AN1306 Thermocouple Circuit Using MCP6V01 and PIC18F2550 Yang Zhen and Ezana Haile Microchip Technology Inc. INTRODUCTION Target Audience This application note is intended for hardware and firmware design engineers who need to accurately measure Type-K Thermocouple voltage and convert it to degree Celsius (°C). Goals • Accurately measure Type-K Thermocouple Electromotive Force (EMF) • Provide Low-Cost and accurate thermocouple solution Description This application note shows how to use a difference amplifier system to measure EMF voltage at the cold junction of thermocouple in order to accurately measure temperature at the hot junction. This can be done by using the MCP6V01 auto-zeroed op amp because of its extremely low input offset voltage (VOS) and very high common mode rejection ratio (CMRR). This solution minimizes cost by using resources internal to the PIC18F2550, such as 10-bit ADC and 4-bit adjustable reference, to achieve less than 0.1°C resolution from a measurement range of -100°C to 1000°C. THERMOCOUPLE OVERVIEW Thermocouples are constructed of two dissimilar metals such as Chromel and Alumel (Type-K). The two dissimilar metals are bonded together on one end of the wires with a weld bead, or Hot Junction. The junction point is the temperature sensor. Temperature difference between the Hot Junction and the open junction, Cold Junction, generates measurable voltage between the two terminals of the open junction. This voltage is commonly called the Electromotive Force (EMF) voltage, or Seebeck Effect. This EMF voltage does not require excitation current or voltage. If the difference in temperature between the open and closed end of the Thermocouple wires increases, then the EMF voltage increases proportionally. The Type-K thermocouple used in the circuit is from OMEGA with part number 5SRTC-TT-K-24-36. The EMF voltage and temperature range of Type-K thermocouple are shown in Figure 1. The voltage shown is referenced to 0°C. 60 50 40 EMF (mV) Author: 30 20 10 0 -10 -300 -100 100 300 500 700 900 1100 1300 Temperature (°C) Related Reference Design Board The measurements for this application note were made on the MCP6V01 Thermocouple Auto-Zeroed Reference Design Board which is discussed in the user’s guide (DS51738). This board is further described by: • Order Number: MCP6V01RD-TCPL • Assembly Number: 114-00169 © 2009 Microchip Technology Inc. FIGURE 1: Temperature. EMF Voltage vs. From Figure 1, it can be summarized that the EMF voltage has relatively small magnitude (millivolts). Consequently, the signal conditioning portion of the electronics requires an analog gain stage. In addition, the signal conditioning circuit must have absolute reference voltage in order to measure temperature with absolute accuracy. DS01306A-page 1 AN1306 SYSTEM BLOCK DIAGRAM range is segmented into 16 smaller ranges. This gives a greater range (-100°C to +1000°C) and better accuracy. Figure 2 shows the system block diagram of the solution. The difference amplifier uses MCP6V01 autozeroed op amp to amplify the thermocouple’s EMF voltage. The MCP1541 provides a reference voltage of 4.1V which references the PIC18F2550’s internal 10-bit ADC. The 2nd order RC low-pass filter reduces noise and aliasing at the ADC input. The CVREF is an internal comparator voltage reference of PIC18F2550, which is a 16-tap resistor ladder network that provides a selectable reference voltage. It has low accuracy and high variable output resistance. The buffer amplifier eliminates the output impedance loading effect and produces the voltage VSHIFT that shifts the VOUT1. The MCP9800 senses temperature at the thermocouple connector, or cold-junction. It should be located as close as possible to the connector on the PCB. This measurement is used to perform cold junction compensation for the thermocouple measurement. The VSHIFT is brought back into the PIC18F2550, sampled and calibrated by the internal ADC, then used to adjust measured VOUT1, so that the temperature The Thermal Management Software is used to perform data acquisition to show the real-time