Circuit Note CN-0209 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.com/CN0209. Devices Connected/Referenced ADP1720 High Voltage, Low Dropout Linear Regulator ADG442 LC2MOS Quad SPST Switches AD8275 G = 0.2, Level Translation ADC Driver AD8676 Dual, Rail-to-Rail Output Op Amp REF194 Precision, 4.5 V Voltage Reference AD7193 24-Bit Σ-Δ ADC AD8617 Low Noise, Rail-to-Rail Input/Output Amp ADuM1400 Quad-Channel Digital Isolator ADT7310 16-Bit Digital Temperature Sensor ADuM1401 Quad-Channel Digital Isolator ADG1414 Serially-Controlled Octal SPST Switches Fully Programmable Universal Analog Front End for Process Control Applications EVALUATION AND DESIGN SUPPORT Circuit Evaluation Boards CN-0209 Circuit Evaluation Board (EVAL-CN0209-SDPZ) System Demonstration Platform (EVAL-SDP-CB1Z) Design and Integration Files Schematics, Layout Files, Bill of Materials CIRCUIT FUNCTION AND BENEFITS The circuit shown in Figure 1 provides a fully programmable universal analog front end (AFE) for process control applications. The following inputs are supported: 2-, 3-, and 4wire RTD configurations, thermocouple inputs with cold junction compensation, unipolar and bipolar input voltages, and 4 mA-to-20 mA inputs. Today, many analog input modules use wire links (jumpers) to configure the customer input requirements. This requires time, knowledge, and manual intervention to configure and reconfigure the input. This circuit provides a software controllable switch to configure the modes along with a constant current source to excite the RTD. The circuit is also reconfigurable to set common-mode voltages for the thermocouple configuration. A differential amplifier is used to condition the analog input voltage range to the Σ-Δ ADC. The circuit provides industry-leading performance and cost. Because of the voltage gain provided by the AD8676 and AD8275, the design is particularly suitable for small signal inputs, all types of RTDs, or thermocouples. The AD7193 is a 24-bit Σ-Δ ADC that can be configured to have four differential inputs or eight pseudo differential inputs. The ADuM1400 and ADuM1401 provide all the necessary signal isolation between the microcontroller and the ADC. The circuit also includes standard external protection and is compliant with the IEC 61000 specifications. CIRCUIT DESCRIPTION This circuit provides a fully programmable universal analog front end (AFE) for process control applications supporting 2-, 3-, and 4-wire RTD configurations, thermocouple inputs with cold junction compensation, unipolar and bipolar input voltages, and 4 mA-to-20 mA inputs as shown in the configuration diagram of Figure 2. The ADG1414, a serially controlled octal SPST switch, is used to configure the selected measurement mode. Voltage Measurement This circuit supports the measurement of unipolar and bipolar signal ranges up to ±10 V. The input signal goes through a signal conditioning stage before conversion by the AD7193 ADC. The AD8676 amplifier buffers the inputs before the gain stage. The AD8275 is used to level shift the input signal and provides gain so that it matches the input range of the AD7193. The AD8275 output is biased with a common-mode voltage connected to its REF pin. This voltage is generated by the REF194 precision 4.5 V reference. RTD Measurement As shown in the connection table, this circuit supports 2-, 3-, and 4-wire RTD configurations. In this case, the transducer is a 1000 Ω platinum (Pt) RTD (resistive temperature device). The most accurate arrangement is a 4-lead RTD configuration. In the application shown, an external 200 µA current source provides the excitation current for the RTD, and the AD7193 is operated at gain of 