Circuit Note CN-0287 Devices Connected/Referenced 4-Channel, 4.8 kHz, Ultralow Noise, AD7193 24-Bit Sigma-Delta ADC with PGA 0.5C Accurate, 16-Bit Digital SPI ADT7310 Temperature Sensor Precision Micropower, Low Noise AD8603 CMOS R-to-R Input/Output Operational Amplifiers Circuits from the Lab reference designs are engineered and tested for quick and easy system integration to help solve todays 4.096 V, Micropower High Accuracy ADR3440 analog, mixed-signal, and RF design challenges. For more Voltage Reference information and/or support, visit www.analog.com/CN0287. CMOS, Low Voltage, 3-Wire Serially- ADG738 Controlled, Matrix Switch ADG702 CMOS Low Voltage 2 SPST Switch AD5201 33-Position Digital Potentiometer 3 kV RMS Dual Channel Digital ADuM1280 Isolators Quad-Channel, 2.5 kV Isolators with ADuM5401 Integrated DC-to-DC Converter Isolated 4-Channel, Thermocouple/RTD Temperature Measurement System with 0.5C Accuracy The RTD excitation current are is programmable for optimum EVALUATION AND DESIGN SUPPORT noise and linearity performance. Circuit Evaluation Boards CN-0287 Circuit Evaluation Board (EVAL-CN0287-SDPZ) RTD measurements achieve 0.1C accuracy (typical), and System Demonstration Platform (EVAL-SDP-CB1Z) Type-K thermocouple measurements achieve 0.05C typical Design and Integration Files accuracy because of the 16-bit ADT7310 digital temperature Schematics, Layout Files, Bill of Materials sensor used for cold-junction compensation. The circuit uses a four-channel AD7193 24-bit sigma-delta ADC with on-chip CIRCUIT FUNCTION AND BENEFITS PGA for high accuracy and low noise. The circuit shown in Figure 1 is a completely isolated 4-channel Input transient and overvoltage protection are provided by low temperature measurement circuit optimized for performance, leakage transient voltage supressors (TVS) and Schottky diodes. input flexibility, robustness, and low cost. It supports all types of The SPI-compatible digital inputs and outputs are isolated thermocouples with cold junction compensation and any type (2500 V rms), and the circuit is operated on a fully isolated of RTD (resistance temperature detector) with resistances up to power supply. 4 k for 2-, 3-, or 4-wire connection configurations. Rev. C Circuits from the Lab reference designs 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 One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A. determining its suitability and applicability for your use and application. Accordingly, in no event shall Tel: 781.329.4700 www.analog.com Analog Devices 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 20132014 Analog Devices, Inc. All rights reserved. CN-0287 Circuit Note +5V V : ANALOG GROUND DD AD5201 0 +4.096V +5V A SHDN COLD JUNCTION COMPENSATION : DIGITAL GROUND LOGIC W +5V CLK SCLK CONTROL B SCLK CT CT SDI DIN PWR-ON ADG738 CS INT INT PRESET CS SCLK CS ADT7310 CS V ADT7310 SS GND SCLK SCLK DIN 5.6V DOUT DOUT ZENER DIODE GND +5V DIN DIN +5V +5V SYNC SCLK DIN DOUT +4.096V +5V R3 FORCE V ADG738 V+ OUT SENSE D S1 1k C2 C2 AD8603 ADR3440 1.69k 300 +5V C2 V FORCE GND S2 SENSE 1.69k +5V 300 ADG702 S7 REFIN1(+) REFIN1(0) S 1.69k 300 D +5V P3 +5V +5V ISO S8 IN CH 1 P2 V V DD1 DD2 1.69k 300 1 GND GND +5V 1 2 GND ISO AD7193 V V IA OA INT INT ISO 1nF V V 2 AIN1 IB OB CT CT ISO 1.69k 300 +5V JP1 27nF ADuM1280 1 3 AIN2 1.69k 300 +5V AD7193 CS 2 CS +5V ISO 1nF V V THERMOCOUPLE: AIN3 DD2 DD1 3 SCLK SCLK 4 RTD 2,3W GND GND 2 1 AIN4 GND DOUT DOUT ISO V V OA IA ADT7310 CS ADT7310 CS ISO V V DIN DIN OB IB AIN5 ADG738 CS ADG738 CS ISO CH 4 AIN6 ADuM1280 +5V 1 +5V AV DD +5V 1nF +5V ISO 0.1F 10F V V AIN7 ISO DD1 2 300 1.69k AGND +5V GND GND 27nF ISO 1 JP4 GND ISO V V OA IA 1 RTD 4W DIN DIN AIN8 ISO 3 300 V V 1.69k OA IB 1nF 2 +5V SCLK SCLK ISO +5V DV V V DD OC IC 3 AD7193 CS AD7193 CS ISO 4 V V ID OD REFIN2(+) DOUT DOUT ISO 1.69k 300 4.02k DGND 0.1% ADuM5401 10ppm AINCOM REFIN2() Figure 1. 4-Channel Thermocouple and RTD Circuit (Simplified Schematic: All Connections and Decoupling Not Shown) signal generated is typically from several microvolts to tens of CIRCUIT DESCRIPTION millivolt depending on the temperature difference. Temperature Measurement Introduction For example, K-type thermocouples are capable of measuring Thermocouples and RTDs (resistance temperature detectors) 200C to +1350C with an output range of approximately 10 are the most frequently used sensors for temperature mV to +60 mV. It is important for the signal chain to maintain measurement in industrial applications. Thermocouples are as high impedance and low leakage as possible to achieve the able to measure very high temperatures up to about +2300C highest accuracy for the voltage measurement. In order to and also have a fast response time (measured in fractions of a convert this voltage to an absolute temperature, the cold junction second). RTDs are capable of higher accuracy and stability than temperature must be accurately known. Traditionally 1C to 2C thermocouples, and the resistance of long wire lengths has been considered sufficient, although since the cold junction (hundreds of meters) to a remote RTD can be compensated for measurement error contributes directly to the absolute with 3- or 4-wire connections. temperature error, a higher accuracy cold junction temperature A thermocouple consists of two wires of different metals joined measurement is beneficial at one end. This end is placed at the temperature which is to be An RTD is made from a pure material, such as platinum, nickel measured, refered to as the measurement junction. The other or copper, that has a predictable change in resistance as the end is connected to a precision voltage measurement unit, and temperature changes.The most widely used RTD is platinum this connection is referred to as the reference junction or alternately (Pt100 and Pt1000). the cold junction. The temperature difference between the measurement junction and the cold junction generates a voltage One method used to accurately measure the resistance is to (known as the Seebeck effect voltage) that is related to the measure the voltage across the RTD generated by a constant difference between the temperatures of the two junctions. The current source. Errors in the current source can be cancelled by referring the measurement to the voltage generated across a Rev. C Page 2 of 9 10926-001