Circuit Note CN-0214 Devices Connected/Referenced Circuits from the Lab™ reference circuits are engineered and tested for quick and easy system integration to help solve today’s analog, mixed-signal, and RF design challenges. For more information and/or support, visit www.analog.com/CN0214. ADuC7060/ ADuC7061 ARM7 Based Microcontroller with Dual 24-Bit Σ-Δ ADCs ADP3333 2.5 V Low Dropout Linear Regulator ADP7102-2.5 50 mA, High Voltage, Micropower Linear Regulator OP193 Precision, Low Power Op Amp USB-Based Thermocouple Temperature Measurement System Using ARM7 EVALUATION AND DESIGN SUPPORT converter (DAC), and a 1.2 V internal reference, as well as an ARM7 core, 32 kB flash, 4 kB SRAM, and various digital peripherals such as UART, timers, SPI, and I2C interfaces. Circuit Evaluation Boards ADuC7061 Mini Kit (EVAL-ADUC7061MKZ) Design and Integration Files Schematics, Layout Files, Bill of Materials In the circuit, the ADuC7060/ADuC7061 are connected to a thermocouple and a 100 Ω platinum resistance temperature detector (RTD). The RTD is used for cold junction compensation. CIRCUIT FUNCTION AND BENEFITS In the source code, an ADC sampling rate of 4 Hz is chosen. When the ADC input programmable gain amplifier (PGA) is configured for a gain of 32, the noise-free code resolution of the ADuC7060/ADuC7061 is greater than 18 bits. This circuit uses the ADuC7060/ADuC7061 precision analog microcontroller in an accurate thermocouple temperature monitoring application. The ADuC7060/ADuC7061 integrates dual 24-bit sigma-delta (Σ-Δ) analog-to-digital converters (ADCs), dual programmable current sources, a 14-bit digital-to-analog 2.5V USB HEADER ADP3333-2.5 BEAD 5V IN BEAD 10Ω OUT 4.7µF 4.7µF 4.7µF 0.1µF 0.1µF FT232R D– D+ TxD RxD AVDD IEXC0 GND SHIELD 0.1µF BEAD DVDD 10Ω 100Ω PtRTD S2 1kΩ S1 1kΩ RESET NTRST/BM ADC0 0.01µF 10Ω ADC1 0.01µF ADuC7060/ADuC7061 FERRITE BEADS: 1kΩ @ 100MHz TAIYO YUDEN BK2125HS102-T 2.5V OP193 RREF VREF+ 5.62kΩ 0.1% TxD RxD AGND DGND 09869-001 DAC0 P1.0/SIN ADC3 THERMOCOUPLE JUNCTION P1.1/SOUT VREF– Figure 1. ADuC7060/ADuC7061 as a Temperature Monitor Controller with a Thermocouple Interface (Simplified Schematic, All Connections Not Shown) Rev. D Circuits from the Lab™ circuits from Analog Devices have been designed and built by Analog Devices engineers. Standard engineering practices have been employed in the design and construction of each circuit, and their function and performance have been tested and verified in a lab environment at room temperature. However, you are solely responsible for testing the circuit and determining its suitability and applicability for your use and application. Accordingly, in no event shall Analog Devices be liable for direct, indirect, special, incidental, consequential or punitive damages due to any cause whatsoever connected to the use of any Circuits from the Lab circuits. (Continued on last page) One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A. Tel: 781.329.4700 www.analog.com Fax: 781.461.3113 ©2011–2013 Analog Devices, Inc. All rights reserved. CN-0214 Circuit Note The following features of the ADuC7060/ADuC7061 are used in this application: • • • • • • • • Primary 24-bit Σ-Δ ADC with PGA. The PGA is set for a gain of 32 in the software for this application. The primary ADC was continuously switched between sampling the thermocouple and the RTD voltages. Programmable excitation current sources for forcing a controlled current through the RTD. The dual current sources are configurable in 200 µA steps from 0 µA to 2 mA. For this example, a 200 µA setting was used to minimize the error introduced by the RTD self-heating. Internal 1.2 V reference for the ADC in the ADuC7060/ ADuC7061. For measuring the thermocouple voltage, the internal voltage reference was used due to its precision. External voltage reference for the ADC in the ADuC7060/ ADuC7061. For measuring the RTD resistance, a ratiometric setup was used where an external reference resistor (RREF) was connected across the external VREF+ AND VREF− pins. A 14-bit DAC. The DAC was used to set the thermocouple common-mode voltage to 850 mV above ground. An ARM7TDMI® core. The powerful 16-/32-bit ARM7 core with integrated 32 kB flash and SRAM memory runs the user code that configures and controls the ADC, processes the ADC conversions from the RTD, and controls the communications over the UART/USB interface. A UART. The UART was used as the communication interface to the host PC. Two external switches are used