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

CIRCUIT FUNCTION AND BENEFITS
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.

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.

_{Download Full Block Diagram Below}