, allowing the user to configure key operating parameters, including RTD current (Figure 8).
Figure 8: Microchip Technology’s TMPSNS-RTD1 Pt100 RTD Evaluation Board supports two RTDs and allows the user to configure key operating parameters. (Image source: Microchip Technology)
This evaluation board block diagram shows how to build a complete RTD interface channel, function by function, so that the user can understand the circuit and then make adjustments as needed (Figure 9). The evaluation board has an internal RTD and can also connect an external two-, three-, or four-wire Pt100 RTD, as well as a low-current current source to minimize self-heating. The voltage across the RTD can be amplified using the MCP6S26 Programmable Gain Amplifier (PGA). This PGA boosts the RTD voltage and also allows the user to digitally program the amplifier gain and extend the sensor output range. In addition, the differential amplifier drives a 12-bit differential analog-to-digital converter (ADC). Finally, the microcontroller reads the converter output data using the SPI interface and sends it to the host PC via the USB interface.
Figure 9: This TMPSNS-RTD1 Pt100 RTD evaluation board block diagram shows the AFE and associated signal paths from the RTD excitation/sensing via the SPI interface. (Image source: Microchip Technology)
The associated user guide includes complete installation and setup information, as well as step-by-step instructions for an intuitive PC-based graphical user interface (GUI). The GUI allows the user to set parameters such as number of samples, sample rate, PGA gain, internal RTD current and external current (Figure 10).
Figure 10: Through the PC-based GUI provided by the application, users of the TMPSNS-RTD1 Pt100 RTD evaluation board can adjust key operating points and evaluate the resulting performance. (Image source: Microchip Technology)
To complete the documentation, the user guide includes a full detailed bill of materials (BOM), schematics, top and bottom printed circuit board layouts, and silkscreen.
Epilogue
Temperature measurement is an essential function, and RTDs are a common and widely used sensor in this application, even though the correct use may seem complicated. However, it provides high accuracy and repeatability over a wide temperature range when the proper circuitry is used for excitation and detection. As with any high-performance sensor, device characteristics must be understood to obtain optimum performance. As mentioned above, using ICs with different levels of functional integration, users can build RTD-based systems that eliminate surprises and deliver superior performance.
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