Circuit Note CN-0354 Devices Connected/Referenced Precision Thermocouple Amplifier with AD8495 Cold Junction Compensation Circuits from the Lab® reference designs 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/CN0354. AD7787 Low Power, 24-Bit Σ-Δ ADC REF194 Precision Micropower, Low Dropout Voltage Reference ADG1609 4.5 Ω RON, 4-Channel ±5 V, +12 V, +5 V, and +3.3 V Multiplexer ADR3412 Micropower, High Accuracy Voltage Reference ADM8829 Switched Capacitor Voltage Inverter Low Power Multichannel Thermocouple Measurement System with Cold Junction Compensation EVALUATION AND DESIGN SUPPORT cold junction compensation (0°C to 50°C) and converts the thermocouple output to a voltage with a precise scale factor of 5 mV/°C. The error is less than 2°C, over a measurement range of −25°C to +400°C, and is primarily due to the thermocouple nonlinearity. A nonlinearity correction algorithim reduces the error to less than 0.5°C over a 900°C measurement range. Noise free resolution is less than 0.1°C. Circuit Evaluation Boards CN-0354 Evaluation Board (EVAL-CN0354-PMDZ) System Demonstration Platform (EVAL-SDP-CB1Z) SDP-I-PMOD Interposer Board (PMD-SDP-IB1Z) Design and Integration Files Schematics, Layout Files, Bill of Materials The signal is then digitized by a 24-bit Σ-Δ ADC, and the digital value is provided on an SPI serial interface. With the PMOD form factor for rapid prototyping, the design requires minimal PC board area and is ideal for applications that require precise thermocouple temperature measurements. CIRCUIT FUNCTION AND BENEFITS The circuit shown in Figure 1 is a flexible, 4-channel, low power thermocouple measurement circuit with an overall power consumption of less than 8 mW. The circuit has a multiplexed front end, followed by an instrumentation amplifier that performs ALUMEL REFERENCE JUNCTION CHROMEL 5V P2 0.01µF + V1 – ALUMEL CHROMEL ISOTHERMAL BLOCK SENSE 100Ω Cu Cu Cu Cu Cu Cu Cu Cu A0 A1 EN 1.1kΩ AIN1(–) SERIAL INTERFACE AND CONTROL LOGIC Σ-Δ ADC DOUT/RDY SCLK DOUT/RDY CS AIN2 CLOCK GND GND OUT AD8495 0.01µF 1.2V P1 1 −5V BUF MUX IN-AMP REF ADG1609 AIN1(+) 1nF 1µF 100Ω 1MΩ AD7787 VDD COLD JUNCTION COMPENSATION 5V REFIN 5V REF194 IN EN OUT ADR3412 4.5V 1.2V IN OUT EN GND 2 3 ADM8829 ISOTHERMAL REGION IN OUT CAP+ CAP− GND −5V 4.7µF 4.7µF 12641-001 + V2 – Cu Cu 4.5V 5V VDD Figure 1. Multichannel K Type Thermocouple Measurement System (Simplified Schematic: All Connections and Decoupling Not Shown) Rev. A Circuits from the Lab® reference designs 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 toanycausewhatsoeverconnectedtotheuseofanyCircuitsfromtheLabcircuits. (Continuedonlastpage) 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 ©2014–2015 Analog Devices, Inc. All rights reserved. CN-0354 Circuit Note The thermocouple voltage generated at the output of the isothermal block is proportional to the difference between the temperature at the measurement thermocouple and the temperature of the isothermal block (cold junction). The signal is amplified by the AD8495, a precision instrumentation amplifier that is laser trimmed to provide a precise 5 mV/°C output for a K type thermocouple. The AD8495 also provides cold junction compensation over a range of 0°C to 50°C. The fifth thermocouple, shown in Figure 1, is added to cancel the voltage generated by any temperature differential that exists between the isothermal block and the AD8495 cold junction compensation circuit. With the multiplexer enabled, the constantan (alumel) copper junction, formed by the thermocouple connection at the isothermal block, is in series with a copper constantan (alumel) junction, formed by the reference thermocouple connection. This series combination contributes equal but opposite voltages because the block is isothermal. In this condition, the AD8495 internal cold junction compensation compensates for the reference junction at the IC, which must remain between 0°C and 50°C. Therefore, the fifth thermocouple connection eliminates the requirement of mounting the AD8495 directly on the isothermal block, as is normally the case. Thermocouple Signal Conditioning Thermocouples are used in temperature measurements requiring high temperature ranges. They are preferred over resistance temperature detectors (RTDs) for this reason, as well as for their lower cost. However, thermocouples