16 to maximize the dynamic range in the circuit. The AD8617 amplifier is configured as a current source when the RTD measurement mode is selected. It is reconfigured in closed-loop to set the common-mode voltage when the thermocouple measurement is selected. The AD8617 is a dual low noise amplifier so that it can drive both input channels 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 suitability and applicability for your use and application. Accordingly, in no event shall 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) 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. CHANNE L 1 CHANNE L 2 J2-3 J2-2 J2-4 J2-1 J1-3 J1-2 J1-1 J1-4 DIN Rev. A | Page 2 of 5 P3 P2 ADG1414 SCLK SUPPLY +15V SCLK IN4 D4 IN3 D3 IN2 D2 IN1 D1 -15V -15V SYNC GND2 S4 S3 S2 S1 GND2 D8 S8 ADG442 D7 S7 DIN D6 S6 D5 D4 S5 D3 S4 D2 S2 S3 D1 S1 ADG1414 AVDD -15 AGND AIN2_0 AIN1_0 GND2 10uF 10uF GND2 -INB +INB +INA -INA GND2 -15V -V AD8676 +V OUTB OUTA GND2 1/2 AD8617 +15V +2.25V +5V GND2 ADP1720 V IN VOUT 1/2 AD8617 +15V +2.25V -15V 0.1uF 0.1uF 11.5k 11.5k 0.1uF GND2 REF1 REF2 +4.5V +5V J2-6 J2-5 J1-6 J1-5 GND2 +5V 10uF CURRENT CHANNE L 1&2 AVDD +15 P2 P2 4-20_2 4-20_1 AD8275 10uF AD7193 DIN DOUT SCLK DOUT/RDY CS DIN SCLK DVDD ADT7310 +2.25V 0.1uF +5V GND2 1k 1k AGND BPDSW DGND GND2 AIN8 AIN7 AIN6 AIN5 AIN4 AIN3 AIN2 AIN1 P3 AIN2_0 GND2 0.1uF REFIN1+ REFIN1- AVDD +4.5V 4-20_2 4-20_1 P3 200 200 GND2 REF194 +4.5V VOUT AIN1_0 V IN INT1 INT2 DOUT AD7193 DOUT AD7193 DIN SCLK SCLK DIN ADG1414 ADT7310 DIN CS DOUT 0.1uF GND2 +5V 10k Vdd2 Gnd2 Voa Vob Voc Vid Ve2 Gnd2 ADuM1401 Vdd1 Gnd1 Via Vib Vic Vod Ve1 Gnd1 10k Vdd1 Gnd1 Via Vib Vic Vid Ve1 Gnd1 ADuM1400 Vdd2 Gnd2 Voa Vob Voc Vod Ve2 Gnd2 GND2 GND INT CT GND2 ADT7310 SCLK VDD +5V GND1 +3.3V INT2 INT1 INT1_I INT2_I DOUT_I AD7193_I SCLK_I DIN_I ADG1414_I ADT7310_I CN-0209 Circuit Note Figure 1. Universal Programmable Analog Front End for Process Control Applications (Simplified Schematic: All Connections and Decoupling Not Shown) 09839-001 INTERFACE MICROCONTROLLER Circuit Note CN-0209 CONNECTION TABLE RTD 2 WIRES RTD 3 WIRES RTD JX-1 JX-2 RTD 4 WIRES RTD JX-3 JX-4 TC JX-1 JX-2 RTD JX-3 JX-4 DIFFERENTIAL INPUT VOLTAGE TC Va Vb JX-1 JX-2 JX-3 JX-4 SINGLE INPUT VOLTAGE V Uab = Va–Vb JX-1 JX-2 JX-3 JX-4 JX-2 JX-3 JX-2 JX-3 CURRENT CURRENT INPUIT JX-6 09839-002 JX-5 Figure 2. Analog Input Configuration Table available on the board. The resistor configuring the current source must have a low temperature coefficient to avoid drift errors in the measurement circuit. The sense resistor must have a low temperature coefficient to avoid temperature drift errors in the measurement circuit. Thermocouple Measurement In a thermocouple application, the voltage generated by the thermocouple is measured with respect to an absolute reference, provided externally to the ADC. The cold junction compensation is implemented using the ADT7310 16-bit temperature sensor. Because the signal from the thermocouple is small, and to maximize the dynamic range in the circuit, the AD7193 is operated at its highest gain range of 128. Because the input channel is buffered, large decoupling capacitors can be placed on the front end, if required, to eliminate any noise pickup that may be present in the thermocouple leads. The common-mode voltage for the thermocouple measurement is provided by the AD8617 amplifier. Regulator and Reference Selection 4 mA-to-20 mA Current Measurement This circuit also supports 4 mA-to-20 mA current measurement. The current is converted to a voltage using an on-board sense resistor. To use the full dynamic range of the ADC in the current measurement mode, a 200 Ω resistor is used. The ADP1720 was chosen as