Circuit Note CN-0172 Devices Connected/Referenced Circuits from the Lab reference circuits are engineered and 0.25C Accurate, 16-Bit Digital SPI ADT7320 tested for quick and easy system integration to help solve todays Temperature Sensor analog, mixed-signal, and RF design challenges. For more 3-Channel, Low Noise, Low Power, 24-Bit information and/or support, visit www.analog.com/CN0172. AD7793 - ADC with On-Chip In-Amp and Reference 3-Channel Thermocouple Temperature Measurement System with 0.25C Accuracy This type of application is popular where a cost-effective, accurate EVALUATION AND DESIGN SUPPORT temperature measurement is required over the wide temperature Circuit evaluation boards ranges offered by thermocouples. CN-0172 circuit evaluation board (EVAL-CN0172-SDPZ) System Demonstration Platform (EVAL-SDP-CB1Z) CIRCUIT DESCRIPTION CN0172 breakout board (included with EVAL-CN0172- The circuit in Figure 1 is designed to measure the temperature SDPZ board) of three K type thermocouples at the same time using the AD7793 Design and integration files 24-bit - ADC. The reference junction temperature is measured Schematics, Layout Files, Bill of Materials, Software using the ADT7320, a 0.25C accurate, 16-bit digital SPI temperature sensor. Thermocouple Voltage Measurement CIRCUIT FUNCTION AND BENEFITS The interface between the thermocouple and the AD7793 ADC The function of the circuit shown in Figure 1 is to provide a high is a thermocouple connector and filter. Each connector (J1, J2, accuracy multichannel thermocouple measurement solution. and J3) connects directly to a set of differential ADC inputs. Achieving a precision thermocouple measurement requires a The filter on the inputs to the AD7793 reduces any noise pickup signal chain of precision components that amplifies the small in the thermocouple leads before the signal reaches the AIN(+) thermocouple voltage, reduces noise, corrects nonlinearity, and and AIN() inputs of the ADC. The AD7793 has an on-chip provides accurate reference junction compensation (commonly multiplexer, buffer, and instrumentation amplifier to amplify the referred to as cold junction compensation). This circuit addresses small voltage from the thermocouple measurement junction. all these challenges for measuring thermocouple temperature with better than 0.25C accuracy. Cold Junction Measurement The ADT7320 precision 16-bit digital temperature sensor is used The circuit shown in Figure 1 shows how three K type thermo- couples are connected to the AD7793 precision 24-bit sigma- to measure the reference (cold) junction temperature with an delta (-) analog-to-digital converter (ADC) to measure the accuracy of 0.25C over the 20C to +105C temperature thermocouple voltage. Because the thermocouple is a differential range. The ADT7320 is fully calibrated at the factory, and no user device rather than an absolute temperature measurement device, calibration is required. The ADT7320 contains an internal band the reference junction temperature must be known to get an gap reference, a temperature sensor, and a 16-bit - ADC to accurate absolute temperature reading. This process is known measure and digitize the temperature to 0.0078C resolution. as reference junction compensation, commonly referred to as Both the AD7793 and ADT7320 are controlled by an SPI interface cold junction compensation. In this circuit, the ADT7320 using the System Demonstration Platform (EVAL-SDP-CB1Z). precision 16-bit digital temperature sensor is used for the cold Both AD7793 and ADT7320 can also be controlled by a micro- junction reference measurement and provides the required controller. accuracy. Rev. A 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 whatsoever connected to the use of any Circuits from the Lab circuits. (Continued on last page) Fax: 781.461.3113 20122013 Analog Devices, Inc. All rights reserved. CN-0172 Circuit Note FILTERING REFERENCE (COLD) JUNCTION COMPENSATION FOR CHANNEL 1 (J1) R1 J1 1k VDD 5 J4 14 CHANNEL 1 + AIN1(+) 16 15 14 13 DV DD 1 THERMOCOUPLE C3 C1 R2 2 1 VBIAS MEASUREMENT 0.1F 0.01F SCLK 12 1k V DD JUNCTION 6 13 3 2 11 AIN1() DOUT C1 AV GND DD U2 C2 17 0.1F 4 3 THERMOCOUPLE C5 C4 EPAD 0.01F DIN ADT7320 10 REFERENCE U1 10F 0.1F + CT 5 4 JUNCTION CS 9 AD7793 INT 6 R3 J2 5 6 7 8 1k 12 7 + AIN2(+) GND CHANNEL 2 C10 C8 THERMOCOUPLE VBIAS R7 0.1F 0.01F MEASUREMENT 1k 8 JUNCTION REFERENCE (COLD) JUNCTION AIN2() C6 COMPENSATION FOR CHANNEL 2 (J2) THERMOCOUPLE SPI DOUT 0.01F 15 J5 REFERENCE DOUT/RDY 16 15 14 13 1 JUNCTION 2 1 SCLK 12 R8 V DD 3 2 J3 SPI DIN 11 1k DOUT C1 16 GND 9 U3 + AIN3(+)/REFIN(+) DIN 17 0.1F 4 3 CHANNEL 3 EPAD DIN ADT7320 C13 C11 10 THERMOCOUPLE VBIAS R9 CT 5 0.1F 0.01F 4 CS MEASUREMENT SPI CLK 1k 9 1 10 6 INT JUNCTION AIN3()/REFIN() SCLK C7 THERMOCOUPLE 5 6 7 8 0.01F REFERENCE 2 JUNCTION 4 IOUT 1 CLK R20 R11 REFERENCE (COLD) JUNCTION 1k 1k CS1 COMPENSATION FOR CHANNEL 3 (J3) 11 3 IOUT 2 CS J6 16 15 14 13 1 VDD VDD = 3.3V 2 1 SCLK SPI DOUT 12 V DD CS1 3 2 11 DOUT C1 GND U4 SPI DIN 17 0.1F 4 3 CS2 EPAD ADT7320 DIN 10 5 CT 4 CS SPI CLK CS3 9 6 INT CS4 5 6 7 8 1 2 3 4 5 6 7 8 9 CS2 CS3 CS4 SPI DIN SPI DOUT SPI CLK J7 VDD 1 2 3 4 5 6 1 2 3 4 5 6 1 2 3 4 5 6 J9 J10 J11 NOTES: 1. EPAD = EXPOSED PADDLE OF THE LFCSP PACKAGE (ADT7320) 2. THE REFERENCE (COLD) JUNCTION COMPENSATION SENSOR BOARDS CONNECTIONS TO THE REST OF THE CIRCUIT: A) CHANNEL 1 (J1): J4 TO J9 B) CHANNEL 2 (J2): J5 TO J10 C) CHANNEL 3 (J3): J6 TO J11 Figure 1. Multichannel Thermocouple Measurement System (Simplified Schematic: All Connections and Decoupling Not Shown) Figure 2. EVAL-CN0172-SDPZ Circuit Evaluation Board Rev. A Page 2 of 6 SPICLK SPIDIN SPIDOUT CS1 CS2 CS3 CS4 VDD GND NC NC NC NC NC NC NC NC NC NC NC NC NC NC NC NC NC NC NC NC NC NC NC NC 09240-002 09240-001