are nonlinear, which means that the voltage generated by a thermocouple changes at different rates and at different temperatures. For example, a J type thermocouple changes by 52 µV/°C at 25°C and by 55 µV/°C at 150°C. K type thermocouples tend to be more linear, at approximately 41 µV/°C, when temperatures are above 0°C. The voltage response of a thermocouple to a temperature gradient is generally described by a polynomial of greater than the sixth-order. Figure 2 shows the Seebeck coefficient of the different types of thermocouples over their operating temperature ranges. Figure 2 shows that the K type thermocouple has the widest temperature range and can measure temperatures up to 1250°C. 100 The output of the AD8495 is filtered by the 1.1 kΩ/1 nF singlepole filter that has a −3 dB cutoff frequency of 145 kHz. This filter minimizes broadband noise at the AD7787 ADC input. The AD7787 is a 24-bit, low noise, low power Σ-Δ ADC for low frequency measurement applications, such as thermocouple measurement systems. Its internal clock eliminates the need for an external clock and makes the output data rate user configurable, which can reduce current consumption because it functions at a lower internal clock frequency. It contains a Σ-Δ ADC with one differential input and one single-ended input, either of which can be buffered or unbuffered after passing through the multiplexer. The AD7787 operates from an internal clock. Therefore, the user does not have to supply a clock source to the device. The output data rate from the device is software programmable and can be varied from 9.5 Hz to 120 Hz, with the rms noise equal to 1.1 µV at the lower update rate. The AD7787 operates at an update rate of 9.5 Hz in this circuit. The internal clock frequency can be divided by a factor of 2, 4, or 8, which leads to a reduction in the current consumption. The update rate, cutoff frequency, and settling time all scale with the clock frequency. 80 E J T 60 40 K 20 0 –400 –200 0 200 400 600 800 1000 1200 1400 TEMPERATURE (°C) 12641-002 The four thermocouple inputs to the circuit are terminated at the isothermal block, P2. The ADG1609 complementary metal oxide semiconductor (CMOS) analog multiplexer switches the four thermocouple channels to a single signal conditioning block to handle the four thermocouple inputs. The switches exhibit a break-before-make switching action and an inherent low charge injection to minimize transients when switching between channels. The AD7787 operates with a power supply from 2.5 V to 5.25 V. When operating from the 3 V supply, the power dissipation for the device is 225 µW maximum. It is packaged in a 10-lead MSOP. SEEBECK COEFFICIENT (µV/°C) CIRCUIT DESCRIPTION Figure 2. Seebeck Coefficient of Thermocouple vs. Temperature Due to the nonlinear characteristic of thermocouples, complex signal processing and signal conditioning are required to obtain a precise temperature reading, and the AD8495 provides an ideal solution. The AD8495 is trimmed to provide cold junction compensation over a reference junction range of 0°C to 50°C and provides a linear 5 mV/°C transfer function. Over a measurement range of −25°C to +400°C, the maximum output error is ±2°C. To calculate the output voltage, use the following equation: TMJ = VOUT − VREF 5 mV/°C The AD8495 operates on a single 5 V supply in the circuit. Because of the PNP transistor input structure, the input voltage can be as low as −200 mV, thereby allowing the measurement of negative temperatures. However, the output voltage must be offset to process negative temperatures, which is accomplished using the reference voltage input pin (REF). Rev. A | Page 2 of 5 Circuit Note CN-0354 With the REF pin grounded using the P1 jumper, the minimum temperature that the system can measure is 5°C. The P1 jumper also allows the REF pin bias voltage to connect to a 1.2 V ADR3412 reference, thereby allowing measurement temperatures as low as −235°C. The corresponding temperature range for either condition is specified in Table 1. In both cases, the temperature span is 875°C. It is important to drive the REF pin with a low impedance source, such as a voltage reference or a buffer amplifier, to prevent errors. Table 1. Measurement Ranges for REF = 0 V and 1.2 V with Single 5 V Supply Temperature Range 5°C to 880°C −235°C to +640°C Open Thermocouple Detection The AD8495 can detect an open or broken thermocouple. The inputs of the AD8495 are the bases of PNP transistors; therefore, the bias current always flows out of the inputs. If either input is open, the output goes to one of the supply rails. Connecting the negative input to ground through a 1 MΩ resistor causes the AD8495 output to rail high in an open thermocouple condition. In cases where any of the four channels are not used, short the inputs to prevent the AD8495 from railing to the positive supply if that channel is connected. The 1 MΩ resistor also provides a bias current return path to ground. The AD8495 has a high common-mode rejection, thereby minimizing the common-mode noise picked up by long thermocouple leads. The high impedance inputs of the amplifier make it easy to add extra filtering for additional electromagnetic interference/radio frequency interference (EMI/RFI) protection. Table 2. Circuit Current Consumption Part Number AD8495 AD7787 ADG1609 REF194 ADR3412 ADM8829 Maximum Current Consumption 250 µA 160 µA 1 µA 45 µA 100 µA 1000 µA Note that the total current is 1556 µA. Test Results The AD8495 has a maximum temperature error due to the thermocouple nonlinearity of ±2°C from −25°C to +400°C, for a reference junction temperature of 0°C to 50°C. Wider temperature ranges or better accuracy requires a linearity correction algorithm that can be implemented in the software. Nonlinearity correction is discussed in the AN-1087 Application Note, Thermocouple Linearization When Using the AD8494/AD8495/AD8496/AD8497. Figure 3 shows the linearity error of the circuit, with and without the correction algorithm implemented. 2.0 1.5 1.0 The ADM8829 switched capacitor voltage inverter provides the −5 V required by the ADG1609 multiplexer to accommodate negative temperatures. Rev. A | Page 3 of 5 –0.5 AD8495 CN-0354 CN-0354 WITH NONLINEARITY CORRECTION –1.0 –1.5 900 800 12641-003 TEMPERATURE (°C) 700 600 500 400 300 –2.0 200 The REF194 supplies the 4.5 V reference for the AD7787. The ADR3412 provides the 1.2 V optional offset voltage for the REF input of the AD8495. Jumper P1 allows the REF pin to be connected either to 1.2 V or to ground. 0 100 The circuit in Figure 1 is powered from a single 5 V supply that powers the ADG1609, the AD8495, the VDD pin of the AD7787, the REF194, the ADR3412, and the ADM8829. 0.5 0 Power Considerations ERROR (°C) REF Pin Voltage 0 V or Grounded 1.2 V Table 2 shows the current consumed by each device in the circuit, based on the data sheet specifications. The design consumes a maximum of 1.56 mA. Note that the maximum current consumption is due to the ADM8829 switched capacitor voltage inverter. This inverter can be eliminated if a negative supply is available to drive the VSS pin of the ADG1609, thereby reducing the total current to approximately 556 µA. Figure 3. Linearity Error with and Without the Correction Algorithm Implemented CN-0354 Circuit Note The noise of the system was tested in a controlled temperature environment to check the noise free code resolution of the system. Figure 4 shows the noise distribution, which spans approximately 1098 codes. For a 24-bit resolution and a span of 900°C, 1 LSB = 900°C/224 = 0.07°C resolution. Equipment Required The following equipment is needed: 200 • • • • • • 175 Software Installation 16,777,216 Noise Free Resolution = log 2 Code Spread 225 NUMBER OF OCCURENCES CIRCUIT EVALUATION AND TEST The CN-0354 evaluation kit includes self installing software on a CD. The software is compatible with Windows® XP (SP2), Windows Vista (32-bit and 64-bit), and Windows 7 (32-bit and 64-bit). If the setup file does not run automatically, run the setup.exe file from the CD. Install the evaluation software before connecting the evaluation board and the SDP board to the USB port of the PC to ensure that the evaluation system is correctly recognized when connected to the PC. The software allows full configuration of the serial interface. It is important that the configurations of the masters and the slaves match for proper operation. 