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ANALOG DEVICES CN-0206 handbook

1. Ambient 0 C Temperature 20 C Thermocouple Reading C 20 0 Thermistor Reading C 20 3 20 3 Resultant Reading C 0 3 20 3 Peak to Peak Noise C 0 02 0 02 From Table 1 the thermocouple is reporting the correct value while the thermistor has a 0 3 C error This is the accuracy of the system when linearization is not included Including linearization for the thermocouple and the thermistor would improve the accuracy of the system and it would also allow the system to measure a wider range of temperatures If the difference between the minimum and maximum temperature readings is measured for every 10 readings the peak to peak noise in terms of temperature is 0 02 C Therefore the actual peak to peak resolution is very close to the expected value COMMON VARIATIONS The AD7793 is a low noise low power ADC Other suitable ADCs are the AD7792 and AD7785 Both parts have the same feature set as the AD7793 However the AD7792 is a 16 bit ADC while the AD7785 is a 20 bit ADC CIRCUIT EVALUATION AND TEST Test data was taken using the board shown in Figure 3 Complete documentation for the system can be found in the CN 0206 Design Support package at www analog com CN0206 DesignSupport LEARN MORE CN 0206 Design Support Package www analog com CN0206 DesignSupport Kester Walt 1999 Sensor Signal Conditioning Section 7 Analog Devices MT 004 Tutorial The Good the Bad and the Ugly A
2. pin of the AD7793 When the 1 mA excitation current is used the output compliance equals AVDD 1 1 V From the previous calculations this specification is met since the maximum voltage at IOUT1 equals the voltage across the precision resistor plus the voltage across the thermistor which equals 2 V 1 04 V 3 04 V The AD7793 is configured to operate with an output data rate of 16 7 Hz For every ten conversions read from the thermocouple one conversion is read from the thermistor The resultant temperature equals Temperature Thermocouple Temperature Cold Junction Temperature The conversions from the AD7793 are processed by the ADuC832 analog microcontroller and the resultant temperature is displayed on the LCD display The thermocouple design is operated from 6 V 2 x 3 V Lithium Ion batteries A diode reduces the 6 V to a level suitable for the AD7793 and the ADuC832 analog microcontroller An RC filter is placed between the ADuC832 power supply and the AD7793 power supply so that the power supply digital noise to the AD7793 is minimized Figure 2 shows the relationship between voltage generated across the thermocouple and temperature for a T type thermocouple The circled area is the region from 0 C to 60 C where the transfer function is approximately linear 20 15 S E u 10 W W z g 5 fe S Q APPROXIMATELY E 0 LINEAR REGION x 5 10 S 300 200 100 0 100 200 300 400 TEMP
3. signal so that it has a common mode voltage of AVDD 2 This ensures that the input voltage limits are met with significant margin The thermistor has a value of 1 KQ at 25 C The typical resistance at 0 C is 815 Q and 1040 Q at 30 C Assuming a linear transfer function between 0 C and 30 C the relationship between cold junction temperature and thermistor resistance R is Cold Junction Temperature 30 x R 815 1040 815 The 1 mA excitation current on the AD7793 is used to supply the thermistor and the 2 kQ precision resistor The reference voltage is generated using this external precision 2 KQ resistor This architecture gives a ratiometric configuration the excitation current is used to supply the thermistor and to generate the reference voltage Therefore any deviation in the value of the excitation current does not alter the accuracy of the system The AD7793 operates at a gain of 1 when sampling the thermistor channel For a maximum cold junction of 30 C the maximum voltage generated across the thermistor is 1 mA x 1040 Q 1 04 V The precision resistor is chosen so that the maximum voltage generated across the thermistor multiplied by the PGA gain is less than or equal to the voltage generated across the precision resistor For a conversion value of ADC_CODE the corresponding thermistor resistance R equals R ADC_CODE 0x800000 x 2000 2 One other consideration is the output compliance of the IOUT1
4. ANALOG DEVICES Circuit Note CN 0206 Devices Connected Referenced Circuits from the Lab reference circuits are engineered and Circuits from the Lab Ba analog mixed signal and RF design challenges For more Reference Circuits tested for quick and easy system integration to help solve today s AD7793 3 Channel Low Noise Low Power 24 Bit Z A ADC with On Chip In Amp and Reference information and or support visit www analog com CN0206 ADuC832 Precision Analog Microcontroller Complete Thermocouple Measurement System Using the AD7793 24 Bit Sigma Delta ADC EVALUATION AND DESIGN SUPPORT Design and Integration Files Schematics Layout Files Bill of Materials CIRCUIT FUNCTION AND BENEFITS The circuit shown in Figure 1 is a complete thermocouple system based on the AD7793 24 bit sigma delta ADC The AD7793 is a low power low noise complete analog front end for high precision measurement applications The device includes a PGA internal reference internal clock and excitation currents thereby greatly simplifying the thermocouple system design The system noise is approximately 0 02 C peak to peak THERMOCOUPLE CONNECTOR QO AIN1 THERMOCOUPLE G 0 01pF eae 1kQ O AIN1 THERMISTOR KTY81 110 1kQ AT 25 C COLD JUNCTION AIN2 k REFIN 0 1 10ppm REFIN DVpp AD7793 CLK GND The AD7793 consumes only 500 pA maximum ma