to force the part into flash boot mode. By holding S1 low and toggling S2, the ADuC7060/ ADuC7061 enters boot mode instead of normal user mode. In boot mode, the internal flash can be reprogrammed through the UART interface. Both the thermocouple and the RTD generate small signals; therefore, a PGA is required to amplify those signals. As the auxiliary ADC on the ADuC7060/ADuC7061 does not have a PGA, both were connected to the primary ADC and switching between the two was done in software. In addition to the decoupling shown in Figure 1, the USB cable must have a ferrite bead for added electromagnetic interference (EMI)/radio frequency interference (RFI) protection. The ferrite bead used in this circuit was the Taiyo Yuden, #BK2125HS102-T, which has an impedance of 1000 Ω at 100 MHz. The circuit must be constructed on a multilayer printed circuit board (PCB) with a large area ground plane. Proper layout, grounding, and decoupling techniques must be used to achieve optimum performance (see Tutorial MT-031, Grounding Data Converters and Solving the Mystery of "AGND" and "DGND," Tutorial MT-101, Decoupling Techniques, the ADuC7060 Evaluation Board layout and ADuC7061 Evaluation Board layout). The EVAL-ADuC7061MKZ PCB is shown in Figure 2. 09869-002 CIRCUIT DESCRIPTION Figure 2. EVAL-ADuC7061MKZ Board Used for this Circuit. Code Description The source code used to test the circuit can be downloaded as a zip file from www.analog.com/CN0214-SourceCode. The UART is configured for a baud rate of 9600, eight data bits, no parity, and no flow control. If the circuit is connected directly to a PC, a communication port viewing application such as the Hilgraeve HyperTerminal can be used to view the results sent by the program to the UART, as shown in Figure 3. The thermocouple used in this application is a Type T (copperconstantan) that has a temperature range of −200°C to +350°C. Its sensitivity is approximately 40 µV/°C, which means that the ADC in bipolar mode, with a PGA gain of 32, can cover the entire temperature range of the thermocouple. The RTD was used for cold junction compensation. The one used in this circuit was a platinum 100 Ω RTD, Enercorp PCS 1.1503.1. It is available in a 0805 surface-mount package. This RTD has a temperature variation of 0.385 Ω/°C. The USB interface to the ADuC7060/ADuC7061 is implemented with an FT232R UART-to-USB transceiver that converts USB signals to the UART directly. 09869-003 Note that the reference resistor, RREF, must be a precision 5.62 kΩ (±0.1%) resistor. Figure 3. Output of HyperTerminal Communication Port Viewing Application Rev. D | Page 2 of 5 Circuit Note CN-0214 To get a temperature reading, the temperature of the thermocouple and RTD must be measured. The RTD temperature is converted to its equivalent thermocouple voltage via the lookup table provided in the source code. These two voltages are added together to give the absolute value at the thermocouple. 0.30 0.25 ERROR (°C) 0.20 First, measure the voltage between the two wires of the thermocouple (V1). The RTD voltage is then measured and converted to a temperature via the lookup table. This temperature is then converted to its equivalent thermocouple voltage (V2). V1 and V2 are then added to give the overall thermocouple voltage, and this value is then converted to the final temperature measurement. 0.15 0.10 0.05 Initially, this was done using a simple linear assumption that the voltage on the thermocouple was 40 µV/°C. It can be seen from Figure 4 that this gives an acceptable error only for a small range around 0°C. –0.05 –210 –140 –70 0 70 140 210 280 09869-005 0 350 TEMPERATURE (°C) Figure 5. Error When Using Piecewise Linear Approximation Using 52 Calibration Points and Ideal Measurements 1.0 –10 0.9 0 0.8 0.7 ERROR (°C) ERROR (°C) –20 –40 0.6 0.5 0.4 –60 0.3 0.2 –80 –140 –70 0 70 140 210 280 350 TEMPERATURE (°C) 0 –210 09869-004 –100 –210 –140 –70 0 70 140 TEMPERATURE (°C) Figure 4. Error When Using Simple Linear Approximation A better way of calculating the thermocouple temperatures is to use a sixth-order polynomial for the positive temperatures and a seventh-order polynomial for the negative temperatures. This requires mathematical operations that add to computational time and code size. A suitable compromise is to calculate the respective temperatures for a fixed number of voltages. These temperatures are stored in an array, and values in between are calculated using a linear interpolation between the adjacent