Circuit Note CN-0172 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/CN0172. ADT7320 ±0.25°C Accurate, 16-Bit Digital SPI Temperature Sensor AD7793 3-Channel, Low Noise, Low Power, 24-Bit Σ-Δ ADC with On-Chip In-Amp and Reference 3-Channel Thermocouple Temperature Measurement System with 0.25°C Accuracy EVALUATION AND DESIGN SUPPORT Circuit evaluation boards CN-0172 circuit evaluation board (EVAL-CN0172-SDPZ) System Demonstration Platform (EVAL-SDP-CB1Z) CN0172 breakout board (included with EVAL-CN0172SDPZ board) Design and integration files Schematics, Layout Files, Bill of Materials, Software This type of application is popular where a cost-effective, accurate temperature measurement is required over the wide temperature ranges offered by thermocouples. CIRCUIT DESCRIPTION The circuit in Figure 1 is designed to measure the temperature of three K type thermocouples at the same time using the AD7793 24-bit Σ-Δ ADC. The reference junction temperature is measured using the ADT7320, a ±0.25°C accurate, 16-bit digital SPI temperature sensor. CIRCUIT FUNCTION AND BENEFITS Thermocouple Voltage Measurement The function of the circuit shown in Figure 1 is to provide a high accuracy multichannel thermocouple measurement solution. Achieving a precision thermocouple measurement requires a signal chain of precision components that amplifies the small thermocouple voltage, reduces noise, corrects nonlinearity, and provides accurate reference junction compensation (commonly referred to as cold junction compensation). This circuit addresses all these challenges for measuring thermocouple temperature with better than ±0.25°C accuracy. The interface between the thermocouple and the AD7793 ADC is a thermocouple connector and filter. Each connector (J1, J2, and J3) connects directly to a set of differential ADC inputs. The filter on the inputs to the AD7793 reduces any noise pickup in the thermocouple leads before the signal reaches the AIN(+) and AIN(−) inputs of the ADC. The AD7793 has an on-chip multiplexer, buffer, and instrumentation amplifier to amplify the small voltage from the thermocouple measurement junction. The circuit shown in Figure 1 shows how three K type thermocouples are connected to the AD7793 precision 24-bit sigmadelta (Σ-Δ) analog-to-digital converter (ADC) to measure the thermocouple voltage. Because the thermocouple is a differential device rather than an absolute temperature measurement device, the reference junction temperature must be known to get an accurate absolute temperature reading. This process is known as reference junction compensation, commonly referred to as cold junction compensation. In this circuit, the ADT7320 precision 16-bit digital temperature sensor is used for the cold junction reference measurement and provides the required accuracy. The ADT7320 precision 16-bit digital temperature sensor is used to measure the reference (cold) junction temperature with an accuracy of ±0.25°C over the −20°C to +105°C temperature range. The ADT7320 is fully calibrated at the factory, and no user calibration is required. The ADT7320 contains an internal band gap reference, a temperature sensor, and a 16-bit Σ-Δ ADC to measure and digitize the temperature to 0.0078°C resolution. Cold Junction Measurement Both the AD7793 and ADT7320 are controlled by an SPI interface using the System Demonstration Platform (EVAL-SDP-CB1Z). Both AD7793 and ADT7320 can also be controlled by a microcontroller. Rev. A 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 ©2012–2013 Analog Devices, Inc. All rights reserved. CN-0172 Circuit Note FILTERING 5 AD7793 R20 1kΩ R11 1kΩ SPI_DOUT 4 6 5 4 IOUT 1 CLK 11 IOUT 2 CS J6 CS1 3 CS3 4 8 9 GND CS4 7 VDD 6 CS3 CS2 CS1 SPIDIN SPIDOUT SPICLK 5 NC NC NC NC 8 VDD GND ADT7320 EPAD CT U4 3 DIN 4 CS 5 NC 6 4 INT 9 16 15 14 13 1 SCLK 2 DOUT CS4 3 7 C1 0.1µF REFERENCE (COLD) JUNCTION COMPENSATION FOR CHANNEL 3 (J3) CS1 3 1 2 SPI_CLK 6 12 11 17 10 2 VDD VDD = 3.3V CS2 2 VDD GND ADT7320 EPAD CT U3 3 DIN 4 CS 5 SPI_CLK 10 AIN3(–)/REFIN(–) SCLK 1 SPI_DIN 1 NC NC 3 SPI_DIN NC C7 0.01µF DIN 1 SCLK 2 DOUT 5 CS2 CS3 NC R9 1kΩ C11 0.01µF 8 16 15 14 13 1 2 16 C1 0.1µF INT 9 6 7 12 11 17 10 C1 0.1µF INT 9 NC VBIAS – THERMOCOUPLE REFERENCE JUNCTION C13 0.1µF 9 AIN3(+)/REFIN(+) J5 NC DOUT/RDY R8 1kΩ J3 + SPI_DOUT 12 11 17 10 REFERENCE (COLD) JUNCTION COMPENSATION FOR CHANNEL 2 (J2) AIN2(–) 