5. ERATURE C Figure 2 Thermocouple EMF vs Temperature Rev 0 Page 2 of 4 When the system is at room temperature the thermistor should indicate the value of the ambient temperature The thermocouple indicates the relative temperature with respect to the cold junction temperature i e the temperature difference between the cold junction thermistor and the thermocouple Therefore at room temperature the thermocouple should indicate 0 C If the thermocouple is placed in an ice bucket the thermistor continues to measure the ambient cold junction temperature The thermocouple should indicate the negative of the thermistor value so that the overall temperature equals zero Finally for an output data rate of 16 7 Hz and a gain of 128 the rms noise of the AD7793 equals 0 088 uV The peak to peak noise is 6 6 x RMS Noise 6 6 x 0 088 uV 0 581 uV If the thermocouple has a sensitivity of precisely 40 uV C the thermocouple should measure the temperature to a resolution of 0 581 uV 40 uV 0 014 C Figure 3 shows the actual test board The system was evaluated by measuring the thermistor temperature the thermocouple temperature and the resolution at room temperature and when the thermocouple was placed in an ice bucket The results are shown in Table 1 caaqanaeanacees nag Marke Leg u Se 09776 003 Figure 3 Thermocouple System Using the AD7793 Table 1 Test Results for Thermocouple System
6. al inddental 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 Analog Devices Inc All rights reserved CIRCUIT DESCRIPTION A type T thermocouple is used in the circuit This thermocouple made from copper and constantan measures temperature from 200 C to 400 C It generates a typical temperature dependent voltage of 40 uV C A thermocouple does not have a linear transfer function For a temperature range of 0 C to 60 C the response is quite linear However for wider temperature ranges a linearization routine is required The circuit tested does not include linearization Therefore the useful measurement range of the circuit is from 0 C to 60 C For this temperature range the thermocouple generates a voltage from 0 mV to 2 4 mV The internal 1 17 V reference is used for the thermocouple conversions So the AD7793 is configured for a gain of 128 Since the AD7793 operates from a single power supply the signal generated by the thermocouple must be biased above ground so that it is within the acceptable range of the ADC For a gain of 128 the absolute voltage on the analog inputs must be between GND 300 mV and AVDD 1 1 V The bias voltage generator onboard the AD7793 biases the thermocouple
7. king it suitable for any low power application such as smart transmitters where the complete transmitter must consume less than 4 mA The AD7793 also has a power down option In this mode the complete ADC along with its auxiliary functions are powered down so that the part consumes 1 yA maximum Since the AD7793 provides an integrated solution for thermocouple design it interfaces directly to the thermocouple For the cold junction compensation a thermistor along with a precision resistor is used These are the only external components required for the cold junction measurement other than some simple R C filters for EMC considerations 6VLITHIUMION BATTERY i 2 x 3V CELLS AVpp ADuC832 ANALOG MICRO CONTROLLER 09776 001 Figure 1 Thermocouple Measurement System with Cold Junction Compensation Simplified Schematic All Connections and Decoupling Not Shown Rev 0 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 speci
8. s 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 Analog Devices Inc All rights reserved Trademarks and ANALOG registered trademarks are the property of their respective owners CN09776 0 10 11 0 DEVICES www analog com Rev 0 Page 4 of 4
9. spects of ADC Input Noise Is No Noise Good Noise 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 Rev 0 Page 3 of 4 MT 101 Tutorial Decoupling Techniques Analog Devices Data Sheets and Evaluation Boards AD7793 Data Sheet AD7793 Evaluation Board ADuC832 Data Sheet ADuC832 Evaluation System REVISION HISTORY 10 11 Revision 0 Initial Version 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 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 infringement

ANALOG DEVICES CN-0206 handbook

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