1 Precision Temperature-Sensing With RTD Circuits Author: INTRODUCTION Bonnie C. Baker Microchip Technology Inc. The most widely measured phenomena in the process control environment is temperature. Common elements, such as Resistance Temperature Detectors (RTDs), thermistors [ 7], thermocouples [ 6] or diodes are used to sense absolute temperatures, as well as changes in temperature. For an overview and comparison of these sensors, refer to Microchip s AN679, Temperature-Sensing Technologies [ 5]. Of these technologies, the platinum RTD temperaturesensing element is the most accurate, linear and stable over time [ 1] and temperature. RTD element technologies are constantly improving, further enhancing the quality of the temperature measurement. Typically, a data acquisition system conditions the analog signal from the RTD sensor, making the analog translation of the temperature usable in the digital domain. This application note focuses on circuit solutions that use platinum RTDs in their design (see Figure 1). The linearity of the RTD will be presented along with standard formulas that can be used to improve the off-theshelf linearity of the element. For additional information concerning the thermistor temperature sensor, refer to Microchip s AN685, Thermistors in Single Supply Temperature Sensing Circuits [ 7]. Finally, the signalconditioning path for the RTD system will be covered with application circuits from sensor to microcontroller. Precision Current Source < 1 ma V OUT RTD, the most popular element, is made using platinum; typically 100Ω at 0 C FIGURE 1: RTD Temperature-sensing Elements Use Current Excitation. RTD OVERVIEW The acronym RTD is derived from the term Resistance Temperature Detector [ 4]. The most stable, linear and repeatable RTD is made of platinum metal. The temperature coefficient of the RTD element is positive and almost constant. Typical RTD elements are specified with 0 C values of 50, 100, 200, 500, 1000 or 2000Ω. Of these options, the 100Ω platinum RTD is the most stable over time and linear over temperature. The RTD element requires a current excitation. If the magnitude of the current source is too high, the element will dissipate power and start to self-heat. Consequently, care should be taken to insure that less than 1 ma of current is used to excite the RTD element. An approximation to the platinum RTD resistance change over temperature can be calculated by using the constant a = Ω/Ω/ C (European curve, ITS-90). This constant is easily used to estimate the absolute resistance of the RTD at temperatures between -100 C and +200 C (with a nominal error smaller than 3.1 C). EQUATION 1: RTD( T) RTD 0 ( 1 + T α) RTD(T) = the RTD element s resistance at T (Ω), RTD 0 = the RTD element s resistance at 0 C (Ω), T = the RTD element s temperature ( C), α = Ω/Ω/ C If a higher accuracy temperature measurement is required, or a greater temperature range is measured, the standard formula below (Calendar-Van Dusen Equation) can be used in a calculation in the controller engine or be used to generate a look-up table. Figure 2 shows both the RTD resistance and its slope across temperature Microchip Technology Inc. DS00687C-page 1
2 EQUATION 2: RTD( T) = RTD 0 ( 1 + AT + BT 2 + CT 3 ( T 100) ) RTD(T) = the RTD element s resistance at T (Ω), RTD 0 = the RTD element s resistance at 0 C (Ω), T = the RTD element s temperature ( C) and A, B, C = are constants derived from resistance measurements at multiple temperatures. The ITS-90 standard values are: RTD 0 = 100Ω A = C -1 RTD Resistance ( ) B = C -2 C = C -4, T <0 C = 0, T 0 C R RTD d R RTD /d T RTD Omega RTD Table 100 at 0 C European Curve RTD Temperature ( C) Slope of RTD Resistance ( / C) FIGURE 2: The RTD sensing element s temperature characteristic has a positive temperature coefficient that is almost constant. When the RTD element is excited with a current reference, and self-heating is avoided, the accuracy can be ±4.3 C over the temperature range -200 C to 800 C. The accuracy of a typical RTD is shown in Figure 3. Min/Max RTD Temperature Error ( C) FIGURE 3: The platinum RTD temperature sensor s accuracy is better than other sensors, such as the thermocouple and thermistor. The advantages and disadvantages of the RTD temperature sensing element is summarized in Table 1. TABLE 1: -200 Omega RTD Table 100 at 0 C European Curve Advantages Very Accurate and Stable Reasonably Linear Good Repeatability Class A 500 RTD Temperature ( C) 600 Class B RTD TEMPERATURE SENSING ELEMENT ADVANTAGES AND DISADVANTAGES Disadvantages Expensive Solution Requires Current Excitation Danger of Self-Heating Low Resistive Element DS00687C-page Microchip Technology Inc.