points. It can be seen from Figure 5 that the error is drastically reduced using this method. Figure 5 gives the algorithm error using ideal thermocouple voltages. Figure 6 shows the error obtained when using ADC0 on the ADuC7060 to measure 52 thermocouple voltages over the full thermocouple operating range. The overall worst-case error is <1°C. 210 280 350 09869-106 0.1 Figure 6. Error When Using Piecewise Linear Approximation Using 52 Calibration Points Measured by Using ADC0 of the ADuC7060/ADuC7061 The RTD temperature is calculated using the lookup table provided in the source code and is implemented for the RTD the same way as for the thermocouple. Note that the RTD has a different polynomial describing its temperatures as a function of resistance. For details on linearization and maximizing the performance of the RTD, refer to Application Note AN-0970, RTD Interfacing and Linearization Using an ADuC706x Microcontroller. COMMON VARIATIONS The ADP7102 regulator can be used as a newer alternate to the ADP3333. If more GPIO pins are required on the microcontroller, the ADuC7060, which comes in a 48-lead LFCSP or 48-lead LQFP package, is another option. Note that the ADuC7060/ ADuC7061 can be programmed or debugged via a standard JTAG interface. Rev. D | Page 3 of 5 CN-0214 Circuit Note For a standard UART-to-RS-232 interface, the FT232R transceiver can be replaced with a device such as the ADM3202, which requires a 3 V power supply. For a wider temperature range, a different thermocouple can be used, such as a Type J. To minimize the cold junction compensation error, a thermistor can be placed in contact with the actual cold junction instead of on the PCB. Instead of using the RTD and external reference resistor for measuring the cold junction temperature, an external digital temperature sensor can be used. For example, the ADT7410 can connect to the ADuC7060/ADuC7061 via the I2C interface. For more details on cold junction compensation, see Sensor Signal Conditioning, Analog Devices, Chapter 7, "Temperature Sensors." source, as shown in Figure 7. To evaluate the entire range of a Type T thermocouple, the calibrator was used to set the equivalent thermocouple voltage at 11 points between −200°C to +350°C for the negative and positive ranges of the Type T thermocouple (see the ITS-90 Table for Type T Thermocouple). To evaluate the accuracy of the lookup algorithm, 551 voltage readings, equivalent to temperatures in the range of −200°C to +350°C spaced at +1°C, were passed onto the temperature calculation functions. Errors were calculated for the linear method and the piecewise linear approximation method as can be seen in Figure 4 and Figure 5. RTD Measurement Test To evaluate the RTD circuit and linearization source code, the RTD on the PCB was replaced with an accurate, adjustable resistance source. The instrument used was the 1433-Z decade resistor. RTD values went from 90 Ω to 140 Ω, representing an RTD temperature range of −25°C to +114°C. The setup circuit is shown in Figure 8. ADP3333-2.5 USB HEADER If isolation between the USB connector and this circuit is required, add the ADuM3160/ADuM4160 isolation devices. 5V IN OUT 4.7µF D+ FT232QN GND To test and evaluate the circuit, the thermocouple measurements and the RTD measurements were evaluated separately. 0.1µF SHIELD 0.1µF 4.7µF RxD 0.1µF 1433-Z DECADE RESISTOR DVDD IEXC0 10Ω ADC0 0.01µF BEAD 10Ω The basic test setup is shown in Figure 7. Note that the thermocouple is connected to J2-8 and J2-9, and J2-5 must be connected to J2-8. The EVAL-ADuC7061MKZ board receives its power from the USB connection to the PC. ADC1 0.01µF ADuC7061 OP193 VREF+ RREF 5.6kΩ 0.1% VREF– EVAL-ADuC7061MKZ P1.0/SIN P1.1/SOUT J2-9 RxD TxD Figure 8. Test Setup Used to Calibrate and Test the RTD Output Voltage Range from −25°C to +114°C J2-8 J2-5 The error results for the RTD tests are shown in Figure 9. 