Circuit Note CN-0206 Devices Connected/Referenced Circuits from the Lab reference circuits are engineered and tested for quick and easy system integration to help solve todays 3-Channel, Low Noise, Low Power, 24-Bit, - analog, mixed-signal, and RF design challenges. For more AD7793 ADC with On-Chip In-Amp and Reference information and/or support, visit www.analog.com/CN0206. Thermocouple Temperature Measurement System with Less Than 500 A Current Drain The AD7793 consumes only 500 A maximum, making it suitable EVALUATION AND DESIGN SUPPORT for any low power application, such as smart transmitters where Circuit Evaluation Boards the complete transmitter must consume less than 4 mA. The CN-0206 Circuit Evaluation Board (EVAL-CN0206-SDPZ) AD7793 has a power-down option. In this mode, the complete System Demonstration Platform (EVAL-SDP-CB1Z) ADC, along with its auxiliary functions, is powered down so Design and Integration Files that the part consumes 1 A maximum. Schematics, Layout Files, Bill of Materials Because the AD7793 provides an integrated solution for thermo- CIRCUIT FUNCTION AND BENEFITS couple design, it interfaces directly to the thermocouple. For the The circuit shown in Figure 1 is a complete thermocouple system cold junction compensation, a thermistor along with a precision based on the AD7793 24-bit sigma-delta (-) analog-to-digital resistor is used. These are the only external components required converter (ADC). The AD7793 is a low power, low noise, complete for the cold junction measurement other than some simple RC analog front end for high precision measurement applications. filters for EMC considerations. The device includes a programmable gain amplifier (PGA), an internal reference, an internal clock, and excitation currents, thereby greatly simplifying the thermocouple system design. +5V + 0.1F 10F THERMOCOUPLE CONNECTOR DV AV DD DD 1k + AIN1(+) 0.01F 0.1F AD7793 THERMOCOUPLE 1k AIN1() 0.01F SCLK IOUT2 DIN IOUT1 DOUT/RDY AIN2(+) THERMISTOR CS COLD JUNCTION KTY81-110 1k AT 25C AIN2() REFIN(+) 2k 0.1% 10ppm REFIN() CLK GND Figure 1. Thermocouple Measurement System with Cold Junction Compensation (Simplified Schematic: All Connections and Decoupling Not Shown) Rev. B 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 One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A. suitability and applicability for your use and application. Accordingly, in no event shall Analog Devices Tel: 781.329.4700 www.analog.com be liable for direct, indirect, special, incidental, consequential or punitive damages due to any cause Fax: 781.461.3113 20112013 Analog Devices, Inc. All rights reserved. whatsoever connected to the use of any Circuits from the Lab circuits. (Continued on last page) 09776-001CN-0206 Circuit Note When reading the thermocouple voltage, the ADC uses the CIRCUIT DESCRIPTION external 2 V reference and is configured for a gain of 64. This A Type T thermocouple is used in the circuit. This thermocouple defines the analog input voltage range as 31.25 mV (V /Gain). REF (made from copper and constantan) measures temperature from For a gain of 64, the absolute voltage on the analog inputs must 200C to +400C. It generates a typical temperature dependent be between GND + 300 mV and AVDD 1.1 V. voltage of 40 V/C. Because the AD7793 operates from a single power supply, the The thermocouple response is approximately linear over a small signal generated by the thermocouple must be biased above portion of its entire temperature range, from 0C to 60C (see ground so that it is within the acceptable range of the ADC. The Figure 2). To allow accurate measurements over the entire bias voltage generator on-board the AD7793 biases the thermo- temperature range, the CN0206-SDP-0 evaluation software couple signal so that it has a common-mode voltage of AV /2. DD implements a linearization routine. 20 ADC Channel 2 Configuration, Thermistor TYPE T THERMOCOUPLE The second channel of the ADC monitors the voltage generated 15 across a thermistor being driven by one of the current output pins of the AD7793. A 1mA excitation current drives the series 10 pair of thermistor and precision resistor (2 k , 0.1%), as shown in Figure 1. 5 The thermistor value varies from 0C (815 ) to 30C (1040 ) , APPROXIMATELY producing a voltage signal range of 815 mV to 1040 mV. The 0 LINEAR REGION precision resistor produces 2.0 V for use as an external reference. With a gain of 1, the analog input range is 2 V (VREF/Gain). 5 This architecture gives a ratiometric configuration. Any change in the value of the excitation current does not alter the accuracy of 10 300 200 100 0 100 200 300 400 the system. TEMPERATURE (C) Assuming a linear transfer function between 0C and 30C, the Figure 2. Thermocouple EMF vs. Temperature relationship between the cold junction temperature and the Cold Junction Compensation thermistor resistance, R, is Thermocouples measure the temperature difference between Cold Junction Temperature = 30 (R 815)/(1040 815) two points, not an absolute temperature. To measure a single One other consideration is the output compliance of the IOUT1 temperature, maintain one of the junctions (normally the cold pin of the AD7793. When the 1 mA excitation current is used, junction) at a known reference temperature, and the other the output compliance equals AV 1.1 V. This specification is DD junction at the temperature to be sensed. met because the maximum voltage at IOUT1 equals the voltage Having a junction of known temperature is not convenient for across the precision resistor plus the voltage across the thermistor, most applications therefore, a thermally sensitive device is placed which equals 2 V + 1.04 V = 3.04 V. on the printed circuit board (PCB). This thermistor is used to Output Coding measure the temperature of the thermocouple input connection. The output code for an input voltage on either channel is The thermistor fits inside the metallic tab found on the thermo- N 1 couple connection, minimizing any temperature gradients that Code = 2 (AIN Gain/V ) +1 REF may exist. ere: wh Because the voltage from a known cold junction is simulated, the AIN is the analog input voltage. appropriate correction can be applied. See the Thermocouple GAIN is the in-amp setting Linearization section for more detailed information on processing N = 24. and manipulating the thermocouple and thermistor voltages to The EVAL-SDP-CB1Z analog microcontroller processes the produce an accurate temperature reading. conversions from the AD7793. ADC Channel 1 Configuration, Thermocouple Thermocouple Linearization The temperature range for the thermocouple is 200C to +400C. As mentioned in the Circuit Description section, the thermo- The typical temperature dependent voltage generated by the couple is only approximately linear over a small temperature thermocouple is 40 V/C. This voltage generates a thermocouple range. In fact, the thermocouple is highly nonlinear over the voltage range of 8 mV to +16 mV. rest of the temperature range. For this reason, a linearization procedure is implemented in the CN0206-SDP-0 LabVIEW software. Rev. B Page 2 of 5 THERMOCOUPLE EMF (mV) 09776-002