How to use ADUCM360 to accurately monitor thermocouple temperature (Part 1)

Circuit Function and Advantages

This circuit shows how to use the ADuCM360/ADuCM361 precision analog microcontroller in a precision thermocouple temperature monitoring application. The ADuCM360/ADuCM361 integrates dual 24-bit Σ-Δ analog-to-digital converters (ADCs), dual programmable current sources, 12-bit digital-to-analog converters (DACs), 1.2 V internal reference, ARM Cortex-M3 core, 126 kB flash, 8 kB SRAM, and various digital peripherals such as UART, timers, SPI, and I2C interfaces.

In this circuit, the ADuCM360/ADuCM361 is connected to a thermocouple and a 100 Ω platinum resistance temperature detector (RTD). The RTD is used to perform cold junction compensation.

In the source code, the ADC sampling rate is selected to be 4 Hz. When the ADC input programmable gain amplifier (PGA) is configured for a gain of 32, the noise-free code resolution of the ADuCM360/ADuCM361 is greater than 18 bits.

Figure 1. ADuCM360/ADuCM361 as a Temperature Monitoring Controller Interfacing with Thermocouples (Simplified Schematic; All Connections Not Shown)

Circuit Description

The following features of the ADuCM360/ADuCM361 are used in this application:

- In software, a 24-bit sigma-delta ADC with a PGA gain of 32 is configured for thermocouples and RTDs. ADC1 continuously switches between sampling the thermocouple signal and the RTD voltage signal.

- Programmable excitation current sources to drive a controlled current through the RTD. The dual-channel current sources can be configured from 0A to 2mA. This example uses the 200A setting to minimize errors due to RTD self-heating.

- The ADC in the ADuCM360/ADuCM361 has a built-in 1.2 V reference voltage source. Its internal reference voltage source has high accuracy and is suitable for measuring thermocouple voltages.

- The ADC in the ADuCM360/ADuCM361 has an internal external voltage reference. It can measure the RTD resistance; using a ratiometric setup, an external reference resistor (RREF) is connected across the external VREF+ and VREF pins.

- Bias voltage generator (VBIAS). VBIAS is used to set the thermocouple common-mode voltage to AVDD/2.

- ARMCortex-M3 core. The powerful 32-bit ARM core integrates 126kB flash and 8kB SRAM memory to run user code, configure and control the ADC, process ADC conversions through RTD, and control communications over the UART/USB interface.

- UART is used as the communication interface with the PC host.

- Two external switches are used to force the device into 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.

The signals generated by thermocouples and RTDs are very small, so a PGA is needed to amplify these signals.

The thermocouple used in this application is type T (copper-constantan), which has a temperature range of −200°C to +350°C. The sensitivity is approximately 40V/°C, which means that the ADC can cover the entire temperature range of the thermocouple in bipolar mode and with a PGA gain setting of 32.

The RTD is used to perform cold junction compensation. This circuit uses a platinum 100Ω RTD, model Enercorp PCS 1.1503.1. It is available in a 0805 surface mount package. The temperature change rate is 0.385Ω/°C.

Note that the reference resistor, RREF, should be a precision 5.6kΩ (±0.1%) resistor.

The USB interface of the ADuCM360/ADuCM361 is implemented through the FT232R UART to USB transceiver, which converts USB signals directly to UART.

In addition to the decoupling shown in Figure 1, the USB cable itself must also use ferrite beads to enhance EMI/RFI protection. The ferrite beads used in this circuit are Taiyo Yuden #BK2125HS102-T, which has an impedance of 1000Ω at 100 MHz.

This circuit must be constructed on a multilayer printed circuit board (PCB) with a large area ground plane. Proper layout, grounding, and decoupling techniques should be used to achieve optimal performance.

The PCB used to evaluate this circuit is shown in Figure 2.

Figure 2. EVAL-ADuCM360TCZ board used in this circuit.