0 USB CABLE –0.01 WAVTEK 4808 MULTIFUNCTION CALIBRATOR –0.02 –0.03 09869-006 ERROR (°C) PC Figure 7. Test Setup Used to Calibrate and Test the Circuit Over Full Thermocouple Output Voltage Range –0.05 –0.06 –0.07 Two methods were used to evaluate the performance of the circuit. Initially, the circuit was tested with the thermocouple attached to the board and it was used to measure the temperature of an ice bucket, and then the temperature of boiling water. A Wavetek 4808 multifunction calibrator was used to fully evaluate the error, as shown in Figure 4 and Figure 6. In this mode, the thermocouple was replaced with the calibrator as the voltage –0.04 –0.08 09869-009 SEE TEXT DVDD AVDD TxD Thermocouple Measurement Test THERMOCOUPLE JUNCTION DVDD BEAD 10Ω D– CIRCUIT EVALUATION AND TEST AVDD DVDD BEAD 09869-008 When using the external reference resistor as the reference source for the RTD measurement, it is recommended to buffer the input to the VREF+ pin using an op amp in unity-gain mode. This ensures that the input leakage current to the VREF+ pin does not distort the measurement accuracy. In Figure 8, an op amp, OP193, in unity-gain was used for this purpose. This input is not buffered in Figure 1; however, for best results, a buffer is required. –0.09 –0.10 –25 –5 15 35 55 75 95 115 TEMPERATURE (°C) Figure 9. Error in °C of RTD Measurement Using Piecewise Linearization Code and ADC0 Measurements Rev. D | Page 4 of 5 Circuit Note CN-0214 LEARN MORE REVISION HISTORY CN0214 Design Support Package: http://www.analog.com/CN0214-DesignSupport 8/13—Rev. C to Rev. D Changed ADP1720-ADJ to ADP7102 ............................ Universal Changes to Title ................................................................................. 1 ADIsimPower Design Tool. Kester, Walt. 1999. Sensor Signal Conditioning. Analog Devices. Chapter 7, "Temperature Sensors." Kester, Walt. 1999. Sensor Signal Conditioning. Analog Devices. Chapter 8, "ADCs for Signal Conditioning." Looney, Mike. RTD Interfacing and Linearization Using an ADuC706x Microcontroller. AN-0970 Application Note. Analog Devices. MT-022 Tutorial, ADC Architectures III: Sigma-Delta ADC Basics. Analog Devices. MT-023 Tutorial, ADC Architectures IV: Sigma-Delta ADC Advanced Concepts and Applications. Analog Devices. MT-031 Tutorial, Grounding Data Converters and Solving the Mystery of "AGND" and "DGND." Analog Devices. MT-101 Tutorial, Decoupling Techniques. Analog Devices. ITS-90 Table for Type T Thermocouple. Data Sheets and Evaluation Boards ADuC7060 Data Sheet 1/13—Rev. B to Rev. C Changes to Devices Connected/Referenced Section and Figure 1... 1 Changes to Code Description Section and Common Variations Section ................................................................................................ 3 Changes to Thermocouple Measurement Test Section, RTD Measurement Test Section, and Figure 8 ....................................... 4 Changes to Data Sheet and Evaluation Boards Section ............... 5 11/11—Rev. A to Rev. B Change to Devices Connected/Referenced ................................... 1 Changes to Circuit Description....................................................... 1 Changes to Code Description ......................................................... 2 Changes to Common Variations ..................................................... 4 Changes to Circuit Evaluation and Test ......................................... 4 6/11—Rev. 0 to Rev. A Changes to Circuit Description....................................................... 2 Changes to Circuit Evaluation and Test ......................................... 4 4/11—Revision 0: Initial Version ADuC7061 Data Sheet ADuC7060 Evaluation Kit ADuC7061 Evaluation Kit ADM3202 UART to RS-232 Transceiver Data Sheet ADP3333 Data Sheet ADP7102 Data Sheet OP193 Data Sheet (Continued from first page) Circuits from the Lab circuits are intended only for use with Analog Devices products and are the intellectual property of Analog Devices or its licensors. While you may use the Circuits from the Lab circuits in the design of your product, no other license is granted by implication or otherwise under any patents or other intellectual property by application or use of the Circuits from the Lab circuits. Information furnished by Analog Devices is believed to be accurate and reliable. However, Circuits from the Lab circuits are supplied "as is" and without warranties of any kind, express, implied, or statutory including, but not limited to, any implied warranty of merchantability, noninfringement or fitness for a particular purpose and no responsibility is assumed by Analog Devices for their use, nor for any infringements of patents or other rights of third parties that may result from their use. Analog Devices reserves the right to change any Circuits from the Lab circuits at any time without notice but is under no obligation to do so. ©2011–2013 Analog Devices, Inc. All rights reserved. Trademarks and registered trademarks are the property of their respective owners. CN09869-0-8/13(D) Rev. D | Page 5 of 5