Circuit Note
CN-0206
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/CN0206.
Devices Connected/Referenced AD7793 3-Channel, Low Noise, Low Power,  24-Bit Σ-Δ ADC with On-Chip In-Amp  and Reference
ADuC832
Precision Analog Microcontroller
Complete Thermocouple Measurement System Using the
AD7793 24-Bit Sigma-Delta ADC
Rev.0
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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. The AD7793 consumes only 500 µA maximum, making  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 µA 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.
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CN-0206
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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 µV/°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 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 k Ω at +25°C. The typical resistance at 0°C is 815 Ω and 1040 Ω 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 × (R  – 815)/(1040 – 815) The 1 mA excitation current on the AD7793 is used to supply the thermistor and the 2 k Ω precision resistor. The reference voltage is generated using this external precision 2 kΩ 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 × 1040 Ω = 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) × 2000/223
One other consideration is the output compliance of the IOUT1 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 × 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.generated
20
15
10
5
–5
–10–300
–200–1000100200300400
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T H E R M O C O U P L E  E M F  (m V )
TEMPERATURE (°C)
APPROXIMATELY LINEAR REGION
TYPE “T” THERMOCOUPLE
Figure 2. Thermocouple EMF vs Temperature
Circuit Note
CN-0206
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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 µV . The peak-to-peak noise is
6.6 × RMS Noise  = 6.6 × 0.088 µV = 0.581 µV
If the thermocouple has a sensitivity of precisely 40 µV/°C, the thermocouple should measure the temperature to a resolution of
0.581 µV ÷ 40 µV = 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.
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Figure 3. Thermocouple System Using the AD7793
Table 1. T est Results for Thermocouple System
0°C Ambient
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/CN0206-DesignSupport .
LEARN MORE
CN-0206 Design Support Package:
www.analog/CN0206-DesignSupport
Kester, Walt. 1999. S ensor 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: 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.
CN-0206
Circuit Note
Rev. 0 | Page 4 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
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