Analog Devices: CN0221 Reference Design for USB-Based Temperature Monitor Using the ADuCM360 Precision Analog Microcontroller and an External Thermocouple

Kit Overview   This circuit uses the ADuCM360ADuCM360/ADuCM361ADuCM361 precision analog microcontroller in an accurate thermocouple temperature monitoring application. The ADuCM360/ADuCM361 integrates dual 24-bit sigma-delta (Σ-Δ) analog-to-digital 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.   In the circuit, the ADuCM360/ADuCM361 is connected to a thermocouple and a 100 Ω platinum resistance temperature detector (RTD). The RTD is used for cold junction compensation.   In the source code, an ADC sampling rate of 4 Hz is chosen. When the ADC input programmable gain amplifier (PGA) is configured for a gain of 32, the noise-free code resolution of the ADuCM360ADuCM360/ADuCM361ADuCM361 is greater than 18 bits.   The ADuCM360 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. The ADuCM361 contains all the features of the ADuCM360 except that only one 24-bit Σ-Δ ADC (ADC1) is available.   Key Applications: Field Instruments/ Smart Transmitters, Sensors and Sensor Interface, Temperature Controllers

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Design Elements

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Kit Features

The following features of the ADuCM360ADuCM360/ADuCM361ADuCM361 are used in this application: 

• A 24-bit Σ-Δ ADC with a PGA set for a gain of 32 in the software for the thermocouple and RTD.
  • The ADC1 was switched continuously between sampling the thermocouple and the RTD voltages.
• Programmable excitation current sources for forcing a controlled current through the RTD.
  • The dual current sources are configurable in from 0 μA to 2 mA. For this example, a 200 μA setting was used to minimize the error introduced by the RTD self-heating.
• An internal 1.2 V reference for the ADC in the ADuCM360/ADuCM361.
  • It measures the thermocouple voltage; the internal voltage reference was used due to its precision.
• An external voltage reference for the ADC in the ADuCM360/ADuCM361.
  • It measures the RTD resistance; a ratiometric setup was used where an external reference resistor (R_REF) was connected across the external VREF+ and VREF− pins.
• A bias voltage generator (VBIAS).
  • The VBIAS function was used to set the thermocouple common-mode voltage to AVDD/2.
• The ARM Cortex-M3 core. T
  • he powerful 32-bit ARM core with integrated 126 kB flash and 8 kB SRAM memory runs the user code that configures and controls the ADC, processes the ADC conversions from the RTD, and controls the communications over the UART/USB interface.
• The UART was used as the communication interface to the host PC.
• 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/ADuCM361 enters boot mode instead of normal user mode. In boot mode, the internal flash can be reprogrammed through the UART interface.

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Kit Contents

The Analog Devices EVAL-CN0221-EB1ZEVAL-CN0221-EB1Z kit is supplied with below contents:

• Evaluation Board

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