Circuit Note CN-0066 Devices Connected/Referenced Circuit Designs Using Analog Devices Products Apply these product pairings quickly and with confidence. For more information and/or support call 1-800-AnalogD (1-800-262-5643) or visit www.analog.com/circuit. AD7793 24-Bit Σ-Δ ADC ADR441 Precision 2.5 V Reference ADuM5401 Quad-Channel isoPower® Digital Isolator Fully Isolated Input Module Based on the AD7793 24-Bit Σ-Δ ADC and the ADuM5401 Digital Isolator CIRCUIT FUNCTION AND BENEFITS The ADR441 is the chosen reference for this circuit. The ADR441 has excellent accuracy specs of 0.04% and a drift of 3 ppm/°C maximum. This circuit provides a complete solution for a single-supply input circuit design requiring isolation. The AD7793 is a 24-bit Σ-Δ ADC. The ADC includes an on-chip PGA and can, therefore, accept small signal inputs from sensors directly. The PGA gains can be set for 1, 2, 4, 8, 16, 32, 64, or 128. The ADuM5401 provides all the necessary signal isolation and power between the microcontroller and the input. The ADuM5401 is a quad-channel isolator with integrated isoPower technology, based on Analog Devices, Inc., iCoupler® technology. It is used to provide isolation between the field side and the system microcontroller, with an isolation rating of 2.5 kV rms. The ADuM5401 also has an integrated dc-to-dc converter, which can provide 500 mW of regulated isolated power at either 5 V or 3.3 V. This design uses the 5 V provided from the ADuM5401 to supply all the analog circuitry on the input module. All four data wires are used: three for transmit (CS, SCLK, DIN) and one for receive (DOUT), which connects to the standard SPI interface. CIRCUIT DESCRIPTION Sensor outputs are commonly small signal, such as thermocouple or RTD. This design allows these small signal inputs to be directly connected to the AD7793 input, which has an internal PGA with a maximum gain of 128. The design can be used as a complete solution to a non-loop-powered SMART transmitter. 0.1µF 10nF DIFFERENTIAL VOLTAGE INPUT +5VISO VISO GNDISO VOA VOB VOC VID VSEL GNDISO ISO 1kΩ 1kΩ 0.1µF CS SCLK DIN GND DOUT AVDD/DVDD AINx(+) AD7793 AINx(–) REFIN(+) REFIN(–) ISO 10nF 10nF ISO VDD1 GND1 VIA VIB VIC VOD RCOUT GND1 10nF 0.1µF 10µF + 5V MBR0530 CS SCLK DIN DOUT 0V INTERFACE TO MICROCONTROLLER ADuM5401 + 10µF 0.1µF 0.1µF ISO 2.5V ADR441 + 10µF 0.1µF VOUT VIN 0.1µF + 10µF ISO ISO 08348-001 ISO ISO ISO Figure 1. 24-Bit Isolated Single Supply Input Circuit Rev. A “Circuits from the Lab” 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“Circuit from the Lab”. (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 ©2009-2011 Analog Devices, Inc. All rights reserved. CN-0066 Circuit Note The AD7793 is specified for an AVDD of 2.7 V to 5.25 V under normal operation. For loads greater than 10 mA, the output voltage of the ADuM5401 is 4.75 V to 5.25 V, although for lighter loads, it is 4.7 V to 5.4 V. For this reason, the user can either ensure that the isolated circuit draws more than 10 mA during operation or place a diode in series with the AVDD supply of the AD7793 to ensure that the AVDD voltage is within the specification. Figure 2 shows a histogram plot of the AD7793 output performance when a 2.5 reference is connected to its inputs. The measured noise on the output is measured in peak-to-peak resolution. The 20.4-bit peak-to-peak resolution figure means there are 3.6 bits of noise flicker on the output of the ADC with respect to the ADC’s 24-bit resolution. Therefore, the ADC’s noise-free code resolution is 20.4 bits. In terms of contribution to the overall system error, the measured noise of 3.6 bits contributes ~0.00007% error with respect to the full-scale range of the ADC. The integral nonlinearity error of the AD7793 is 0.0015%. 