Circuit Note CN-0382 Devices Connected/Referenced Low Power, 24-Bit, Sigma-Delta ADC with AD7124-4 PGA and Reference AD5421 16-Bit, Loop Powered, 4 mA to 20 mA DAC Circuits from the Lab reference designs are engineered and tested for quick and easy system integration to help solve todays AD5700 Low Power HART Modem analog, mixed-signal, and RF design challenges. For more ADuM1441 SPI Isolator information and/or support, visit www.analog.com/CN0382. Ultralow Quiescent Current, 150 mA, ADP162 CMOS Linear Regulator High Voltage Latch-Up Proof, Triple SPDT ADG5433 Switches Isolated 4 mA to 20 mA/HART Temperature and Pressure Industrial Transmitter Using a Low Power, Precision, 24-Bit, Sigma-Delta ADC 20 mA analog current signal. The HART interface allows EVALUATION AND DESIGN SUPPORT features such as remote calibration, fault interrogation, and Circuit Evaluation Board transmission of process variables, which are necessary in DEMO-AD7124-DZ Evaluation Board applications such as temperature and pressure control. Design and Integration Files Schematics, Layout Files, Bill of Materials, Code Examples The circuit uses the AD7124-4, an ultralow power, precision 24-bit, - analog-to-digital converter (ADC), which includes CIRCUIT FUNCTION AND BENEFITS all the features needed for temperature and pressure systems. The circuit shown in Figure 1 is an isolated smart industrial The circuit also includes the AD5421, a 16-bit, 4 mA to 20 mA, field instrument that interfaces to many types of analog sensors loop powered digital-to-analog converter (DAC) the AD5700, such as temperature (Pt100, Pt1000, and thermocouple) or the industrys lowest power and smallest footprint HART- bridge pressure sensors. The instrument communicates via a compliant IC modem the ADuM1441, which provides ultralow 4 mA to 20 mA analog output and a highway addressable power serial peripheral interface (SPI) isolation the ADG5433 remote transducer (HART) interface. HART is a digital 2-way CMOS switch and the ADP162 low power, 3.3 V regulator in communication in which a 1 mA peak-to-peak frequency shift the isolated power circuitry. keyed (FSK) signal is modulated on top of the standard 4 mA to Rev. A 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 20152016 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) CN-0382 Circuit Note AVDD ISOL DVDD ISOL ADG5433 ADP162A S1B 1000Z 1.5 10 OUT D1 IN S1A 1F 1F 1F EN 10F NC S2B GND D2 S2A IGND GND DGND IGND EP VSS 10 VDD 9V VDD REGOUT 10F 10F DGND EN IN3 IN2 IN1 LOOP+ L* AD5421 REGIN DGND DGND DVDD 3.3V 20M DVDD 3.3V 1M 4700pF VLOOP DVDD DGND LOOP L* 10 DC-DC ENABLE LOOP CIN 100kHz *0.3 AT DC 1k AT 100MHz PRIMARY 0.22F 0.068F IGND AVDD ISOL DVDD ISOL SENSOR SIMULATION DVDD ISOL DVDD 3.3V DGND REFOUT AV IOV DD DD UART V V 0.01F DD1 DD2 AIN0 1M V GND 1k 1 GND RES 0.1F 2 DGND IGND AIN1 DIN V IA V OA 0.01F 68k SCLK V V IB OB MICROCONTROLLER SPI SPI 68k CS V V OC 0.01F IC 1k DVDD 3.3V THERMISTOR AIN2 DOUT/RDY NTC V V OD ID TEMPERATURE 0.1F 10k SENSOR AIN3 EN EN 1 2 0.01F 5.6k GND GND 2 1 DGND AVDD ISOL AD7124-4 0.01F 1k IGND 100 UART RX ADuM1441 AIN6 UART TX UART 0.01F 470 1k IGND DGND PROGRAMMING/ REFIN1(+) DEBUG INTERFACE 5.6k 0.1F REFIN1() J5-1 1k 0.01F J5-2 J5-3 0.01F HART OUT XTAL1 J5-4 AIN4 150k 1k 0.1F ADC IP J5-5 AIN5 1.2M XTAL2 300pF 1k 0.01F J5-6 AD5700 REF 1.2M 150pF 1F J5-7 V CC 0.01F J5-8 AIN7 470 1k DVDD 3.3V 1F DGND PSW AV DGND SS HART VDD 470 470 IGND Figure 1. 4 mA to 20 mA, Loop Powered Field Instrument with HART Interface (Simplified Schematic: All Connections and Decoupling Not Shown) The AD7124-4 can achieve simultaneous 50 Hz and 60 Hz CIRCUIT DESCRIPTION rejection when operating at an output data rate of 25 SPS Analog Front-End Interface (single-cycle settling). The AD7124-4 is a low power, low noise, completely integrated The AD7124-4 establishes the highest degree of signal chain analog front end for high precision measurement applications. integration. The device contains a precision, low noise, low The device contains a low noise, 24-bit, - analog-to-digital drift internal band gap reference, and also accepts an external converter (ADC), and can be configured to have four differential reference, which can be internally buffered. Other differential inputs or seven single-ended or pseudo differential key integrated features include programmable low drift excitation inputs. The on-chip low noise gain stage ensures that signals of current sources, burnout currents, and a bias voltage generator, small amplitude can be interfaced directly to the ADC. which sets the common-mode voltage of a channel to AVDD/2. One of the major advantages of the AD7124-4 is that it gives the The low-side power switch enables the user to power down user the flexibility to employ one of three integrated power bridge sensors between conversions, ensuring the absolute modes. The current consumption, range of output data rates, minimal power consumption of the system. The device also and rms noise can be tailored with the power mode selected. allows the user the option of operating with either an internal The device also offers a multitude of filter options, ensuring that clock or an external clock. the user has the highest degree of flexibility. Rev. A Page 2 of 13 CD TXD RTS SYNC RXD SCLK SDO SDIN FAULT HART OUT HART IN 13355-001