EVAL-CN0221-EB1Z

Circuit Note CN-0221 Devices Connected/Referenced Circuits from the Lab® reference designs are engineered and tested for quick and easy system integration to help solve today’s analog, mixed-signal, and RF design challenges. For more information and/or support, visit www.analog.com/CN0221. ADuCM360/ ADuCM361 Cortex-M3 Based Microcontroller with Dual 24-Bit Σ-Δ ADCs ADP1720-3.3 Low Dropout Linear Regulator USB-Based, Thermocouple Temperature Measurement System Using ARM Cortex-M3 EVALUATION AND DESIGN SUPPORT converter (DAC), and a 1.2 V internal reference, as well as an ARM Cortex-M3 core, 128 kB flash, 8 kB SRAM, and various digital peripherals such as UART, timers, SPIs, and I2C interfaces. Circuit Evaluation Board CN-0221 Evaluation Board (EVAL-ADuCM360TCZ) Design and Integration Files Schematics, Layout Files, Bill of Materials, source code for ADuCM360 In the circuit, the ADuCM360/ADuCM361 is connected to a thermocouple and a 100 Ω platinum resistance temperature detector (RTD). The RTD is used for cold junction compensation. In the source code, an ADC sampling rate of 4 Hz is chosen. 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. CIRCUIT FUNCTION AND BENEFITS This circuit uses the ADuCM360/ADuCM361 precision analog microcontroller in an accurate thermocouple temperature monitoring application. The ADuCM360/ADuCM361 integrates dual 24-bit sigma-delta (Σ-Δ) analog-to-digital converters (ADCs), dual programmable current sources, a 12-bit digital-to-analog 3.3V USB HEADER ADP1720-3.3 BEAD IN 5V BEAD 10Ω OUT GND 4.7µF 4.7µF 10µF 0.1µF FT232R D– D+ RxD AVDD AIN5/IEXC GND SHIELD 0.1µF TxD 0.1µF BEAD IOVDD RESET 10Ω 100Ω PtRTD RESET P2.2/BM AIN0 SD 0.01µF 10Ω AIN1 0.01µF FERRITE BEADS: 1kΩ @ 100MHz TAIYO YUDEN BK2125HS102-T ADuCM360 VREF + RREF 5.6kΩ 0.1% VREF – AIN7/VBIAS TxD RxD AGND 09985-001 J1 P0.1/SIN AIN3 THERMOCOUPLE JUNCTION P0.2/SOUT AIN2 Figure 1. ADuCM360/ADuCM361 as a Temperature Monitor Controller with a Thermocouple Interface (Simplified Schematic, All Connections Not Shown) Rev. C Circuits from the Lab® reference designs from Analog Devices have been designed and built by Analog Devices engineers. Standard engineering practices have been employed in the design and construction of each circuit, and their function and performance have been tested and verified in a lab environment at room temperature. However, you are solely responsible for testing the circuit and determining its suitability and applicability for your use and application. Accordingly, in no event shall Analog Devices be liable for direct, indirect, special, incidental, consequential or punitive damages due toanycausewhatsoeverconnectedtotheuseofanyCircuitsfromtheLabcircuits. (Continuedonlastpage) One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A. Tel: 781.329.4700 www.analog.com Fax: 781.461.3113 ©2012–2014 Analog Devices, Inc. All rights reserved. CN-0221 Circuit Note The following features of the ADuCM360/ADuCM361 are used in this application:         A 24-bit Σ-Δ ADC with a PGA set for a gain of 32 in the software for the thermocouple and RTD. The ADC1 was switched continuously 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 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. An internal 1.2 V reference for the ADC in the ADuCM360/ ADuCM361. It measures the thermocouple voltage; the internal voltage reference was used due to its precision. An external voltage reference for the ADC in the ADuCM360/ADuCM361. It measures the RTD resistance; a ratiometric setup was used where an external reference resistor (RREF) was connected across the external VREF+ and VREF− pins. A bias voltage generator (VBIAS). The VBIAS function was used to set the thermocouple common-mode voltage to AVDD/2. The ARM Cortex-M3 core. The powerful 32-bit ARM core with integrated 128 kB flash and 8 kB 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. 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 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. Note that the reference resistor, RREF, should be a precision 5.6 kΩ (±0.1%). The USB interface to the ADuCM360/ADuCM361 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 must 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. Construct the circuit on a multilayer printed circuit board (PCB) with a large area ground plane. Use proper layout, grounding, and decoupling techniques to achieve optimum performance (see Tutorial MT-031, Grounding Data Converters and Solving the Mystery of "AGND" and "DGND," Tutorial MT-101, Decoupling Techniques, and the ADuCM360TCZ Evaluation Board layout). The PCB used for evaluating this circuit is shown in Figure 2. Both the thermocouple and the RTD generate very small signals; therefore, a PGA is required to amplify those signals. The thermocouple used in this application is a Type T (copperconstantan) that has a temperature range of −200°C to +350°C. Its sensitivity is approximately 40 μV/°C, which means that the ADC in 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, 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. Rev. C | Page 2 of 5 09985-002 CIRCUIT DESCRIPTION Figure 2. EVAL-ADuCM360TCZ Board Used for this Circuit Circuit Note CN-0221 Code Description 20 The link to the source code used to test the circuit can be found in the CN0221 Design Support Package at http://www.analog.com/CN0221-DesignSupport 0 –20 ERROR (°C) The UART is configured for a baud rate of 9600, 8 data bits, no parity, and no flow control. If the circuit is connected directly to a PC, a communication port viewing application, such as a HyperTerminal, can be used to view the results sent by the program to the UART, as shown in Figure 3. –40 –60 –100 –210 –140 –70 0 70 140 210 280 350 TEMPERATURE (°C) 09985-004 –80 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 six-order polynomial for the positive temperatures and a seventh-order polynomial for the negative temperatures. This requires mathematical operations that 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. 0.30 Figure 3. Output of HyperTerminal Communication Port Viewing Application First, the voltage measured between the two wires of the thermocouple (V1). The RTD voltage is measured, converted to a temperature via a look-up table, and then, this temperature is converted to its 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. Rev. C | Page 3 of 5 0.20 ERROR (°C) To get a temperature reading, measure the temperature of the thermocouple and the RTD. The RTD temperature is converted to its equivalent thermocouple voltage via a look-up table (see the ISE, Inc., ITS-90 Table for Type T Thermocouple). These two voltages are added together to give the absolute value at the thermocouple. 0.25 0.15 0.10 0.05 0 –0.05 –210 –140 –70 0 70 140 210 280 350 TEMPERATURE (°C) Figure 5. Error When Using Piecewise Linear Approximation Using 52 Calibration Points and Ideal Measurements 09985-005 09985-003 Figure 4. Error When Using Simple Linear Approximation CN-0221 Circuit Note Figure 6 shows the error obtained when using ADC1 on the ADuCM360 to measure 52 thermocouple voltages over the full thermocouple operating range. The overall worst-case error is