Circuit Note CN-0300 Devices Connected/Referenced Circuits from the Lab reference designs are engineered and Cortex-M3 Based Microcontroller with tested for quick and easy system integration to help solve todays ADuCM360 Dual 24-Bit - ADCs analog, mixed-signal, and RF design challenges. For more information and/or support, visit www.analog.com/CN0300. ADP1720-3.3 Low Dropout Linear Regulator 12-Bit, 4-20mA Loop-Powered Thermocouple Measurement System Using ARM Cortex-M3 a 12-bit digital-to-analog converter (DAC), and a 1.2 V internal EVALUATION AND DESIGN SUPPORT reference, as well as an ARM Cortex-M3 core, 126 kB flash, 8 kB Circuit Evaluation Board SRAM, and various digital peripherals such as UART, timers, CN-0300 Evaluation Board (EVAL-CN0300-EB1Z) includes 2 SPIs, and I C interfaces. Analog Devices J-Link OB emulator (USB-SWD/UART- EMUZ) In the circuit, the ADuCM360 is connected to a Type T Design and Integration Files thermocouple and a 100 platinum resistance temperature Schematics, Layout Files, Bill of Materials, source code for detector (RTD). The RTD is used for cold junction compensation. ADuCM360 The low power Cortex-M3 core converts the ADC readings to a real temperature value. The Type T temperature range supported is CIRCUIT FUNCTION AND BENEFITS 200C to +350C, and this temperature range is converted to This circuit uses the ADuCM360 precision analog microcontroller an output current range of 4 mA to 20 mA. in an accurate thermocouple temperature monitoring application The circuit provides a complete solution for thermocouple and controls the 4 mA to 20 mA output current accordingly. The measurements with a minimum requirement for external ADuCM360 integrates dual 24-bit sigma-delta (-) analog-to- components, and it is loop powered for loop voltages up to 28 V. digital converters (ADCs), dual programmable current sources, 3.3V ADP1720-3.3 VLOOP 1.6 IN OUT 10F 10F GND 10F FERRITE BEAD 600 AT 100MHz MURATA BLM31AJ601SN1L CURRENT 0.1F 0.1F METER INTERFACE BOARD CONNECTOR VLOOP+ AVDD IOVDD RESET RESET IEXC GND NPN DAC 10 BC548 VLOOP SWDIO SWDIO 100 PtRTD SOUT ADC0 RLOOP 47 0.01F SWCLK SWCLK 10 SIN 100k ADC1 NC AIN9 0.01F ADuCM360 100k IOVDD IOVDD AIN8 VREF+ 5.6k R REF 0.1% RESET VREF RESET 10k AIN2 SD P2.2/BM 10nF 10k AIN3 THERMOCOUPLE 10nF DVDD REG JUNCTION 0.47F AIN7/VBIAS AGND Figure 1. ADuCM360 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 One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A. determining its suitability and applicability for your use and application. Accordingly, in no event shall Tel: 781.329.4700 www.analog.com Analog Devices be liable for direct, indirect, special, incidental, consequential or punitive damages due Fax: 781.461.3113 20122013 Analog Devices, Inc. All rights reserved. to any cause whatsoever connected to the use of any Circuits from the Lab circuits. (Continued on last page) P0.2/SOUT P0.1/SIN 10955-001CN-0300 Circuit Note CIRCUIT DESCRIPTION The UART is used as the communication interface to the host PC. This is used to program the on-chip flash. It is also used as The following features of the ADuCM360 are used in this a debug port and for calibrating the DAC and ADC. application: Two external switches are used to force the part into its flash The 12-bit DAC output with its flexible on-chip output boot mode. By holding SD low and toggling the RESET button, buffer is used to control an external NPN transistor, BC548. the ADuCM360 enters boot mode instead of normal user By controlling the V voltage of this transistor, the current BE mode. In boot mode, the internal flash can be reprogrammed passing through a 47 load resistor can be set to the through the UART interface. desired value. When NPN mode is selected, the buffered The J1 connector, an 8-pin dual-in-line connector, connects to on chip 1.2 V reference voltage is present on AIN8. the Analog Devices J-Link OB emulator that is provided with The DAC is 12-bit monotonic however, the accuracy of the CN-0300 support hardware. This allows programming the DAC output is typically around 3 LSBs. In addition, the and debugging of this application board. See Figure 3. bi-polar transistor introduces linearity errors. To improve Both the thermocouple and the RTD generate very small signals the accuracy of the DAC output and to eliminate offset and therefore, a programmable gain amplifier (PGA) is required to gain end-point errors, ADC0 measures, on AIN9, a amplify those signals. feedback voltage reflecting the voltage across the load resistor (R ). Based on this ADC0 reading, the DAC LOAD The thermocouple used in this application is a Type T (copper- output is corrected by the source code. This provides constantan) that has a temperature range of 200C to +350C. 0.5C accuracy on the 4 mA to 20 mA output. Its sensitivity is approximately 40 V/C, which means that the The 24-bit - ADC with a PGA set for a gain of 32 in the ADC in bipolar mode, with a PGA gain of 32, can cover the software for the thermocouple and the RTD. ADC1 switches entire temperature range of the thermocouple. continuously between sampling the thermocouple and the The RTD was used for cold junction compensation. The particular RTD voltages. one used in this circuit was a platinum 100 RTD, Enercorp Programmable excitation current sources force a controlled PCS 1.1503.1. It is available in a 0805, surface-mount package. current through the RTD. The dual current sources are This RTD has a temperature variation of 0.385 /C. configurable in steps from 0 A to 2 mA. For this example, Note that the reference resistor, RREF, must be a precision 5.6 k a 200 A setting is used to minimize the error introduced (0.1%). by the RTD self-heating. An internal 1.2 V reference is provided for the ADC in the Construct the circuit on a multilayer printed circuit board (PCB) with a large area ground plane. Use proper layout, grounding, ADuCM360. When measuring the thermocouple voltage, and decoupling techniques to achieve optimum performance (see the internal voltage reference is used due to its precision. Tutorial MT-031, Grounding Data Converters and Solving the An external voltage reference for the ADC in the ADuCM360. Mystery ofAGN andDGND Tutorial MT-101, Decoupling When measuring the RTD resistance, a ratiometric setup Techniques, and the ADuCM360TCZ Evaluation Board layout). was used where an external reference resistor (RREF) was connected across the external VREF+ and VREF pins. The The PCB used for evaluating this circuit is shown in Figure 2. on-chip reference input buffer is enabled because the reference source in this circuit is high impedance. The on-chip reference buffer means no external buffer is required to minimize input leakage effects. A bias voltage generator (VBIAS). The VBIAS function is used to set the thermocouple common-mode voltage to AVDD Reg/2 (900 mV). Again, this removes the need for external resistors to set the thermocouple common-mode voltage. The ARM Cortex-M3 core. The powerful 32-bit ARM core with integrated 126 kB flash and 8 kB SRAM memory runs Figure 2. EVAL-CN0300-EB1Z Board Used for this Circuit the user code that configures and controls the ADCs and converts the ADC conversions from the thermocouple and RTD inputs to a final temperature value. It also controls the DAC output and continuously monitors this DAC output using the closed-loop feedback from the voltage level on AIN9. For extra debug purposes, it also controls the communications over the UART/USB interface. Rev. C Page 2 of 7 10955-002