CN0214

Application Note AN-0970, RTD Interfacing and Linearization Using an ADuC706x Microcontroller The following features of the ADuC7060/ADuC7061 are used in this application:

• Primary 24-bit Σ-Δ ADC with programmable gain amplifier (PGA): The PGA is set for a gain of 32 in the software for this application. The primary ADC was continuously switched between sampling the thermocouple and the RTD voltages.
• Programmable excitation current sources for forcing a controlled current through the RTD: The dual current sources are configurable in 200 μA steps from 0 μA to 2 mA. For this example, a 200 μA setting was used to minimize the error introduced by the RTD self-heating.
• Internal 1.2 V reference for the ADC in the ADuC7060/ADuC7061: For measuring the thermocouple voltage the internal voltage reference was used due to its precision.
• External voltage reference for the ADC in the ADuC7060/ADuC7061: For measuring the RTD resistance, we used a ratiometric setup where an external reference resistor (R_REF) is connected across the external V_REF+ and V_REF− pins.
• 14-bit DAC: The DAC was used to set the thermocouple common-mode voltage to 850 mV above ground.
• ARM7TDMI^® core: The powerful 16-/32-bit ARM7 core with integrated 32 kB flash and SRAM memory runs the user code that configures and controls the ADC, processes the ADC conversions from the RTD, and controls the communications over the UART/USB interface.
• UART: The UART was used as the communication interface to the host PC.
• Two external switches are used to force the part into its flash boot mode: By holding S1 low and toggling S2, the ADuC7060/ADuC7061 will enter boot mode instead of normal user mode. In boot mode, the internal flash may be reprogrammed through the UART interface.

Both the thermocouple and the RTD generate very small signals; therefore, a PGA is required to amplify those signals. As the auxiliary ADC on the ADuC7060/ADuC7061 does not have a PGA, both were connected to the primary ADC, and switching between the two was done in software.

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

The RTD was used for cold junction compensation. The particular one used in this circuit was a platinum 100 Ω RTD, model number Enercorp PCS 1.1503.1. It is available in a 0805 surface-mount package. This RTD has a temperature variation of 0.385 Ω/°C.

Note that the reference resistor, R_REF, should be a precision 5.62 kΩ (±0.1%).

The USB interface to the ADuC7060/ADuC7061 is implemented with an FT232R UART to USB transceiver, which converts USB signals directly to the UART.

In addition to the decoupling shown Figure 1, the USB cable itself should have a ferrite bead for added EMI/RFI protection. The ferrite beads used in the circuit were Taiyo Yuden, #BK2125HS102-T, which have an impedance of 1000 Ω at 100 MHz.

The circuit must be constructed on a multilayer PC board with a large area ground plane. Proper layout, grounding, and decoupling techniques must be used to achieve optimum performance (see Tutorial MT-031, Grounding Data Convertersand Solving the Mystery of "AGND" and "DGND," Tutorial MT-101, Decoupling Techniques, and the ADuC7060/ADuC7061 Evaluation Board layout).

The EVAL-ADUC7061MKZ PCB is shown in Figure 2.

Code Description

The source code used to test the circuit can be downloaded as a zip file from http://www.analog.com/CN0214-SourceCode.

The UART is configured for a baud rate of 9600, 8 data bits, no parity, no flow control. If the circuit is connected directly to a PC, a communication port viewing application such as HyperTerminal can be used to view the results sent by the program to the UART, as shown in Figure 3.

In order to get a temperature reading, the temperature of the thermocouple and of the RTD must be measured. The RTD temperature is converted to its equivalent thermocouple voltage via a lookup table. These two voltages are added together to give the absolute value at the thermocouple.

First, the voltage measured between the two wires of the thermocouple (V1). The RTD voltage is measured, converted to a temperature via a lookup table and this temperature is then converted to it’s equivalent thermocouple voltage (V2). V1 and V2 are then added to give the overall thermocouple voltage, and this is then converted to the final temperature measurement.

Initially, this was done using a simple linear assumption that the voltage on the thermocouple was 40 μV/°C. It can be seen from Figure 4. that this gives an acceptable error only for a small range around 0°C. A better way of calculating the thermocouple temperatures is to use a 6th order polynomial for the positive temperatures and a 7th order polynomial for the negative temperatures. This requires mathematical operations, which add to computational time and code size. A suitable compromise is to calculate the respective temperatures for a fixed number of voltages. These temperatures are stored in an array, and values in between are calculated using a linear interpolation between the adjacent points. It can be seen from Figure 5 that the error is drastically reduced using this method. Figure 5 gives the algorithm error using ideal thermocouple voltages. Figure 6 shows the error obtained when using ADC0 on the ADuC7060 to measure 52 thermocouple voltages over the full thermocouple operating range. The overall worst case error is <1°C.

The RTD temperature is calculated using lookup tables and is implemented for the RTD the same way as for the thermocouple. Note that the RTD has a different polynomial describing its temperatures as a function of resistance.

For details on linearization and maximizing the performance of the RTD, refer to Application Note AN-0970, RTD Interfacing and Linearization Using an ADuC706x Microcontroller.