Analog Devices: CN0300 Reference Design for Complete Closed-Loop Precision Analog Microcontroller Thermocouple Measurement System

Kit Overview   This EVAL-CN0300-EB1ZEVAL-CN0300-EB1Z circuit uses the ADuCM360ADuCM360 precision analog microcontroller in an accurate thermocouple temperature monitoring application and controls the 4 mA to 20 mA output current accordingly. The ADuCM360 integrates dual 24-bit sigma-delta (Σ-Δ) analog-todigital converters (ADCs), dual programmable current sources, a 12-bit digital-to-analog converter (DAC), and a 1.2 V internal reference, as well as an ARM Cortex-M3 core, 126 kB flash, 8 kB SRAM, and various digital peripherals such as UART, timers, SPIs, and I2C interfaces.   The ADuCM360ADuCM360 is a fully integrated, 3.9 kSPS, 24-bit data acquisition system that incorporates dual high performance, multichannel sigma-delta (Σ-Δ) analog-to-digital converters (ADCs), a 32-bit ARM Cortex-M3 processor, and Flash/EE memory on a single chip. The ADuCM360 is designed for direct interfacing to external precision sensors in both wired and battery-powered applications.   In the circuit, the ADuCM360ADuCM360 is connected to a Type T thermocouple and a 100 Ω platinum resistance temperature detector (RTD). The RTD is used for cold junction compensation. The low power Cortex-M3 core converts the ADC readings to a real temperature value. The Type T temperature range supported is −200°C to +350°C, and this temperature range is converted to an output current range of 4 mA to 20 mA.   The circuit provides a complete solution for thermocouple measurements with a minimum requirement for external components, and it is loop powered for loop voltages up to 28 V.   Key Applications: Field Instruments/ Smart Transmitters, Sensors and Sensor Interface, Temperature Controllers

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Kit Features

The following features of the ADuCM360ADuCM360 are used in this application:

• The 12-bit DAC output with its flexible on-chip output buffer is used to control an external NPN transistor, BC548.
  • By controlling the VBE voltage of this transistor, the current passing through a 47 Ω load resistor can be set to the desired value.
• The DAC is 12-bit monotonic; however, the accuracy of the DAC output is typically around 3 LSBs. In addition, the bi-polar transistor introduces linearity errors.
  • To improve the accuracy of the DAC output and to eliminate offset and gain end-point errors, ADC0 measures a feedback voltage reflecting the voltage across the load resistor (RLOAD).
  • Based on this ADC0 reading, the DAC output is corrected by the source code. This provides ±0.5°C accuracy on the 4 mA to 20 mA output.
• The 24-bit Σ-Δ ADC with a PGA set for a gain of 32 in the software for the thermocouple and the RTD. ADC1 switches continuously between sampling the thermocouple and the RTD voltages.
• Programmable excitation current sources force a controlled current through the RTD. The dual current sources are configurable in steps from 0 μA to 2 mA.
• An internal 1.2 V reference is provided for the ADC in the ADuCM360. When measuring the thermocouple voltage, the internal voltage reference is used due to its precision.
• An external voltage reference for the ADC in the ADuCM360. When measuring the RTD resistance, a ratiometric setup was used where an external reference resistor (RREF) was connected across the external VREF+ and VREF− pins.
  • The on-chip reference input buffer is enabled because the reference source in this circuit is high impedance.
  • The on-chip reference buffer means no external buffer is required to minimize input leakage effects.
• A bias voltage generator (VBIAS). The VBIAS function is used to set the thermocouple common-mode voltage to AVDD/2 (900 mV). Again, this removes the need for external resistors to set the thermocouple common-mode voltage.
• The ARM Cortex-M3 core.
  • The powerful 32-bit ARM core with integrated 126 kB flash and 8 kB SRAM memory runs the user code that configures and controls the ADCs and converts the ADC conversions from the thermocouple and RTD inputs to a final temperature value.
  • It also controls the DAC output and continuously monitors this DAC output using the closed-loop feedback from the voltage level on AIN9.
  • For extra debug purposes, it also controls the communications over the UART/USB interface.
• The UART is used as the communication interface to the host PC. This is used to program the on-chip flash. It is also used as a debug port and for calibrating the DAC and ADC.
• Two external switches are used to force the part into its flash boot mode. By holding SD low and toggling the RESET button, the ADuCM360 enters boot mode instead of normal user mode. In boot mode, the internal flash can be reprogrammed through the UART interface.
• The J1 connector, an 8-pin dual-in-line connector, connects to the USB-SWD/UART board that is provided with the CN0300 support hardware. With the J-Link-Lite board, this allows programming and debugging of this application board.

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Kit Contents

The Analog Devices EVAL-CN0300-EB1ZEVAL-CN0300-EB1Z Evaluation Board kit is supplied with below contents:

• Evaluation Board

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