temperature data. PC (Thermal Management Software) USB PIC18F2550 (USB) Microcontroller I2CTM Port Buffer Amplifier I2C + ALERT 10-Bit ADC Module CVREF VOUT2 x1 2nd Order RC Low-Pass Filter MCP1541 4.1V Voltage Reference 3 VOUT1 Difference Amplifier MCP6V01 VSHIFT Cold Junction Compensation TCJ VREF + VP TTC MCP9800 Temp. Sensor - VM Connector (Cold Junction) FIGURE 2: DS01306A-page 2 Type-K Thermocouple Welded Bead (Hot Junction) System Block Diagram. © 2009 Microchip Technology Inc. AN1306 HARDWARE CIRCUITS Voltage Sensors With Common Mode Noise Any remote voltage sensor with differential output is usually subject of high common-mode noise. An example would be a temperature sensor for an engine, such as a thermocouple sensor. EQUATION 1: V1 + V2 V CM = -----------------2 Where: V DM = V 1 – V 2 Common mode noise is reduced by shielding, PCB layout, and using a difference or instrumentation amplifier. In this application note, we will focus on using difference amplifier to reduce the common mode noise. Difference Amplifier Figure 5 shows a difference amplifier using an op amp. It presents an impedance of R1 to each end of the sensor (V1 and V2) and amplifies the input difference voltage (V1 - V2). An ideal difference amplifier gives an output as: EQUATION 2: VCM = Common Mode Voltage V OUT = G DM × ( V 1 – V 2 ) VDM = Difference Mode Voltage V1, V2 = Differential Outputs of Remote Voltage Sensor R G DM = -----2R1 Where: GDM Figure 3 shows voltage sensors with high common mode noise. VDD V1 = R1 Difference Mode Gain R2 V1 VDD/2 VDD V2 0V + - VDD VCM VDD/2 V2 VDM R1 0V FIGURE 3: Voltage Output Sensor with High Common Mode Noise. FIGURE 5: Figure 4 shows voltage sensor with low common mode noise. • • • • V1 V2 VDD VOUT R2 Difference Amplifier Advantages: Resistive isolation from the source Large input voltage range is possible Rejects common mode noise Simplicity Disadvantages: VDD/2 • Resistive loading of the source • Input stage distortion 0V VDD VCM VDD/2 VDM 0V FIGURE 4: Local Sensors with Low Common Mode Noise. © 2009 Microchip Technology Inc. DS01306A-page 3 AN1306 Equation 3 gives a more practical result for the difference amplifier. Analog Sensor Conditioning Circuit EQUATION 3: It includes three building blocks: V1 + V2 V OUT = G DM × ( V 1 – V 2 ) + G CM × ⎛ -------------------⎞ ⎝ 2 ⎠ R G DM = -----2R1 G DM G CM = ---------------------------CMRR DIFF Figure 6 shows the analog sensor conditioning circuit. • Buffer Amplifier • Difference Amplifier • 2nd Order Low-Pass Filter BUFFER AMPLIFIER GDM = Difference Mode Voltage GCM = Common Mode Voltage • MCP6001 standard op amp used as unity gain buffer • Provides a low impedance adjustable reference voltage CMRRDIFF = Common Mode Rejection Ratio of Difference Amplifier EQUATION 5: Where: CVREF = V SHIFT From the above equation, it can be summarized that a practical difference amplifier amplifies the difference mode voltage by GDM and the common mode voltage by GCM. Where: CVREF = Selectable reference voltage of PIC18F2550 The CMRRDIFF is given by: EQUATION 4: DIFFERENCE AMPLIFIER 1 CMRR DIFF = -----------------------------------------------------1 - + 2 × TOL ----------------------R CMRR OP Where: TOLR = Resistors’ Tolerance CMRROP = Common Mode Rejection Ratio of Operational Amplifier Notice that a difference amplifier with lower TOLR and higher CMRROP will have the higher CMRRDIFF. If the op amp’s CMRR (CMRROP) is given in V/V (e.g., 80 dB is converted to 10,000 V/V), and the resistor tolerance (TOLR) is given in absolute terms (e.g., 0.1% becomes 0.001), then the difference amplifier’s CMRR (CMRRDIFF) will be in V/V (for the example already given, 476 V/V = 54 dB). Equation 3 shows that as CMRRDIFF increases, GCM becomes smaller. For a perfectly symmetrical difference amplifier, as CMRRDIFF approaches infinity, GCM approaches zero. DS01306A-page 4 • • • • VDD = 5.0V, VSS = 0V Uses a MCP6V01 auto-zeroed op amp (U5) Two 0.1% tolerance gain resistors (R8 and R11) Two 0.1% tolerance input resistors for shifting VOUT1 (R9 and R10) • Two 0.1% tolerance input resistors for