Circuit Note CN-0067 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 AD8220 High CMRR Bipolar In-Amp ADuM5401 Quad-Channel isoPower® Digital Isolator AD8601 Single-Supply Op Amp Fully Isolated Input Module Based on the AD7793 24-Bit Σ-Δ ADC, the ADuM5401 Digital Isolator, and a High Performance In-Amp CIRCUIT FUNCTION AND BENEFITS CIRCUIT DESCRIPTION This circuit provides a complete solution for an industrial control input module. This design is suitable for process control programmable logic controllers (PLCs) and distributed control system (DCS) modules that must digitize standard 4 mA to 20 mA current inputs and unipolar input voltage ranges. The AD8220 in-amp is used to level shift the bipolar signals to provide a 0 V to 5 V input signal to the AD7793 ADC. The ADuM5401 provides all the necessary signal isolation and power between the microcontroller and the ADC. The circuit also includes standard external protection and has been tested and verified to be fully compliant with IEC 61000 specifications. For industrial control modules, analog input voltage and current ranges include 0 V to 5 V, 0 V to 10 V, 4 mA to 20 mA, and 0 mA to 20 mA. A resistor divider (R2-R3) on the input is used to attenuate high voltage inputs so that they match the maximum input range of the AD8220 (−0.1 V to +2.9 V when operating on a single +5 V supply). A good variation of this circuit is to use the AD8226, which has a wider maximum input range (−0.1 V to +4.1V). The AD8220 is used to level shift the input signal and provide gain so that it matches the input range of the AD7793. This circuit is specifically designed for unipolar analog inputs. Other boards have been designed for bipolar operation, which requires bipolar supplies (see Scott Wayne, “iCoupler® Digital Isolators Protect RS-232, RS-485, and CAN Buses in Industrial, Instrumentation, and Computer Applications,” Analog Dialogue, October 2005). ALTERNATE INPUT S3 ISO RG RG S4 R2 200pF 100kΩ S1 AINx(+) 0.1µF ISO 0.1µF 0.1µF 10Ω ISO (0.5V) ISO + 10nF +5VISO AVDD/DVDD CS SCLK AD7793 DIN AINx(–) REFIN(+) REFIN(–) GND DOUT 1kΩ REF S2 1kΩ ISO 10µF ADuM5401 0.1µF 10nF ISO AD8220 RG 51kΩ 470pF R1 ISO 250Ω, 0.1% 25ppm/°C + 10µF 0.1µF 10nF ISO ISO 0.1µF ISO AD8601 0.1µF ISO 0.1µF 0.1µF 10µF 10nF 5V + MBR0530 CS SCLK 5V DOUT 0V +2.5VISO 5.1kΩ ISO VDD1 GND1 VIA VIB VIC VOD RCOUT GND1 ISO 25kΩ ISO VISO GNDISO VOA VOB VOC VID VSEL GNDISO INTERFACE TO MICROCONTROLLER 1nF 0.1µF 10µF + 2.5V ADR441 VOUT + 10µF 0.1µF VIN ISO 0.1µF+ 10µF ISO 08349-001 TVS 20V 1N4148 INPUT 600Ω AT 100MHz FERRITE BEAD 20V TVS: R3 SMBJ20CA 330kΩ 1.5nF +5VISO ISO Figure 1. 24-Bit Isolated Industrial Control Voltage Input Module (Simplified Schematic) Rev. B “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-0067 Circuit Note The AD8220 also has the excellent CMRR needed in these applications. Jumpers (shown here as switches for clarity) are used to switch between current and voltage ranges in hardware and to set the gain of the AD8220. For example, when the input is set to receive a 4 mA-to-20 mA current, the switches are configured to provide a 250 Ω load resistor (R1) on the input, providing a full-scale voltage of 5 V. The AD8220 output is biased with a common-mode signal connected to the REF pin. This reference signal is generated from the same reference as the AD7793, divided down to provide a 0.42 V bias voltage that is buffered using the AD8601 and connected to the REF pin of the AD8220. The REF input to the AD8220 should always be driven with a low impedance, heavily decoupled source. Figure 2 shows a noise plot of the input module when a 2.5 V input signal is connected using the 0 V to +10 V input range. The measured noise on the output is in peak-to-peak resolution. The 19.5 bit peak-to-peak noise-free code resolution means there are 4.5 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 19.5 bits. In terms of contributing to the overall system error, the measured noise of 4.5 bits contributes ~0.00014% error with respect to the fullscale range of the ADC. The integral nonlinearity error of the AD7793 is 0.0015%. 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 that can provide 500 mW of regulated isolated power at either 5 V or 3.3 V. This design uses 5 V provided from the ADuM5401 to supply all the analog circuitry on the input module. Four wires are used: three for transmit (CS, SCLK, and DIN) and one for receive (DOUT), which connect to the standard SPI interface. 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. The circuit shown in Figure 1 draws ~4.8 mA. 08349-002 The ADR441 is the chosen reference for this circuit. The ADR441 has excellent accuracy specifications of 0.04% and drift specifications of 3 ppm/°C maximum. Figure 2. Input Module Histogram, 4.7 Hz Update Rate, Gain =1, Input = 2.5 V Reference 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). 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 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 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. Rev. B | Page 2 of 3 Circuit Note CN-0067 Test Item EN and IEC 61000-4-5 Description Power line surge, ±2 kV EN and IEC 61000-4-6 Immunity test on power cord, 10 V/m for 30 minutes Immunity test on I/O cable, 10 V/m for 30 minutes 1 Result 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. Data Sheets and Evaluation Boards LEARN MORE PLC Demo System. 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, Inc. AD7793 Data Sheet. MT-004 Tutorial, The Good, the Bad, and the Ugly Aspects of ADC Input Noise—Is No Noise Good Noise? Analog Devices, Inc. ADR441 Data Sheet. ADuM5401 Data Sheet. AD8220 Data Sheet. AD8226 Data Sheet. MT-022 Tutorial, ADC Architectures III: Sigma-Delta ADC Basics. Analog Devices, Inc. AD8601 Data Sheet. REVISION HISTORY MT-023 Tutorial, ADC Architectures IV: Sigma-Delta ADC Advanced Concepts and Applications. Analog Devices, Inc. Slattery, Colm, Derrick Hartmann, and Li Ke. “PLC Evaluation Board Simplifies Design of Industrial Process Control Systems.” Analog Dialogue (April 2009). Wayne, Scott. “iCoupler Digital Isolators Protect RS-232, RS-485, and CAN Buses in Industrial, Instrumentation, and Computer Applications.” Analog Dialogue (October 2005). 5/11—Rev. A to Rev. B Changes to Circuit Functions 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 2/10—Rev. 0 to Rev. A Changes to Figure 1 ..........................................................................1 Changes to Circuit Description Section.........................................1 7/09—Revision 0: Initial Version (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. While you may use the "Circuits from the Lab" 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". 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" at any time without notice, but is under no obligation to do so. Trademarks and registered trademarks are the property of their respective owners. ©2009–2011 Analog Devices, Inc. All rights reserved. Trademarks and registered trademarks are the property of their respective owners. CN08349-0-5/11(B) Rev. B | Page 3 of 3