This circuit shows how to use the precision analog microcontroller ADuCM360 / ADuCM361 in a precision thermocouple temperature monitoring application . ADuCM360/ADuCM361 integrates dual-channel 24-bit analog-to-digital converter (ADC), dual-channel programmable current source, 12-bit analog-to-analog converter (DAC), 1.2 V internal reference voltage source, ARM Cortex-M3 core, 126 kB Flash memory, 8 kB SRAM, and various digital peripherals such as UART, timers, SPI and I ² C interfaces, etc.

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

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

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

• In the software, a 24-bit Σ-Δ ADC with a gain of 32 times the PGA is configured for the thermocouple and RTD. ADC1 continuously switches between thermocouple signal sampling and RTD voltage signal sampling.
• A programmable excitation current source is used to drive controlled current through the RTD. Dual-channel current sources are configurable from 0 μA to 2 mA. This example uses a 200 μA setting to minimize errors caused by RTD self-heating effects.
• The ADC in the ADuCM360/ADuCM361 has a built-in 1.2V reference voltage source. Its internal reference voltage source is highly accurate and suitable for measuring thermocouple voltages.
• The ADC in the ADuCM360/ADuCM361 has a built-in external voltage reference. It measures RTD resistance; in a ratiometric setting, an external reference resistor (RREF) is connected to the external VREF+ and VREF pins.
• Bias voltage generator (VBIAS). VBIAS is used to set the thermocouple common-mode voltage to AVDD/2.
• ARM Cortex-M3 core. The powerful 32-bit ARM core integrates 126kB flash and 8kBSRAM memory to run user code, configure and control the ADC, handle ADC conversions through the RTD, and control communication over the UART/USB interface.
• The UART is used as a communication interface with the PC host.
• Two external switches are used to force the device into flash boot mode. Keep SD at low level and switch the RESET button at the same time, ADuCM360/ADuCM361 will enter boot mode instead of normal user mode. In boot mode, the internal flash memory can be reprogrammed via the UART interface.

Thermocouples and RTDs produce very small signals, so a PGA is required 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 the ADC can cover the entire temperature range of the thermocouple in bipolar mode and 32x PGA gain setting.

RTD is used to perform cold junction compensation. This circuit uses a platinum 100Ω RTD, model number Enercorp PCS 1.1503.1. It comes 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 ADuCM360/ADuCM361 is implemented through the FT232R UART to USB transceiver, which directly converts USB signals to UART.

In addition to the decoupling shown in Figure 1, the USB cable itself must 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 built on a multilayer printed circuit board (PCB) with a large ground plane. For optimal performance, proper layout, grounding, and decoupling techniques should be used (refer to Tutorial MT-031 - Implementing Data Converter Grounding and Solving the Mysteries of "AGND" and "DGND" , Tutorial MT-101 - —Decoupling techniques , and ADuCM360TCZ evaluation board layout).

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

Code description

The source code link used to test the circuit is in the CN0221 Design Support package: http://www.analog.com/CN0221-DesignSupport

The UART is configured for baud rate 9600, 8 data bits, no polarity, and no flow control. If this circuit is directly connected to the PC, you can use a communication port viewing program such as "HyperTerminal" to view the results sent by the program to the UART, as shown in Figure 3.

Measure the temperature of the thermocouple and RTD to obtain a temperature reading. Convert the RTD temperature to its equivalent thermocouple voltage using a lookup table (see ISE's ITS-90 Type T Thermocouple Table). These two voltages are added to give the absolute temperature value of the thermocouple.

First, V1 is the voltage measured between the two wires of the thermocouple. Using a lookup table, the RTD voltage is measured and converted to a temperature value; this temperature value is then converted to its equivalent thermocouple voltage (V2). V1 and V2 are then added to give the total thermocouple voltage value, which is converted into the final temperature measurement.

Initially, this conversion was based on a simple linear assumption: the thermocouple temperature was 40V/°C. As can be seen from Figure 4, the error caused by such a conversion is only acceptable for a small range of temperatures around 0°C. A better way to calculate the thermocouple temperature is to use a 6th order polynomial for positive temperatures and a 7th order polynomial for negative temperatures. This requires mathematical operations, resulting in increased computation time and codeword size. A suitable compromise is to calculate the corresponding temperatures for a fixed number of voltages and then store these temperatures in an array with values ​​in between using linear interpolation of adjacent points. As can be seen from Figure 5, the error is significantly reduced when using this method. Figure 5 shows the error of the algorithm using ideal thermocouple voltages.

Figure 6 shows the error caused by using ADC1 on the ADuCM360 to measure 52 thermocouple voltages within the full thermocouple operating range. The overall maximum error is <1°C.

Like thermocouples, RTD temperatures can be calculated and implemented using lookup table methods. Note that the polynomial that describes the relationship between RTD temperature and resistance is different from the polynomial that describes the relationship between thermocouples.

For more information on linearization and achieving optimal RTD performance, please refer to application note AN-0970: RTD Interface and Linearization with ADuC706x Microcontrollers .