the 5 V regulator for this circuit. The ADP1720 is a high voltage micropower linear regulator particularly suitable for industrial application. The 4.5 V REF194 was chosen as the reference for this circuit, and the E-grade device has an initial accuracy ±2 mV at 25°C and a drift of 5 ppm/°C maximum. It is a low dropout device and consumes less than 45 μA, with performance specifications over −40°C to +125°C. Isolation The ADuM1400 and the ADuM1401 are quad-channel digital isolators based on Analog Devices’ iCoupler® technology. These are used to provide isolation between the field side and the system microcontroller, with an isolation rating of 2.5 kV rms. Four wires are used through the ADuM1400, all for transmit (SCLK, DIN, ADG1414, ADT7310). Four wires are used through the ADuM1401: one for transmit (AD7193) and three for receive (INT1, INT2, DOUT). The DIN, DOUT, and SCLK lines are connected to the SPORT interface. Rev. A | Page 3 of 5 CN-0209 Circuit Note Table 1. Measured Performance Based on 1000 Samples Input ±10V Input Range 4 mA to 20 mA Range RTD Thermocouple AD7193 Configuration Gain = 1; 50 Hz and 60 Hz rejection; output data rate = 50 Hz Gain = 1; 50 Hz and 60 Hz rejection; output data rate = 2.63 Hz Gain = 16; 50 Hz and 60 Hz rejection; output data rate = 2.63 Hz Gain = 128; 50 Hz and 60 Hz rejection; output data rate = 2.63 Hz Figure 3 shows a histogram plot of the AD7193 output performance when configured in bipolar input mode with the inputs connected to ground. This histogram shows the effects of input-referred noise. The effective resolution achieved in this mode is 19.2 bits. Table 1 shows the performance for other operating modes based on 1000 data samples from the ADC. This design also includes external protection such as standard protection diodes and transient voltage suppressors (TVS devices) to enhance the robustness of the circuit. Refer to the schematics and other resources in the CN0209 Design Support Package: www.analog.com/CN0209-DesignSupport. 200 NUMBER OF OCCURRENCES 180 160 140 RMS Noise (nV) 7940 931 243 220 Effective Resolution (Bits) 19.15 22.24 20.29 19.23 Equipment Needed • PC with a USB port and Windows® XP or Windows Vista® (32-bit) or Windows® 7 (32-bit) • EVAL-CN0209-SDPZ Circuit Evaluation Board • EVAL-SDP-CB1Z SDP Evaluation Board • CN0209 Evaluation Software • Power supply: +15 V and –15 V • RTD temperature sensor • Thermocouple Getting Started Load the evaluation software by placing the CN0209 Evaluation Software disc in the CD drive of the PC. Using "My Computer," locate the drive that contains the evaluation software disc and open the Readme file. Follow the instructions contained in the Readme file for installing and using the evaluation software. Functional Block Diagram See Figure 1 of this circuit note for the circuit block diagram, and the file “EVAL-CN0209-SDPZ-SCH-Rev0.pdf” for the circuit schematics. This file is contained in the CN0209 Design Support Package: www.analog.com/CN0209-DesignSupport. 120 100 80 60 40 0 –0.01699 –0.01698 –0.01697 –0.01696 –0.01695 –0.01694 –0.01693 AMPLITUDE (V) 09839-003 20 Figure 3. Noise Distribution Histogram,1000 Samples AD7193 50 Hz Data Rate, Gain = 1, Input = 4.5 V Reference CIRCUIT EVALUATION AND TEST This circuit uses the EVAL-CN0209-SDPZ circuit board and the EVAL-SDP-CB1Z System Demonstration Platform (SDP) evaluation board. The two boards have 120-pin mating connectors, allowing for the quick setup and evaluation of the circuit’s performance. The EVAL-CN0209-SDPZ board contains the circuit to be evaluated, as described in this note, and the SDP evaluation board is used with the CN0209 evaluation software to capture the data from