15 7 NC 8 C6 0.01µF 6 NC R7 1kΩ 5 12 GND C8 0.01µF NC VBIAS – THERMOCOUPLE REFERENCE JUNCTION 7 AIN2(+) C10 0.1µF NC + 6 NC R3 1kΩ 3 DIN 4 CS NC 4 VDD GND ADT7320 EPAD CT U2 NC 3 C5 C4 + 10µF 0.1µF U1 1 SCLK 2 DOUT NC 13 AVDD 16 15 14 13 1 2 NC CHANNEL 3 THERMOCOUPLE MEASUREMENT JUNCTION 14 DVDD 6 AIN1(–) C2 0.01µF J2 CHANNEL 2 THERMOCOUPLE MEASUREMENT JUNCTION AIN1(+) C1 0.01µF J4 NC – THERMOCOUPLE REFERENCE JUNCTION C3 0.1µF R2 1kΩ VDD NC VBIAS 5 NC J1 + NC CHANNEL 1 THERMOCOUPLE MEASUREMENT JUNCTION REFERENCE (COLD) JUNCTION COMPENSATION FOR CHANNEL 1 (J1) R1 1kΩ 8 CS4 SPI_DIN SPI_DOUT SPI_CLK J7 VDD 1 2 3 4 5 6 1 J9 2 3 4 J10 5 6 1 2 3 4 5 6 J11 09240-001 NOTES: 1. EPAD = EXPOSED PADDLE OF THE LFCSP PACKAGE (ADT7320) 2. THE REFERENCE (COLD) JUNCTION COMPENSATION SENSOR BOARDS CONNECTIONS TO THE REST OF THE CIRCUIT: A) CHANNEL 1 (J1): J4 TO J9 B) CHANNEL 2 (J2): J5 TO J10 C) CHANNEL 3 (J3): J6 TO J11 09240-002 Figure 1. Multichannel Thermocouple Measurement System (Simplified Schematic: All Connections and Decoupling Not Shown) Figure 2. EVAL-CN0172-SDPZ Circuit Evaluation Board Rev. A | Page 2 of 6 Circuit Note CN-0172 Figure 2 shows the EVAL-CN0172-SDPZ circuit evaluation board with three K type thermocouple connectors, the AD7793 ADC, and the ADT7320 temperature sensor mounted on a separate flexible printed circuit board (PCB) between two copper contacts for the reference temperature measurement. Figure 3 shows a side view of the ADT7320 mounted on a separate flexible PCB and inserted between the two copper contacts of the thermocouple connector. The flexible PCB shown in Figure 3 has advantages over small FR4-type PCBs because it is thinner and more flexible. This allows the ADT7320 to be mounted neatly in between the copper contacts of the thermocouple connector to minimize temperature gradients between the reference junction and ADT7320. The advantages of the exposed tip are that it provides the best heat transfer, has the quickest response time, and is low cost and light weight. The disadvantages of the exposed tip are that it is susceptible to mechanical damage and corrosion. As a result, it is not suitable for harsh environments. However in cases where quick response time is needed, the exposed tip is the best choice. If an exposed tip is required in an industrial environment, electrical isolation may be required in the signal chain. This can be addressed by using digital isolators (see www.analog.com/icoupler). Unlike a traditional thermistor or resistance temperature detector (RTD), the ADT7320 is a fully plug-and-play solution that does not require multipoint calibration after board assembly or consume processor and memory resources with calibration coefficients and linearization routines. The ADT7320 avoids self-heating issues that undermine the accuracy of traditional resistive sensor solutions because it only dissipates 700 µW (typical) of power at 3.3 V. Guidelines for Accurate Temperature Measurement 09240-003 The following guidelines ensure that the ADT7320 accurately measures the temperature of the reference junction. Figure 3. Side View of ADT7320 Mounted on a Flexible PCB The small size and thinness of the flexible PCB also allows the ADT7320 to quickly respond to temperature changes at the reference junction. Figure 4 shows the typical thermal response time of the ADT7320. 140 125°C 120 DUT TEMPERATURE (°C) 105°C 100 85°C IT TAKES LESS THAN 2 SECONDS FOR THE DUT TO REACH 63.2% OF ITS FINAL TEMPERATURE SPAN 40 0 5 10 15 20 TIME (s) 25 09240-004 20 0 Decoupling: The ADT7320 must have a decoupling capacitor mounted as close as possible to VDD to ensure accurate temperature measurement. A decoupling capacitor, such as a 0.1 µF high frequency ceramic type is recommended. In addition, use a low frequency decoupling capacitor in parallel with the high frequency ceramic, such as a 10 µF to 50 µF tantalum capacitor. Maximizing Thermal Conduction: The primary thermal path from the reference junction to the ADT7320 is through the plastic package and the backside exposed paddle (GND). Because the copper contacts are connected to the ADC inputs, the backside paddle cannot be connected in this application because it affects the biasing to the ADC inputs. 