3 RTD CURRENT EXCITATION CIRCUIT For best linearity, the RTD sensing element requires a stable current reference for excitation. This can be implemented in a number of ways, one of which is shown in Figure 4. In this circuit, a voltage reference, along with two operational amplifiers, are used to generate a floating 1 ma current source. RTD R W1 R W2 R W3 I RREF 1mA + V RREF V 2 R REF 2.5 kω V REF 2.5V FIGURE 4: A current source for the RTD element can be constructed in a single-supply environment from two op amps and a precision voltage reference. This is accomplished as follows. The op amp A 1 and the resistors R 1 through R 4 form a difference amplifier with a differential gain (G A1 ) of 1 V/V (since the resistors are all equal). A 2.5V precision voltage reference (V REF ) is applied to the input of this difference amplifier. The output of op amp A 2 (V OUT2 V 2 ) serves as the difference amplifier s reference voltage. The voltage at the output of A 1 is shown in Equation 3. A 2 A 1 R 2 R 1 25 kω 25 kω R 3 R 4 25 kω 25 kω A 1 =A 2 =½MCP602 EQUATION 3: Now it is easy to derive the voltage (V RREF ) across the resistor R REF, assuming V OUT2 =V 2 ; see Equation 4. EQUATION 4: The current used to bias the RTD assembly (I RREF ) is constant and independent of the voltage V 2 (which is across the RTD element); see Equation 5. EQUATION 5: V OUTA1 = V REF G A1 + V OUTA2 V OUTA1 = A 1 s output voltage V OUTA2 = A 2 s output voltage V REF = Reference voltage at the input G A1 = Differential Gain = 1 V/V V RREF = V OUTA1 V 2 = V RREF V REF V 2 = Voltage at A 2 s input V RREF = Voltage across R REF I RREF = V RREF R REF = 1 ma I RREF This current is ratio-metric to the voltage reference. The same voltage reference should be used in other portions of the circuit, such as the analog-to-digital (A/D) converter reference. Absolute errors in the circuit will occur as a consequence of the reference voltage, the op amp offset voltages, the output swing of A 1, mismatches between the resistors and the errors in R REF and the RTD element. The temperature drift of these same elements also causes errors; primarily due to the voltage reference, op amp offset drift and the RTD element Microchip Technology Inc. DS00687C-page 3
4 RTD SIGNAL-CONDITIONING PATH Changes in resistance of the RTD element over temperature are usually digitized through an A/D conversion, as shown in Figure 5. The current excitation circuit (see Figure 4) excites the RTD element. The magnitude of the current source can be tuned to 1 ma or less by adjusting R REF. The voltage drop across the RTD element is sensed by A 3, then gained and filtered by A 4. With this circuit, a 3-wire RTD element is selected. This configuration minimizes errors due to wire resistance and wire resistance drift over temperature. Current Generator Circuit R 2 R 1 25 kω 25 kω A 1 =A 2 =A 3 =A 4 =¼MCP609 RTD Sensor = PT100 (100Ω at 0 C) I RREF 1mA R RREF 2.5 kω A 1 V REF =2.5V PIC12C508 A 2 R 3 R 4 25 kω 25 kω Sallen-Key Filter with Gain RTD Sensor R W1 R W2 RTD R W3 Correct for R W R 5 R kω 100 kω R kω A 3 R 8 R kω 107 kω C 8B 390 nf C 8A 68 nf C nf R kω R kω A 4 R kω MCP3201 V +IN REF -IN VSS 3 FIGURE 5: This circuit uses a RTD element to measure temperatures from -200 C to 600 C. A current generator excites the sensor. An op amp (A 3 ) cancels the wire resistance error. Another op amp (A 4 ) gains and filters the signal. A 12-bit converter (MCP3201) converts the voltage across the RTD to digital code for the 8-pin controller (PIC12C508). In this circuit, the RTD element equals 100Ω at 0 C. If the RTD is used to sense temperature over the range of -200 C to 600 C, the resistance produced by the RTD would be nominally between 18.5Ω and 313.7Ω, giving a voltage across the RTD between 18.5 mv and mv. Since the resistance range is relatively low, wire resistance and wire resistance change over temperature can skew the measurement of the RTD element. Consequently, a 3-wire RTD device is used to reduce these errors. The errors contributed by the wire resistances, R W1 and R W3, are subtracted from the signal with op amp A 3. In this configuration, R 1 and R 2 are equal and are relatively high. The value of R 1 is selected to ensure that the leakage currents through the resistor do not introduce errors to the current in the RTD element. The transfer function of this portion of the circuit is: EQUATION 6: V OUTA3 = ( V IN V W1 )( 1 + R 6 R 5 ) V IN ( R 6 R 5 ) where: V IN = V W1 +V RTD +V W3, V Wx = the voltage drop across the wires to and from the RTD and V OUTA3 = the voltage at the output of A 3 If nominal resistor values are assumed, then A 3 s output voltage is significantly simplified: EQUATION 7: R 5 = R 6 R W1 = R W3 V OUTA3 = V RTD DS00687C-page Microchip Technology Inc.
