The circuit shown in Figure 1 is a fully loop-powered thermocouple-based temperature monitoring system that utilizes the PWM functionality of a precision analog microcontroller to regulate the 4 mA to 20 mA output current. This design is ideal for industrial applications where remote sensing and signal transmission are required over long distances.
Figure 1 illustrates the ADuCM360 controlling a 4 mA to 20 mA loop-based temperature monitoring circuit. Although not all connections and decoupling components are shown, the schematic represents a complete system. The ADuCM360 integrates essential functions such as dual 24-bit Σ-Δ ADCs, an ARM Cortex-M3 processor core, and PWM/DAC capabilities, allowing it to manage loop voltages up to 28 V. This integration results in a cost-effective and compact solution for temperature monitoring.
The ADuCM360 is connected to a T-type thermocouple and a 100 Ω platinum RTD. The RTD serves as a cold junction compensator, ensuring accurate temperature readings. The low-power Cortex-M3 core processes the ADC data and converts it into temperature values. T-type thermocouples can measure temperatures ranging from -200°C to +350°C, which corresponds to the 4 mA to 20 mA current range.
This circuit is similar to the one described in Circuit Note CN-0300 but offers improved performance with higher-resolution PWM control. The PWM-based output provides 14-bit resolution, enhancing the accuracy of the current output. For more details on sensor interfacing, ADC configuration, and RTD linearization techniques, refer to Circuit Note CN-0300 and Application Note AN-0970.
The circuit is powered by the ADP1720 linear regulator, which supplies 3.3 V to power the ADuCM360, the OP193 operational amplifier, and the optional ADR3412 reference voltage source.
The temperature monitoring section closely resembles the design in CN-0300, utilizing several key features of the ADuCM360:
- A 24-bit Σ-Δ ADC with built-in PGA, capable of setting a gain of 32 for both the thermocouple and RTD. ADC1 alternates between sampling the thermocouple and RTD voltages.
- A programmable excitation current source drives the RTD, with two current sources configurable from 0 μA to 2 mA. In this example, a 200 μA setting is used to reduce self-heating errors.
- The ADC includes an internal 1.2 V reference, suitable for precise thermocouple voltage measurements.
- An external reference resistor (RREF) is connected to the VREF+ and VREF- pins for ratiometric RTD measurement. Since the reference voltage source is high impedance, the on-chip reference buffer is enabled to minimize leakage effects.
- A bias voltage generator (VBIAS) sets the thermocouple common-mode voltage to AVDD_REG/2 (900 mV), eliminating the need for external resistors.
- The ARM Cortex-M3 core handles data processing, ADC configuration, and temperature conversion using ADC readings. It also controls the PWM output to drive the 4 mA to 20 mA loop and manages communication via UART/USB for debugging purposes.
This integrated approach ensures a reliable, accurate, and efficient temperature monitoring system suitable for industrial environments.
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