Circuit Note CN-0319 Devices Connected/Referenced Precision Analog Microcontroller with ADuCM360 Dual - ADCs, ARM Cortex-M3 Precision Analog Microcontroller with ADuCM361 Circuits from the Lab reference designs are engineered and Single - ADC, ARM Cortex-M3 tested for quick and easy system integration to help solve todays 50 mA, High Voltage, Micropower analog, mixed-signal, and RF design challenges. For more ADP1720 Linear Regulator information and/or support, visit www.analog.com/CN0319. Precision, Micropower Single OP193 Operational Amplifier Micro-Power, High-Accuracy 1.2 V ADR3412 Voltage Reference 14-Bit, 4-20 mA, Loop Powered, Thermocouple Temperature Measurement System Using ARM Cortex-M3 EVALUATION AND DESIGN SUPPORT CIRCUIT FUNCTION AND BENEFITS Circuit Evaluation Boards The circuit shown in Figure 1 is a complete loop powered CN-0319 Circuit Evaluation Board (EVAL-CN0319-EB1Z) thermocouple temperature measurement system where the 4 mA- Design and Integration Files to-20 mA output current is controlled using the PWM function of Schematics, Layout Files, Bill of Materials a precision analog microcontroller. 3.3V ADP1720-3.3 BEAD OUT IN 4V TO 28V 1. 6 10F 10F 10F GND 3.3V 0.1F 0.1F ADR3412 IOVDD AVDD OPTIONAL IEXC DAC V = 1.2V 10 REF RTD AIN0 0.01F R1 ADuCM360 100k 10 AIN1 0.01F DVDD 47k 47k V V + IN REF R REF PWM VLOOP+ 5. 62k BC548 (0.1%) OP193 V REF V 10k 0.1F 0.1F R12 10k AIN2 THERMOCOUPLE DGND 10nF R JUNCTION LOOP V RLOOP 47. 5 10k VLOOP AIN3 R2 10nF 100k AIN7 AGND Figure 1. ADuCM360 Controlling 4 mA-to-20 mA Loop-Based Temperature Monitor Circuit. (Simplified Schematic: All Connections and Decoupling 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 to any cause whatsoever connected to the use of any Circuits from the Lab circuits. (Continued on last page) Fax: 781.461.3113 20132014 Analog Devices, Inc. All rights reserved. 11386-001CN-0319 Circuit Note This circuit provides a low cost solution to temperature monitoring A bias voltage generator (VBIAS). The VBIAS function is because most of the circuit functionality is integrated into the used to set the thermocouple common-mode voltage to ADuCM360 precision analog microcontroller, including dual AVDD REG/2 (900 mV). Again, this removes the need for 24-bit - ADCs, the ARM Cortex-M3 processor core, and external resistors to set the thermocouple common-mode the PWM/DAC features for controlling the 4 mA-to-20 mA voltage. loop for loop voltages up to 28 V. The ARM Cortex-M3 core. The powerful 32-bit ARM core with integrated 126 kB flash and 8 kB SRAM memory runs The ADuCM360 is connected to a Type T thermocouple and a the user code that configures and controls the ADCs and 100 platinum resistance temperature detector (RTD). The converts the ADC conversions from the thermocouple and RTD is used for cold junction compensation. The low power RTD inputs to a final temperature value. It also controls the Cortex-M3 core converts the ADC readings to a temperature PWM output driving the 4mA to 20 mA loop. For extra value. The Type T thermocouple temperature range supported is debug purposes, it also controls the communications over 200C to +350C, and this temperature range is converted to the UART/USB interface. an output current range of 4 mA to 20 mA. This circuit is similar to the circuit in Circuit Note CN-0300 Communication with the additional benefit of the higher resolution of the PWM The 16-bit PWM output is externally buffered using driving the 4 mA-to-20 mA loop. The PWM-based output OP193, and is controlling an external NPN transistor, provides 14 bits of resolution. For details on the temperature BC548. By controlling the V voltage of this transistor, the BE sensor interface to the ADC and on linearization techniques for current passing through a 47.5 load resistor can be set to the RTD measurements, see Circuit Note CN-0300 and the the desired value. This provides better then 0.5C AN-0970 Application Note. accuracy (between 200C to +350C) on the 4 mA-to-20 mA output (see test results). CIRCUIT DESCRIPTION The internal DAC is used to provide the 1.2 V reference to The circuit is powered via the ADP1720 linear regulator, which the OP193. Alternatively, the ADR3412, a 1.2 V precision regulates the loop + supply to 3.3 V for the ADuCM360, the reference could be used, for higher precision over OP193 op amp, and the optional ADR3412 reference. temperature. This external reference consumes similar Temperature Monitor power than the internal DAC (~50 A). See current This part of the circuit is similar to the temperature monitor consumption measurement tests section. circuit described in CN-0300. The following features of the The 4 mA-20 mA loop is controlled by the ADuCM360 on-chip ADuCM360 are used: 16-bit PWM (pulse width modulator). The duty cycle of the The 24-bit - ADC with a PGA set for a gain of 32 in the PWM is configured in software to control the voltage across the software for the thermocouple and the RTD. ADC1 switches 47.5 R LOOP resistor, which in turn sets the loop current. Note continuously between sampling the thermocouple and the the top of RLOOP connects to the ADuCM360 ground. The RTD voltages. bottom of RLOOP connects to the loop ground. Because of this, Programmable excitation current sources force a controlled the current due to the ADuCM360, ADP1720, ADR3412 and current through the RTD. The dual current sources are OP193, plus the current set by the filtered PWM output, flows configurable in steps from 0 A to 2 mA. For this example, across R . LOOP a 200 A setting is used to minimize the error introduced The voltage at the junction of R1 and R2 can be expressed: by the RTD self-heating. VR12 = (VRLOOP + VREF) R2/(R1 + R2) VRLOOP An internal 1.2 V reference is provided for the ADC in the ADuCM360. When measuring the thermocouple voltage, After the loop settles: the internal voltage reference is used due to its precision. VIN = VR12 An external voltage reference for the ADC in the ADuCM360. Since R1 = R2: When measuring the RTD resistance, a ratiometric setup V = (V + V )/2 V = V /2 V /2 was used where an external reference resistor (RREF) was IN RLOOP REF RLOOP REF RLOOP connected across the external VREF+ and VREF pins. The V = V 2V RLOOP REF IN on-chip reference input buffer is enabled because the reference = 0, at which point Full-scale current will flow when VIN source in this circuit is high impedance. The on-chip reference VRLOOP = VREF. Hence, full-scale current is VREF/RLOOP, or 24 mA. buffer means no external buffer is required to minimize input When VIN = VREF/2, no current will flow. leakage effects. The OP193 amplifier impedance at VIN is high and will not load the PWM filtered output. The amplifier output varies only a small amount of about 0.7 V. Rev. C Page 2 of 7