5 The voltage signal at the output of A 3 is filtered with a 2 nd order, low pass filter created with A 4, R 8, C 8A, C8 B, R 9 and C 9. It is designed to have a Bessel response and a bandwidth of 10 Hz. R 10 and R 11 set a gain of 7.47 V/V. It reduces noise and prevents aliasing of higher frequency signals. This filter uses a Sallen-Key topology specially designed for high gain; see [ 10]. The capacitor divider formed by C 8A and C 8B improve this filter s sensitivity to component variations; the filter can be unproduceable without this improvement. R 12 isolates A 4 s output from the capacitive load formed by the series connection of C 8A and C 8B ; it also improves performance at higher frequencies. The voltage at A 4 s output is nominally between 0.138V and 2.343V, which is less than V REF (2.5V). The 12-bit A/D converter (MCP3201) gives a nominal temperature resolution of 0.22 C/LSb. CONCLUSION Although the RTD requires more circuitry in the signalconditioning path than the thermistor or the silicon temperature sensor, it ultimately provides a highprecision, relatively accurate result over a wider temperature range. If this circuit is properly calibrated, and temperature correction coefficients are stored in the PIC, it can achieve ±0.01 C accuracy. REFERENCES RTD Temperature Sensors  Evaluating Thin Film RTD Stability, SENSORS, Hyde, Darrell, Oct. 1997, pg. 79.  Refresher on Resistance Temperature Devices, Madden, J.R., SENSORS, Sept. 1997, pg. 66.  Producing Higher Accuracy From SPRTs (Standard Platinum Resistance Thermometer), MEASUREMENT & CONTROL, Li, Xumo, June 1996, pg  Practical Temperature Measurements, OMEGA Temperature Measurement Handbook, The OMEGA Made in the USA Handbook, Vol. 1, pp. Z-33 to Z-36 and Z-251 to Z-254. Other Temperature Sensors  AN679, Temperature Sensing Technologies, DS00679, Baker, Bonnie, Microchip Technology Inc.  AN684, Single-Supply Temperature Sensing with Thermocouples, DS00684, Baker, Bonnie, Microchip Technology Inc.  AN685, Thermistors in Single-Supply Temperature-Sensing Circuits, DS00685, Baker, Bonnie, Microchip Technology Inc. Sensor Conditioning Circuits  AN682, Using Operational Amplifiers for Analog Gain in Embedded System Design, DS00682, Baker, Bonnie, Microchip Technology Inc.  AN990, Analog Sensor Conditioning Circuits An Overview, DS00990, Kumen Blake, Microchip Technology Inc. Active Filters  Kumen Blake, Transmit Filter Handles ADSL Modem Tasks, Electronic Design, June 28, Microchip Technology Inc. DS00687C-page 5
6 NOTES: DS00687C-page Microchip Technology Inc.
7 Note the following details of the code protection feature on Microchip devices: Microchip products meet the specification contained in their particular Microchip Data Sheet. Microchip believes that its family of products is one of the most secure families of its kind on the market today, when used in the intended manner and under normal conditions. There are dishonest and possibly illegal methods used to breach the code protection feature. All of these methods, to our knowledge, require using the Microchip products in a manner outside the operating specifications contained in Microchip s Data Sheets. Most likely, the person doing so is engaged in theft of intellectual property. Microchip is willing to work with the customer who is concerned about the integrity of their code. Neither Microchip nor any other semiconductor manufacturer can guarantee the security of their code. Code protection does not mean that we are guaranteeing the product as unbreakable. Code protection is constantly evolving. We at Microchip are committed to continuously improving the code protection features of our products. Attempts to break Microchip s code protection feature may be a violation of the Digital Millennium Copyright Act. If such acts allow unauthorized access to your software or other copyrighted work, you may have a right to sue for relief under that Act. Information contained in this publication regarding device applications and the like is provided only for your convenience and may be superseded by updates. It is your responsibility to ensure that your