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CN0267 Datasheet(PDF) 2 Page - Analog Devices |
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CN0267 Datasheet(HTML) 2 Page - Analog Devices |
2 / 8 page CN-0267 Circuit Note Rev. B | Page 2 of 8 CIRCUIT DESCRIPTION Analog Front-End Interface The ADuCM360 analog front-end incorporates dual, high performance 24-bit sigma-delta (Σ-Δ) analog-to-digital converters (ADCs). It also integrates programmable gain instrumentation amplifiers, a precision band-gap reference, programmable current sources, a flexible multiplexer, and many other features. It allows a direct interface to multiple analog sensors, such as pressure sensor bridges, resistive temperature sensors, thermocouples, and many other types of sensors used in the industry. The circuit in Figure 1 shows an example connection for a primary bridge type sensor and a secondary resistive temperature sensor; however the ADuCM360 flexible front-end allows many other configurations to accommodate any type of precision analog sensor application. Primary Sensor Input The ADuCM360 on-chip ADC0 measures the field instrument primary sensor, shown as a bridge transducer in Figure 1. The sensor connects to the analog input pins, AIN0 and AIN1, via an RC filter network for improved system electromagnetic immunity. The common-mode filter bandwidth is approximately 16 kHz, and the differential-mode bandwidth is 800 Hz. The ADuCM360 VREF+ and VREF− voltage reference inputs sense the bridge excitation voltage and enable the circuit to work in a ratiometric mode, making the measurement independent of the exact value of the sensor power supply voltage. The on-chip ground switch can dynamically disconnect the bridge excitation and save power when required by the application. Secondary Sensor Input The circuit uses a platinum (Pt) 100 Ω resistive temperature device (RTD) as a secondary sensor. The RTD can sense the temperature of the primary sensor and thus allow for temperature compensation of the primary sensor if required. The ADuCM360 programmable current source supplies the RTD via the AIN4 pin. The ADC1 on the ADuCM360 measures the voltage across the RTD using the AIN3 and AIN2 pins configured as a differential input. The exact value of the current flowing through the RTD is sensed by a precision resistor (RREF) and is measured by the ADC1 using the AIN7 pin. The ADC1 uses the on-chip, band-gap voltage reference. Digital Data Processing, Algorithm, and Communications All the field instrument digital functions are provided by the ADuCM360 32-bit ARM Cortex™ M3 RISC processor, with integrated 128 k bytes of nonvolatile flash/EE memory, 8 k bytes of SRAM, and an 11-channel direct memory access (DMA) controller that supports wired (2× SPI, UART, I²C) communication peripherals. The demonstration software performs the initialization and configuration, processes data from the analog inputs, controls the analog output, and performs the HART communication. Analog Output The AD5421 integrates a low power precision 16-bit DAC with a 4 mA to 20 mA, loop powered output driver and provides all functions required for the field instrument analog output. The AD5421 interfaces with the ADuCM360 controller via the SPI interface. The AD5421 also includes a range of diagnostic functions related to the 4 mA to 20 mA loop. The auxiliary ADC can measure the voltage across the instruments loop terminals via the 20 MΩ/1 MΩ resistive divider connected to the VLOOP pin. The ADC can also measure the chip temperature via the integrated sensor. The ADuCM360 controller can configure and read all the diagnostics of the AD5421, but the AD5421 can also operate autonomously. As an example, if the communication between the controller and the AD5421 fails, the AD5421 automatically sets its analog output to a 3.2 mA alarm current after a defined period. This alarm current indicates to the host that the field instrument failed to operate. The software controls any change of the output current from one value to another to prevent disturbance of the HART communication. (See the Analog Rate of Change section). HART Communication The AD5700 integrates a complete HART FSK modem. The modem is connected to the ADuCM360 controller via a standard UART interface, complemented by request to send (RTS) and carrier detect (CD) signals. The HART output is scaled to the required amplitude by the 0.068 µF/0.22 µF capacitive divider and coupled to the AD5421 CIN pin, where it is combined with the DAC output to drive and modulate the output current. The HART input is coupled from LOOP+ via a simple passive RC filter to the AD5700 ADC_IP pin. The RC filter works as the first stage, band-pass filter for the HART demodulator and also improves the system electromagnetic immunity, which is important for robust applications working in harsh industrial environments. The AD5700 low power oscillator generates the clock for the HART modem with a 3.8664 MHz external crystal connected directly to the XTAL1 and XTAL2 pins. |
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Similar Description - CN0267 |
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