80 60 Power Supply: If the ADT7320 is powered from a switching regulator, noise generated above 50 kHz may affect the temperature accuracy specification. To prevent this, use an RC filter between the power supply and VDD. Carefully choose the value of components used to ensure that the peak value of the supply noise is less than 1 mV. Figure 4. ADT7320 Typical Thermal Response Time This solution is flexible and allows other types of thermocouples to be used, such as J type or T type thermocouples. The K type was selected for this circuit note because of its popularity. The actual thermocouple chosen has an exposed tip. The measurement junction is outside the probe wall and is exposed to the target medium. Rev. A | Page 3 of 6 CN-0172 Circuit Note Guidelines for Accurate Voltage Measurement COMMON VARIATIONS The following guidelines ensure that the AD7793 accurately measures the voltage at the thermocouple measurement junction. For applications that require less accuracy and precision, the AD7792 16-bit Σ-Δ ADC can be used instead of the AD7793 24-bit Σ-Δ ADC. For the reference temperature measurement, the ±0.5°C accurate ADT7310 digital temperature sensor can be used instead of the ±0.25°C accurate ADT7320. Both the AD7792 and ADT7310 are available with an SPI interface. Decoupling: The AD7793 must have decoupling capacitors mounted as close as possible to both AVDD and DVDD to ensure accurate voltage measurement. Decouple AVDD with a 10 µF tantalum capacitor in parallel with a 0.1 µF ceramic capacitor to GND. In addition, decouple DVDD with a 10 µF tantalum capacitor in parallel with a 0.1 µF ceramic capacitor to GND. Refer to Tutorial MT-031 and Tutorial MT-101 for more discussion on grounding, layout techniques, and decoupling techniques. Filtering: The differential inputs of the AD7793 act to remove most of the common-mode noise on the thermocouple lines. Differential low-pass filters comprising R1, R2, and C3, for example, placed at the front end of the AD7793 reduce noise pickup that can be present in the thermocouple leads. The C1 and C2 capacitors provide additional common-mode filtering. Because the AIN(+) and AIN(−) analog inputs to the ADC are differential, most of the voltages in the analog modulator are common-mode voltages. The excellent common-mode rejection (100 dB minimum) of the AD7793 further removes commonmode noise on these inputs. Other Challenges Resolved with this Solution The following summarizes how other challenges mentioned earlier with thermocouples are resolved with this solution. Thermocouple Voltage Amplification: The output voltage of the thermocouple changes by only a few µV per degree. In this case, the popular K type thermocouple changes 41 µV/°C. This small signal requires a large gain stage before the ADC conversion. The AD7793 internal programmable gain amplifier (PGA) provides a gain up to 128. For this solution, a gain of 16 was used to allow the AD7793 to run its own internal full-scale calibration using its internal reference. Correction for Thermocouple Nonlinearity: The AD7793 provides excellent linearity across a wide temperature range (−40°C to +105°C), requiring no correction or calibration by the user. To determine the actual thermocouple temperature, the reference temperature measurement is first converted into an equivalent thermoelectric voltage using equations provided by the National Institute of Standards and Technology (NIST). This voltage is added to the thermocouple voltage measured by the AD7793, and the summation is then translated back into a thermocouple temperature, again using NIST equations. An alternative approach involves using look-up tables. However, to get the same accuracy, the size of the lookup table could be substantial which would require additional memory resources from the host controller. All the processing is done in the software using the EVAL-SDP-CB1Z. CIRCUIT EVALUATION AND TEST The system described uses the EVAL-CN0172-SDPZ and the EVAL-SDP-CB1Z. The CN0172 Breakout Board is included with the EVAL-CN0172-SDPZ board. Equipment Needed The following equipment is needed: • • • • • • • • • • An Oil bath The EVAL-CN0172-SDPZ circuit evaluation board The CN0172 Breakout Board (Included with the EVALCN0172-SDPZ board) The EVAL-SDP-CB1Z circuit evaluation board The CN0172 evaluation board software A Datron 4808 calibrator A Hart Scientific 1590 super thermometer A Hart Scientific precision probe GPIB cables (3) A PC and Windows XP or later running LabVIEW with a GPIB card and