application meets with your specifications. MICROCHIP MAKES NO REPRESENTATIONS OR WARRANTIES OF ANY KIND WHETHER EXPRESS OR IMPLIED, WRITTEN OR ORAL, STATUTORY OR OTHERWISE, RELATED TO THE INFORMATION, INCLUDING BUT NOT LIMITED TO ITS CONDITION, QUALITY, PERFORMANCE, MERCHANTABILITY OR FITNESS FOR PURPOSE. Microchip disclaims all liability arising from this information and its use. Use of Microchip devices in life support and/or safety applications is entirely at the buyer s risk, and the buyer agrees to defend, indemnify and hold harmless Microchip from any and all damages, claims, suits, or expenses resulting from such use. No licenses are conveyed, implicitly or otherwise, under any Microchip intellectual property rights. Trademarks The Microchip name and logo, the Microchip logo, Accuron, dspic, KEELOQ, KEELOQ logo, MPLAB, PIC, PICmicro, PICSTART, rfpic, SmartShunt and UNI/O are registered trademarks of Microchip Technology Incorporated in the U.S.A. and other countries. FilterLab, Linear Active Thermistor, MXDEV, MXLAB, SEEVAL, SmartSensor and The Embedded Control Solutions Company are registered trademarks of Microchip Technology Incorporated in the U.S.A. Analog-for-the-Digital Age, Application Maestro, CodeGuard, dspicdem, dspicdem.net, dspicworks, dsspeak, ECAN, ECONOMONITOR, FanSense, In-Circuit Serial Programming, ICSP, ICEPIC, Mindi, MiWi, MPASM, MPLAB Certified logo, MPLIB, MPLINK, mtouch, PICkit, PICDEM, PICDEM.net, PICtail, PIC 32 logo, PowerCal, PowerInfo, PowerMate, PowerTool, REAL ICE, rflab, Select Mode, Total Endurance, WiperLock and ZENA are trademarks of Microchip Technology Incorporated in the U.S.A. and other countries. SQTP is a service mark of Microchip Technology Incorporated in the U.S.A. All other trademarks mentioned herein are property of their respective companies. 2008, Microchip Technology Incorporated, Printed in the U.S.A., All Rights Reserved. Printed on recycled paper. Microchip received ISO/TS-16949:2002 certification for its worldwide headquarters, design and wafer fabrication facilities in Chandler and Tempe, Arizona; Gresham, Oregon and design centers in California and India. The Company s quality system processes and procedures are for its PIC MCUs and dspic DSCs, KEELOQ code hopping devices, Serial EEPROMs, microperipherals, nonvolatile memory and analog products. In addition, Microchip s quality system for the design and manufacture of development systems is ISO 9001:2000 certified Microchip Technology Inc. DS00687C-page 7
8 WORLDWIDE SALES AND SERVICE AMERICAS Corporate Office 2355 West Chandler Blvd. Chandler, AZ Tel: Fax: Technical Support: Web Address: Atlanta Duluth, GA Tel: Fax: Boston Westborough, MA Tel: Fax: Chicago Itasca, IL Tel: Fax: Dallas Addison, TX Tel: Fax: Detroit Farmington Hills, MI Tel: Fax: Kokomo Kokomo, IN Tel: Fax: Los Angeles Mission Viejo, CA Tel: Fax: Santa Clara Santa Clara, CA Tel: Fax: Toronto Mississauga, Ontario, Canada Tel: Fax: ASIA/PACIFIC Asia Pacific Office Suites , 37th Floor Tower 6, The Gateway Harbour City, Kowloon Hong Kong Tel: Fax: Australia - Sydney Tel: Fax: China - Beijing Tel: Fax: China - Chengdu Tel: Fax: China - Hong Kong SAR Tel: Fax: China - Nanjing Tel: Fax: China - Qingdao Tel: Fax: China - Shanghai Tel: Fax: China - Shenyang Tel: Fax: China - Shenzhen Tel: Fax: China - Wuhan Tel: Fax: China - Xiamen Tel: Fax: China - Xian Tel: Fax: China - Zhuhai Tel: Fax: ASIA/PACIFIC India - Bangalore Tel: Fax: India - New Delhi Tel: Fax: India - Pune Tel: Fax: Japan - Yokohama Tel: Fax: Korea - Daegu Tel: Fax: Korea - Seoul Tel: Fax: or Malaysia - Kuala Lumpur Tel: Fax: Malaysia - Penang Tel: Fax: Philippines - Manila Tel: Fax: Singapore Tel: Fax: Taiwan - Hsin Chu Tel: Fax: Taiwan - Kaohsiung Tel: Fax: Taiwan - Taipei Tel: Fax: Thailand - Bangkok Tel: Fax: EUROPE Austria - Wels Tel: Fax: Denmark - Copenhagen Tel: Fax: France - Paris Tel: Fax: Germany - Munich Tel: Fax: Italy - Milan Tel: Fax: Netherlands - Drunen Tel: Fax: Spain - Madrid Tel: Fax: UK - Wokingham Tel: Fax: /02/08 DS00687C-page Microchip Technology Inc.
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