the thermocouple output (R6 and R7) The difference amplifier is powered in single supply configuration and VDD should have a local bypass capacitor (i.e., 0.01 µF to 0.1 µF). VOUT1 must be kept within the ADC’s allowed voltage range, which is scaled by the gain of MCP6V01. The low tolerance gain setting resistors are matched to provide symmetry for good common mode rejection. The MCP6V01 auto-zeroed op amp less than 2 µV input offset voltage and high common-mode rejection ratio makes it ideal for thermocouple sensing applications. © 2009 Microchip Technology Inc. AN1306 2ND ORDER RC LOW-PASS FILTER The transfer function set by the difference amplifier is: • Fast enough to quick changes in temperature • Double pole for anti-aliasing and removing highfrequency noise • No DC offset and simple architecture EQUATION 6: V OUT1 = G 1 × V TH + G 2 × ( 0 – V SHIFT ) + V REF = G 1 × V TH – G 2 × V SHIFT + V REF The pole set by the low-pass filer is: Where: VTH = VP - VM ; EMF Voltage from Thermocouple VREF = 4.1V ; Reference Voltage VSHIFT = CVREF VOUT1 = Output Voltage of Difference Amplifier G1 = R11/R7 = R8/R6 = 1000 V/V G2 = R11/R10 = R8/R9 = 17.86 V/V EQUATION 7: 1 - = --------------------1 - = 3.19Hz f P = --------------------2 π R 12 C 6 2 π R 13 C 7 R10 5.6 kΩ R11 100 kΩ VREF R7 100Ω U5 MCP6V01 VP R12 499Ω R13 499Ω VOUT2 VM CVREF VSHIFT R9 5.6 kΩ U4 MCP6001 C6 100 nF R6 100Ω R8 100 kΩ C7 100 nF 2nd order low-pass filter Difference Amplifier Buffer Amplifier FIGURE 6: Analog Sensing Circuit Diagram. © 2009 Microchip Technology Inc. DS01306A-page 5 AN1306 VSHIFT Operation Description In this application note, CVRSS = 1 is set for VREF+ and CVRSS = 0 is set for VREF-. The MCP1541 provides an absolute reference voltage 4.1V. (VREF+ = 4.1V and VREF- = 0V). PIC18F2550’S COMPARATOR VOLTAGE REFERENCE BLOCK DIAGRAM The comparator voltage reference is a 16-tap resistor ladder network that provides a selectable reference voltage. Although its primary purpose is to provide a reference for the analog comparators, it may also be used independently of them. A block diagram of the module is shown in Figure 7. The resistor ladder is segmented to provide two ranges of CVREF values and has a power-down function to conserve power when the reference is not being used. The module’s supply reference can be provided from either device VDD/VSS or an external voltage reference. CVRSS = 1 VREF+ VDD 8R CVRSS = 0 CVR3 : CVR0 CVREN R R 16 Steps 16-to-1 Mux R R CVREF VSHIFT R R R CVRR 8R CVRSS = 1 VREFCVRSS = 0 FIGURE 7: DS01306A-page 6 PIC18F2550 Comparator Voltage Reference Block Diagram. © 2009 Microchip Technology Inc. AN1306 VSHIFT OPERATION CONCEPTUAL DIAGRAM This solution minimizes cost by using resources internal to the PIC to achieve high accuracy and high resolution thermocouple solution. This solution eliminates the need for a high end and costly instrumentation system to measure temperature using thermocouple. Further savings could be achieved by using a voltage reference internal to the PIC instead of the external MCP1541. Figure 8 shows the VSHIFT operation conceptual diagram. VSHIFT is also connected to the PIC18F2550 ADC channels along with VOUT2, which is uses to calculate Thermocouple EMF voltage. The 10-bit ADC and the 4-bit adjustable reference voltage provide a 14-bit measurement resoution. The MCP1541 provides an absolute reference to the ADC and difference amplifier circuit. • 14-bit Resolution, 10-bit ADC: - PIC18F2550’s CVREF (4-bit Adjustable Reference Voltage) - PIC18F2550’s internal 10-bit ADC - The firmware automatically searches for correct CVREF level VREF ×1 CVREF VSHIFT Difference V OUT1 VP Amplifier MCP6V01 VM (VOUT1 without VSHIFT) (Voltages not to Scale) VOUT1 VSHIFT V P – VM FIGURE 8: 1100 1000 900 800 700 600 500 400 300 200 100 0 -100 TTC (°C) VSHIFT Operation Conceptual Diagram. © 2009 Microchip Technology Inc. DS01306A-page 7 AN1306 Automatic Reference Voltage Search Figure 9 shows a screen capture from an osciloscope while the PIC18F2550 searches a reference voltage VSHIFT. Channel 1 (yellow trace) is the MCP6V01 output VOUT1 and Channel 2 is VSHIFT. VSHIFT is adjusted until the output is scaled within a voltage range of 0.2V to 4V, as shown in Table 1. The search is sequenced by first setting CVREF levels 0, 15, 1, 14, 2, 13, ... 