14-Bit, 4-20 mA, Loop Powered, Thermocouple Temperature Measurement System Using ARM Cortex-M3 (CN0319)

The circuit shown in Figure 1 is a complete loop powered thermocouple temperature measurement system where the 4 mA-to-20 mA output current is controlled using the PWM function of a precision analog microcontroller.


Figure 1. ADuCM360 Controlling 4 mA-to-20 mA Loop-Based Temperature Monitor Circuit. (Simplified Schematic: All Connections and Decoupling Not Shown)

This circuit provides a low cost solution to temperature monitoring because most of the circuit functionality is integrated into the ADuCM360 precision analog microcontroller, including dual 24-bit Σ-Δ ADCs, the ARM Cortex™-M3 processor core, and the PWM/DAC features for controlling the 4 mA-to-20 mA loop for loop voltages up to 28 V.


The ADuCM360 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 temperature value. The Type T thermocouple 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.


This circuit is similar to the circuit in Circuit Note CN-0300 with the additional benefit of the higher resolution of the PWM driving the 4 mA-to-20 mA loop. The PWM-based output provides 14 bits of resolution. For details on the temperature sensor interface to the ADC and on linearization techniques for the RTD measurements, see Circuit Note CN-0300 and the AN-0970 Application Note.

Возможности

  • 4-20mA thermocouple temperature monitor
  • Cold junction compensation
  • Loop powered

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14-Bit, 4-20 mA, Loop Powered, Thermocouple Temperature Measurement System Using ARM Cortex-M3  CN0319

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Производитель: Analog Devices Inc.
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