the EVAL-CN0209-SDPZ circuit board. Setup Connect the 120-pin connector on the EVAL-CN0209-SDPZ circuit board to the connector marked “CON A” on the EVAL-SDP-CB1Z evaluation (SDP) board. Nylon hardware should be used to firmly secure the two boards, using the holes provided at the ends of the 120-pin connectors. With power to the supply off, connect a +15 V power supply to the pin marked “+15 V,” a −15V power supply to the pin marked “−15 V”and “GND” on the board. Connect the USB cable supplied with the SDP board to the USB port on the PC. Note: Do not connect the USB cable to the mini USB connector on the SDP board at this time. Test Apply power to the ±15 V supply connected to the EVAL-CN0209-SDPZ circuit board. Launch the evaluation software and connect the USB cable from the PC to the USB mini-connector on the SDP board. Rev. A | Page 4 of 5 Circuit Note CN-0209 Once USB communications are established, the SDP board can now be used to send, receive, and capture serial data from the EVAL-CN0209-SDPZ board. Voltage Measurement If you want to measure the noise of the voltage measurement circuit, connect both inputs J3 and J4 to the ground. Then, click on the button of the matching channel of the software: either V1 (if you are using channel 1) or V2 (if you are using channel 2). If you want to measure a voltage, connect both inputs J3 and J4 as shown in Figure 2, the analog input configuration table. Then, click on the matching button of the software as previously explained. Results are displayed as a waveform and a histogram. You have the option to select the scale of the voltage result between µV, mV, and V using the switching button. RTD Measurement If you want to measure the temperature through an RTD temperature sensor, connect inputs J1, J2, J3, and J4 as shown in Figure 2. There are three different configurations of connection as you are using RTD 2-, 3-, or 4-wire. Then, click on the matching button of the software (RTD1 for channel 1, RTD2 for channel 2). The switching button above the waveform allows you to display the result in Fahrenheit, Celsius, or Kelvin. LEARN MORE 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. MT-101 Tutorial, Decoupling Techniques, Analog Devices. Sensor Signal Conditioning, Analog Devices, 1999, Section 7. Data Sheets and Evaluation Boards ADuM1400 Data Sheet ADuM1401 Data Sheet AD7193 Data Sheet AD7193 Evaluation Board AD8275 Data Sheet REF194 Data Sheet AD8676 Data Sheet AD8617 Data Sheet ADG442 Data Sheet Thermocouple Measurement ADG1414 Data Sheet If you want to measure the temperature through a thermocouple, connect inputs J1, J2, J3, and J4 as shown in Figure 2. Select the type of thermocouple you are using (B, E, J, K, R, S, T, N). Then, click on the TC button of the software (TC1 for channel 1, TC 2 for channel 2). ADT7310 Data Sheet REVISION HISTORY The switching button above the waveform allows you to display the result in Fahrenheit, Celsius, or Kelvin. 4/11—Revision 0: Initial Version 11/11—Rev. 0 to Rev. A Changes to Figure 2 .......................................................................... 3 Current Measurement If you want to measure a current, connect both inputs J5 and J6, as shown in Figure 2. Then, click on the matching button (I1 for channel 1, I2 for channel 2). You can select the scale of the current result between µA, mA, and A using the switching button. Information regarding the SDP board can be found in the SDP User Guide. (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 infringements 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 registered trademarks are the property of their respective owners. CN09839-0-11/11(A) Rev. A | Page 5 of 5