6, 9, and 7. The voltage at level 7 set the output to equal approximately 0.7V. Then, EMF is calculated by measuring VSHIFT and VOUT2. Must be: 0.2V < VOUT1 < 4.0V End at High T TC VOUT1 Hunt for Correct VSHIFT Start at Low TTC VSHIFT FIGURE 9: TABLE 1: Voltage vs. Time Plot VSHIFT OPERATION CHANGING POINTS # Ref Approximate VSHIFT ADC (Code) VOUT1 (V) VTH (mV) Approximate Temp Range (°C) 0 0 50 to 1000 0.200 to 4.000 -3.900 to -0.096 -102 to +2 1 0.208333 50 to 1000 0.200 to 4.000 -0.180 to 3.624 -4 to +88 2 0.416667 50 to 1000 0.200 to 4.000 3.541 to 7.344 +86 to +180 3 0.625000 50 to 1000 0.200 to 4.000 7.261 to 11.065 +178 to +272 4 0.833333 50 to 1000 0.200 to 4.000 10.981 to 14.785 +270 to +361 5 1.041667 50 to 1000 0.200 to 4.000 14.701 to 18.505 +359 to +449 6 1.250000 50 to 1000 0.200 to 4.000 18.422 to 22.225 +447 to +537 7 1.458333 50 to 1000 0.200 to 4.000 22.142 to 25.946 +535 to +624 8 1.666667 50 to 1000 0.200 to 4.000 25.862 to 29.666 +622 to +712 9 1.875000 50 to 1000 0.200 to 4.000 29.582 to 33.386 +710 to +802 10 2.083333 50 to 1000 0.200 to 4.000 33.303 to 37.106 +800 to +894 11 2.291667 50 to 1000 0.200 to 4.000 37.023 to 40.827 +892 to +988 12 2.500000 50 to 1000 0.200 to 4.000 40.743 to 44.547 +986 to +1083 13 2.708333 50 to 1000 0.200 to 4.000 44.463 to 48.267 +1081 to +1184 14 2.916667 50 to 1000 0.200 to 4.000 48.184 to 51.987 +1182 to +1277 15 3.125000 50 to 1000 0.200 to 4.000 51.904 to 55.707 +1275 to +1372 DS01306A-page 8 © 2009 Microchip Technology Inc. AN1306 FIRMWARE AND SOFTWARE Firmware The firmware uses the PIC18F2550 USB PIC® Microcontroller to compute Thermocouple temperature and transfer temperature data to PC via the USB interface. The firmware has two major functions, maintain USB interface with PC and measure/compute temperature. The firmware uses USB HID interface and does not require PC side driver software. Once the USB is connected to a PC the USB module is initialized, and the Thermocouple temperature conversion is started upon a successful USB initialization. The Thermocouple measurement routine starts by measuring the thermocouple output voltage from the MCP6V01. If the output voltage is out of range as shown in the Table 1 then the reference voltage is adjusted automatically as shown in Figure 9. Once the corresponding VSHIFT value is determined, both VOUT2 and VSHIFT are digitized using the 10bit ADC. From these voltages, the Thermocouple EMF is calculated. The EMF voltage is converted to temperature in degree Celcius (°C) using the 9th order equation provided by ITS-90 standard (www.nist.org). The temperature value is cold-junction compensated using the MCP9800 temperature sensor. EQUATION 8: EMF CALCULATION EMF = ( V OUT2 + V SHIFT • Gain – V REF ) Where: The temperature data is stored in memory in IEEE Standard for Floating-Point Arithmetic (IEEE 754). When a temperature data is requested from the PC the floating point data is converted to Binary Code Decimal (BCD) and each byte is loaded in the USB data transfer buffer. Along with the temperature data, VOUT2, VSHIFT and the cold-junction temperature are loaded. The PC Graphical User Interface (GUI) converts the BCD data to floating point number which represents temperature. The temperature data is displayed and plotted on the graphical display. Additionally, the GUI displays EMF voltage, thermocouple output and cold junction temperature. Start Initialize USB Perform USB tasks Perform Thermocouple Measurement Adjust VSHIFT Measure VOUT2 is 0.2V < VOUT2< 4V? EMF = Thermocouple voltage (mV) VOUT2 = MCP6V01 Filtered Output (V) VSHIFT = Adjustable reference voltage (V) Measure VSHIFT Gain = Difference Amplifier Gain (R8/R9) VREF = Absolute reference voltage, MCP1541 output (V) EQUATION 9: COLD JUNCTION COMPENSATION T = T CJ – T HJ Where: Calculate EMF (mV) Also see Equation 8 Convert EMF