150 125 100 75 50 25 12641-004 0709C3 0709EC 070A15 070A3E 070A67 070A90 070AB9 070AE2 070B0B 070B34 070B5D 070B86 070BAF 070BD8 070C01 070C2A 070C53 070C7C 070CA5 070CCE 070CF7 070D20 070D49 070D72 070D9B 070DC4 070DED 0 ADC CODE IN HEX The EVAL-CN0354-PMDZ evaluation board The EVAL-SDP-CB1Z system demonstration platform The PMD-SDP-IB1Z, SDP-I-PMOD, interposer board The CN0354 Evaluation Software 6 V wall wart power supply PC (Windows 32-bit or 64-bit) Figure 4. Noise Distribution Histogram of Circuit with the AD7787 Input Buffer Disabled, Output Data Rate = 9.5 Hz Software operation is described in the CN-0354 Software User Guide. Power Supply Requirements Calculate the noise free code resolution as follows: The EVAL-CN0354-PMDZ evaluation board must be powered by a 5 V power supply. It is recommended that the supply provide at least 2 mA. If powered directly from the PMD-SDP-IB1Z interposer board, ensure that the supply is well filtered and free of digital noise. 16,777,216 Noise Free Resolution = log 2 = 13.9 Bits 1098 COMMON VARIATIONS The CN-0354 circuit was proven to work with good stability and accuracy with the devices selected for the design. This design can also be implemented using the AD594/AD595, which are also single chip thermocouple, signal conditioning amplifiers, with an output of 10 mV/°C. The AD8495 has other variants, such as the AD8494, the AD8496, and the AD8497, for different thermocouple types, different ambient temperature ranges, and different measurement temperature ranges, as shown in Table 3. Table 3. AD849x with ±2°C Accuracy Temperature Ranges Device AD8494 AD8495 AD8496 AD8497 Thermocouple Type J K J K Maximum Error ±2°C ±2°C ±2°C ±2°C Ambient Temperature Range 0°C to 50°C 0°C to 50°C 25°C to 100°C 25°C to 100°C Rev. A | Page 4 of 5 Measurement Temperature Range −35°C to +95°C −25°C to +400°C 55°C to 565°C −25°C to +295°C Circuit Note CN-0354 Test Setup Functional Diagram A functional diagram of the test setup is shown in Figure 5, and a photo of the EVAL-CN0354-PMDZ board is shown in Figure 6. EVAL-CFTL-6V-PWRZ 6V WALL WART PC Marcin, Joe. AN-369. Thermocouple Signal Conditioning Using the AD594/AD595. Analog Devices. 120 PINS EVAL-SDP-CB1Z SDP BOARD MT-035 Tutorial. Op Amp Inputs, Outputs, Single-Supply, and Rail-to-Rail Issues. Analog Devices. 12641-005 12 SDP TO PMOD PINS INTERPOSER BOARD SDP CONNECTOR PMOD P2 USB J1 MT-023 Tutorial. ADC Architectures IV: Sigma-Delta ADC Advanced Concepts and Applications. Analog Devices. Malik, Reem. AN-1087. Thermocouple Linearization When Using the AD8494/AD8495/AD8496/AD8497. Analog Devices. USB CABLE EVAL-CN0354-PMDZ BOARD MT-022 Tutorial. ADC Architectures III: Sigma-Delta ADC Basics. Analog Devices. PMD-SDP-IB1Z MT-031 Tutorial. Grounding Data Converters and Solving the Mystery of AGND and DGND. Analog Devices. Figure 5. Test Setup Functional Diagram MT-101 Tutorial. Decoupling Techniques. Analog Devices. 12641-006 NIST (National Institute of Standards and Technology) Table for Type K Thermocouple. Figure 6. EVAL-CN0354-PMDZ Evaluation Board Data Sheets and Evaluation Boards AD8495 Data Sheet AD7787 Data Sheet LEARN MORE ADG1609 Data Sheet CN0354 Design Support Package: http://www.analog.com/CN0354-DesignSupport REF194 Data Sheet Corrigan, Theresa. AN-1024. How to Calculate Settling Time and Sampling Rate of a Multiplexer. Analog Devices. ADM8829 Data Sheet ADR3412 Data Sheet Duff, Matthew and Towey, Joseph. Two Ways to Measure Temperature Using Thermocouples Feature Simplicity, Accuracy, and Flexibility. Analog Dialogue, 44-10, October 2010. Kester, Walt, et. al. “Temperature Sensors”, Chapter 7 in Sensor Signal Conditioning. Analog Devices, 1999. REVISION HISTORY 2/15—Rev. 0 to Rev. A Change to Circuit Function and Benefits Section ........................ 1 Change to Table 1 .............................................................................. 3 9/14—Revision 0: Initial Version (Continued from first page) Circuits from the Lab reference designs 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 reference designs 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 reference designs. Information furnished by Analog Devices is believed to be accurate and reliable. However, Circuits from the Lab reference designs 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 reference designs at any time without notice but is under no obligation to do so. ©2014–2015 Analog Devices, Inc. All rights reserved. Trademarks and registered trademarks are the property of their respective owners. CN12641-0-2/15(A) Rev. A | Page 5 of 5