an USB 2.0 port Setup and Test The test setup shown in Figure 5 was used to evaluate the performance of the multichannel thermocouple solution. A Datron calibrator was used to provide a precise voltage source for the three thermocouple inputs. The temperature of the oil baths was measured with the super thermometer and controlled via the GPIB bus. LabVIEW software for the CN0172 controls the EVALCN0172-SDPZ via the USB port, the EVAL-SDP-CB1Z, Breakout Board, and the SPI bus. The power for the EVAL-SDP-CB1Z is obtained from the USB bus, and the 3.3 V output of the EVALSDP-CB1Z supplies power for the EVAL-CN0172-SDPZ. If oil bath measurements are not required, the EVAL-CN0172SDPZ can be used to measure the three thermocouple temperatures by using the USB interface with a PC and the software provided on the CD. Information and details regarding test setup and calibration, and how to use the evaluation software for data capture can be found in the CN0172 User Guide found at: www.analog.com/CN0172UserGuide. Complete schematics and layouts for the EVAL-CN0172-SDPZ can be found in the CN-0172 Design Support Package: www.analog.com/CN0172-DesignSupport. Rev. A | Page 4 of 6 Circuit Note CN-0172 HART SCIENTIFIC PRECISION PROBE OIL BATH FOR REFERENCE (COLD JUNCTION) DATRON 4808 CALIBRATOR (VOLTAGE SOURCE) CH1 CH2 VOUT CH3 EVAL-CN0172-SDPZ GPIB BUS SPI BUS AND +3.3V POWER SUPPLY 9-PIN CONNECTOR 120-PIN CONNECTOR USB PC SDP BOARD CN0172 BREAKOUT BOARD 09240-005 HART SCIENTIFIC 1590 SUPER THERMOMETER Figure 5. Test Setup Functional Block Diagram Test Results Figure 6 shows a plot of the thermocouple solution error over various thermocouple temperatures using various fixed values of cold junction (CJ) temperatures. Total solution error ≤ ±0.25°C is achieved over a wide temperature range. Note that the accuracy of the solution can be further improved by performing a system calibration of the AD7793 ADC. Figure 7 shows a plot of the thermocouple solution error over various CJ temperatures using various fixed values of thermocouple temperatures. Total solution error ≤ ±0.25°C is achieved over a wide temperature range. 0.25 0.20 0.15 0.25 0.10 0.15 ERROR (°C) 0.10 0.05 0 –0.05 0.05 –0.10 0 –0.15 –0.05 –0.20 –0.10 –0.20 –0.25 –270 CJ = –20°C CJ = +20°C CJ = +60°C CJ = +100°C –70 130 –0.25 –20 CJ = 0°C CJ = +40°C CJ = +80°C CJ = +105°C 330 530 0 20 40 60 TC = –70°C TC = +480°C TC = +880°C 80 COLD JUNCTION TEMPERATURE (°C) 730 THERMOCOUPLE TEMPERATURE (°C) 930 1130 09240-006 –0.15 TC = –270°C TC = +280°C TC = +680°C TC = +1080°C Figure 6. Error vs. Thermocouple Temperature for Fixed Cold Junction (CJ) Temperature Rev. A | Page 5 of 6 100 09240-007 ERROR (°C) 0.20 Figure 7. Error vs. Cold Junction Temperature for Fixed Thermocouple Temperatures CN-0172 Circuit Note LEARN MORE Data Sheets and Evaluation Boards CN-0172 Design Support Package: http://www.analog.com/CN0172-DesignSupport CN-0172 Circuit Evaluation Board (EVAL-CN0172-EB1Z) Standard Development Platform Board (EVAL-SDP-CB1Z) Duff, Matthew, and Joe Towey, Two Ways to Measure Temperature Feature Simplicity, Accuracy and Flexibility , Analog Dialogue,Vol 44, October 2010. ADT7320 Data Sheet and Evaluation Board ADT7310 Data Sheet and Evaluation Board AD7793 Data Sheet and Evaluation Board Thermocouple 101: What is a Thermocouple? ADI Video. AD7792 Data Sheet and Evaluation Board McNamara, Donal, Temperature Measurement Theory and Practical Techniques, AN-892 Application Note, Analog Devices. REVISION HISTORY AD779x Instrumentation Converters, Frequently Asked Questions. Changes to Title .................................................................................1 8/13—Rev. 0 to Rev. A ADT7320/ADT7420 Digital Temperature Sensors, Frequently Asked Questions. 12/12—Revision 0: Initial Version Kester, Walt. 1999. Sensor Signal Conditioning. Section 7. Analog Devices. MT-004 Tutorial, The Good, the Bad, and the Ugly Aspects of ADC Input Noise—Is No Noise Good Noise? Analog Devices. MT-022 Tutorial, ADC Architectures III: Σ-Δ ADC Basics, Analog Devices. MT-023 Tutorial, ADC Architectures IV: Σ-Δ 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. (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. ©2012–2013 Analog Devices, Inc. All rights reserved. Trademarks and registered trademarks are the property of their respective owners. CN09240-0-8/13(A) Rev. A | Page 6 of 6