to ×C Also see ITS-90 Standard Measure Cold-junction temperature and Compensate Sensor Also see Equation 9 T = Absolute Thermocouple temperature (°C) TCJ = Cold-Junction temperature, MCP9800 output (°C) THJ = Hot-Junction temperature or Thermocouple temperature from ITS-90 standard (°C) Save Temperature If requested, send temperature data to PC via USB FIGURE 10: © 2009 Microchip Technology Inc. Top Level Flow Chart. DS01306A-page 9 AN1306 Thermal Management Software GUI The GUI is a measurement tool which enables user to see the changes in temperature graphically by displaying the Thermocouple raw output data along with linearized temperature data. It also enables user to calibrate the system. Temperature can also be measured over an extended period of time by clicking the Start Acquisition button or Play button. The measurement interval is controlled by the software timer. When the timer ticks a command is sent to the hardware to acquire temperature data then the firmware transfers the last successfully converted temperature data. FIGURE 11: DS01306A-page 10 Additionally, user can calibrate the Thermocouple sensor by using the calibration option from the GUI. This feature can be enabled by clicking on the Enable Calibration check box. Once enabled, user can type in the thermocouple calibration temperature and click the Calibrate! button. When calibrated, the temperature difference between the thermocouple and calibration temperature is stored in the PICmicro EEPROM. The difference is also shown in the “Calibration Offset” display of the GUI. Once calibrated, the offset is subtracted from temperature measurements. In addition, clicking the Reset button clears the calibration offset value to 0 (the EEPROM content is set to 0). Graphical User Interface. © 2009 Microchip Technology Inc. AN1306 SUMMARY REFERENCES This application note shows hardware and firmware design engineers how to use a PICmicro® Microcontroller and a difference amplifier system to measure Type-K Thermocouple voltage to accurately measure temperature from -100°C to 1000°C using a 10-bit ADC and 4-bit adjustable reference voltage.  “The OMEGA® Made in the USA Handbook™”, Vol. 1, OMEGA Engineering, Inc., ©2002.  “The OMEGA® Made in the USA Handbook™”, Vol. 2, OMEGA Engineering, Inc., ©2002.  AN990, “Analog Sensor Conditioning Circuits – An Overview”, Kumen Blake, Microchip Technology Inc., DS00990, ©2005.  AN1258, “Op Amp Precision Design: PCB Layout Techniques”, Kumen Blake, Microchip Technology Inc., DS01258, ©2009.  AN679, “Temperature Sensing Technologies”, Bonnie Baker, Microchip Technology Inc., DS00679, ©1998.  AN684, “Single Supply Temperature Sensing with Thermocouples”, Bonnie Baker, Microchip Technology Inc., DS00684, 1998.  MCP6V01/2/3 Data Sheet, “Auto-Zeroed Op Amps”, Microchip Technology Inc., DS22058, ©2008.  PIC18F2455/2550/4455/4550 Data Sheet, “28/40/44-Pin, High-Performance, Enhanced Flash, USB Microcontrollers with nanoWatt Technology”, Microchip Technology Inc., DS39632, ©2007.  “MCP6V01 Thermocouple Auto-Zeroed Reference Design”, Yang Zhen, Microchip Technology Inc., DS51738, ©2008.  AN1297, “Microchip’s Op Amp SPICE Macro Models”, Yang Zhen, Microchip Technology Inc. DS01297, ©2009. © 2009 Microchip Technology Inc. DS01306A-page 11 AN1306 NOTES: DS01306A-page 12 © 2009 Microchip Technology Inc. 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. 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Analog-for-the-Digital Age, Application Maestro, CodeGuard, dsPICDEM, dsPICDEM.net, dsPICworks, dsSPEAK, ECAN, ECONOMONITOR, FanSense, HI-TIDE, In-Circuit Serial Programming, ICSP, Mindi, MiWi, MPASM, MPLAB Certified logo, MPLIB, MPLINK, mTouch, Octopus, Omniscient Code Generation, PICC, PICC-18, PICDEM, PICDEM.net, PICkit, PICtail, PIC32 logo, REAL ICE, rfLAB, Select Mode, Total Endurance, TSHARC, UniWinDriver, 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. © 2009, Microchip Technology Incorporated, Printed in the U.S.A., All Rights Reserved. Printed on recycled paper. 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