08348-002 This circuit is from a portion of the PLC demo system. The PLC demo system has been successfully tested to the IEC 61000 standards shown in Table 1 (see Colm Slattery, Derrick Hartmann, and Li Ke, “PLC Evaluation Board Simplifies Design of Industrial Process Control Systems,” Analog Dialogue (April 2009) for more discussion of external protection techniques). Figure 2. Input Module Histogram of the AD7793, 512 Samples, 4.17 Hz Data Rate, Gain = 1, Input = 2.5 V Reference Table 1. Conformance to IEC Specifications 1 Test Item EN55022 EN and IEC 61000-4-2 EN and IEC 61000-4-3 EN and IEC 61000-4-4 EN and IEC 61000-4-5 EN and IEC 61000-4-6 1 Description Radiated emission Class A, 3 meter anechoic chamber Electrostatic discharge (ESD) ±8 kV VCD Electrostatic discharge (ESD) ±8 kV HCD Radiated immunity 80 MHz to 1 GHz 18 V/m, vertical antenna polarization Radiated immunity 80 MHz to 1 GHz 18 V/m, horizontal antenna polarization Electrically fast transient (EFT) ±4 kV power port Electrically fast transient (EFT) ±2 kV analog I/O ports Power line surge, ±2 kV Immunity test on power cord, 10 V/m for 30 minutes Immunity test on I/O cable, 10 V/m for 30 minutes Result Passed and met −6 dB margin. Maximum deviations in Input Channel 2, Input Channel 3, and Input Channel 4 are respectively −8 ppm, 10 ppm, and 13 ppm when there is interference. Maximum deviations in Input Channel 2, Input Channel 3, and Input Channel 4 are respectively −8 ppm, 10 ppm, and 13 ppm when there is interference. Maximum deviations in Input Channel 2, Input Channel 3, and Input Channel 4 are respectively 0.05%, 0.004%, and −0.13%. Performance automatically resorted to ≤0.05% after interference. Class B. Maximum deviations in Input Channel 2, Input Channel 3, and Input Channel 4 are respectively −0.09%, 0.003%, and −0.02%. Performance automatically resorted to ≤0.05% after interference. Class B. Passed Class B. Passed Class B. No board or part damage occurred, no performance degrade, passed with Class A. Maximum deviations in Input Channel 2, Input Channel 3, and Input Channel 4 are respectively 9.3%, 11%, and 3.4%. Passed Class B. Maximum deviations in Input Channel 2, Input Channel 3, and Input Channel 4 are respectively 4.5%, 4.7%, and 1.4%. Performance automatically resorted to ≤0.05% when interference stopped. A sample was tested during initial release of the PLC Demo system (V07) and met the test compliances listed in this table. These results should be viewed as typical data taken at 25°C. For these tests, the DAC outputs were connected to the ADC inputs, that is, DAC_CH2 to ADC_CH2, DAC_CH3 to ADC_CH3, and DAC_CH4 to ADC_CH4. The DAC outputs were set to 5 V, 6 V, and 10 mA, respectively. The ADC channels correspond to the circuit as in CN0067. Rev. A | Page 2 of 3 Circuit Note CN-0066 LEARN MORE Data Sheets and Evaluation Boards Cantrell, Mark. AN-0971 Application Note, Recommendations for Control of Radiated Emissions with isoPower Devices. Analog Devices. ADuM5401 Evaluation Board. Chen, Baoxing. 2006. iCoupler Products with isoPower Technology: Signal and Power Transfer Across Isolation Barrier Using Microtransformers. Analog Devices. PLC Demo System. AD7793 Data Sheet. ADR441 Data Sheet. ADuM5401 Data Sheet. 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. Slattery, Colm, Derrick Hartmann, and Li Ke. “PLC Evaluation Board Simplifies Design of Industrial Process Control Systems.” Analog Dialogue (April 2009). REVISION HISTORY 5/11—Rev. 0 to Rev. A Changes to Circuit Function and Benefits Section....................... 1 Changes to Figure 1 .......................................................................... 1 Changes to Circuit Description Section......................................... 2 Changes to Table 1 ............................................................................ 2 Changes to Learn More Section ...................................................... 3 7/09—Revision 0: Initial Version Wayne, Scott. “iCoupler Digital Isolators Protect RS-232, RS-485, and CAN Buses in Industrial, Instrumentation, and Computer Applications.” Analog Dialogue (October 2005). (Continued from first page) "Circuits from the Lab" are intended only for use with Analog Devices products and are the intellectual property of Analog Devices or its licensors. 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All rights reserved. Trademarks and registered trademarks are the property of their respective owners. CN08348-0-5/11(A) Rev. A | Page 3 of 3