Circuit Note CN-0376 Devices Connected/Referenced 8-Channel, Low Noise, Low Power, 24-Bit, AD7124-4 Sigma-Delta ADC with PGA and Reference Circuits from the Lab reference designs are engineered and tested for quick and easy system integration to help solve todays ADuM5010 2.5 kV rms, Isolated DC-to-DC Converter analog, mixed-signal, and RF design challenges. For more information and/or support, visit www.analog.com/CN0376. ADuM1441 Micropower Quad-Channel Digital Isolator 36 V, 1 A, Synchronous, Step-Down ADP2441 DC-to-DC Regulator Channel-to-Channel Isolated Temperature Input (Thermocouple/RTD) for PLC/DCS Applications EVALUATION AND DESIGN SUPPORT CIRCUIT FUNCTION AND BENEFITS Circuit Evaluation Boards The circuit shown in Figure 1 provides a dual-channel, channel- CN-0376 Circuit Evaluation Board (EVAL-CN0376-SDPZ) to-channel isolated, thermocouple or RTD input suitable for System Demonstration Platform (EVAL-SDP-CB1Z) programmable logic controllers (PLC) and distributed control Design and Integration Files systems (DCS). The highly integrated design utilizes a low Schematics, Layout Files, Bill of Materials power, 24-bit, - analog-to-digital converter (ADC) with a rich analog and digital feature set that requires no additional signal conditioning ICs. Each channel can accept either a thermocouple or a RTD input. The entire circuit is powered from a standard 24 V bus supply. Each channel measures only 27 mm 50 mm. DIGITAL P2 INTERFACE SDP PMOD DUPLICATE CHANNEL OTHER FIELD SUPPLY +24V GND V REF ADuM5010 REFOUT AVDD AVDD FERRITE ADP2441 V 3.3V DD V V DC-TO-DC ISO DDP AIN0 (IOUT) 10F CONVERTER 1F 10k I I 3k EXC EXC 10F P1 REFIN1(+) V GND 1nF SEL P R 10nF REF FERRITE 1 3.92k 16.9k I 3k 1nF EXC GND IOV ISO DD (4-WIRE) 3k REFIN1() AD7124-4 (3-WIRE) THERMOCOUPLE 3k AIN+ AIN1 1nF 10nF ADuM1441 ADC RTD 3k 1nF V V DD2 DD1 0.1F AIN AIN3 GND GND V 0.1F 2 1 REF 3k EN EN 2 1 AIN4 (IOUT/VBIAS) DIGITAL 3k V INTERFACE REF CS V V OA IA RETURN DIN V V OB IB SDP AIN5 NTC SCLK V V OC IC PMOD 10k OTHER DOUT V V ID OD AIN6 1 AV DGND 5.62k SS 1 0.1% TOLERANCE Figure 1. PLC/DCS Channel-to-Channel Isolated Temperature Input (Simplified Schematic: All Connections and Decoupling Not Shown) Rev. 0 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 Fax: 781.461.3113 2015 Analog Devices, Inc. All rights reserved. to any cause whatsoever connected to the use of any Circuits from the Lab circuits. (Continued on last page) 13011-001CN-0376 Circuit Note Terminal Connections CIRCUIT DESCRIPTION Figure 2 shows the terminal connections for each of the two The AD7124-4 24-bit, - ADC with programmable gain array input channels. These terminals correspond to P1 and P2 in the (PGA) and voltage reference provides the complete set of hardware (see Figure 1). The thermocouple as well as 2-, 3-, or features to implement a flexible input capable of connection to 4-wire RTD connections are shown. either thermocouple or RTD sensors. Features include on-chip I reference, PGA, excitation currents, bias voltage generator, and EXC (4-WIRE) flexible filtering with enhanced 50 Hz and 60 Hz rejection (2-WIRE/3-WIRE) THERMOCOUPLE options. The AD7124-4 is in a small 5 mm 5 mm LFCSP AIN+ package, making it ideal in channel-to-channel isolated designs RTD where space is a premium. It also includes multiple diagnostic AIN (2-WIRE) functions that are available to the user. (3-WIRE/4-WIRE) The ADuM5010 isolated dc-to-dc converter provides 3.3 V RETURN isolated power via integrated isoPower technology. The Figure 2. Terminal Connections (Per Channel) ADuM1441 isolates the serial peripheral interface (SPI) for the Input Filtering AD7124-4. The ADuM1441 micropower isolator consumes As shown in Figure 3, the input common-mode noise filtering is only 4.8 A per channel when idle, resulting in an energy provided by R1, C1 and R2, C2, and has a 50 kHz cutoff frequency, efficient solution. approximately. Differential noise filtering is provided by R1, R2, The ADP2441 36 V, step-down, dc-to-dc regulator accepts an and C3 and has a 2.5 kHz cutoff frequency, approximately. It is industrial standard 24 V supply, with wide tolerance on the particularly important to filter out any interference at the - input voltage. The ADP2441 steps the input voltage down to modulator frequency (307 kHz in full-power mode). It is suggested 3.3 V to power all of the controller-side circuitry. to adjust the cutoff of these filters to meet system bandwidth System Overview requirements, with the cutoff of the common-mode filters being Channel-to-channel isolation is advantageous in automation approximately 10 the cutoff of the differential filter. systems, because faults on a particular input channel have no negative impact on other channels in the system. However, AIN0 (IOUT) 3k channel-to-channel isolated input modules present a significant REFIN1(+) 1nF R 10nF REF design challenge in terms of complexity, space constraints, and 1 3.92k I 3k 1nF EXC system cost. (4-WIRE) 3k REFIN1() (2-WIRE/3-WIRE) THERMOCOUPLE R1 3k Both thermocouple or RTD inputs are commonly used in AIN+ C3 AIN1 C1 1nF industrial automation systems therefore, it is advantageous to 10nF RTD AD7124-4 C2 1nF design a temperature input module that handles both. This AIN3 AIN R2 3k 3k flexibility minimizes the design effort required for the two input AIN4 (IOUT/VBIAS) (2-WIRE) (3-WIRE/4-WIRE) 3k module variants, and also offers flexibility to the module user. V REF R RETURN RETURN The AD7124-4 significantly reduces the design complexity, AIN5 NTC 10k providing a system-on-chip capable of performing all the AIN6 1 necessary measurement functions for both thermocouple and 5.62k RTD sensors. 1 0.1% TOLERANCE Each channel of the circuit in Figure 1 measures only 27 mm by Figure 3. Front-End Filtering and Circuitry (Simplified) 50 mm, and this area can be further reduced by using both sides Input Protection of the printed circuit board (PCB) for populating components. This small size is achieved because the AD7124-4 is in a small To protect the input from an overvoltage condition, 3 k resistors 5 mm 5 mm LFCSP package and integrates almost all the were placed on every input path to the AD7124-4. This resistor required functions except the isolation and additional front-end value limits the current from a 30 V dc overvoltage to less than 10 mA. filtering and protection. The isolation circuit consumes only 2 87 mm for both the data and power isolation together, with a Consider the condition for 30 V connected between AIN+ and minimum combined width of 12.5 mm. AIN. Looking in from AIN+, the 30 V sees R1 (3 k) , followed by internal ESD protection diodes, followed by 3 k looking out from AIN3 in parallel with 3 k looking out from AIN4. Ignoring the internal ESD protection diodes, the total resistance seen between AIN+ and AIN is 3 k + 3 k 3 k = 4.5 k. The current through the AD7124-4 is therefore limited to 30 V 4.5 k = 6.7 mA. Rev. 0 Page 2 of 7 13011-002 13011-003