AN929

AN929 MICROCHIP | Alldatasheet

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 2004 Microchip Technology Inc. DS00929A-page 1 AN929 INTRODUCTION This application note shows how to select a tempera- ture sensor and conditioning circuit to maximize the measurement accuracy and simplify the interface to the microcontroller. Practical circuits and interface techniques will be provided for embedded applications with thermocouples, Resistive Temperature Detectors (RTDs), thermistors and silicon integrated circuit temperature sensors. The attributes of each tempera- ture sensor and the advantages of analog, frequency, ramp rate, duty cycle, serial and logic output solutions will be discussed. An analog output thermocouple circuit will be compared with a frequency output RTD oscillator circuit, along with design examples using serial and analog output silicon Integrated Circuit (IC) sensors. In addition, a Programmable Gain Amplifier (PGA) circuit will be shown that can increase the effectiveness of the Analog-to-Digital Converter (ADC) bit resolution of a non-linear thermistor sensor. DEFINITIONS The following terms are used in this application note:

• Accuracy is the difference between the true and measured temperature  Common Mode Rejection Ratio (CMRR) is defined as the ability of the amplifier to reject a signal which is common to both inputs  Input Offset Voltage (VOS ) is the voltage that must be applied to an amplifier to produce a zero volt output  Local sensors are located on the same PCB as the microcontroller  Precision is the ability to measure a small temperature gradient and determines the interchangeability of the sensor  Remote sensors are located at a distance from the microcontroller’s PCB  Repeatability is the sensor’s ability to reproduce previously measured values  Stability is defined as the long-term drift of the sensor over a period of time SENSOR-TO-MICROCONTROLLER INTERFACE OPTIONS The sensor interface option is selected by evaluating the complexity of the sensor circuitry, in addition to the required hardware and software trade-offs of the micro- controller. The available sensor interface options that are proportional to temperature include: A n a l o g  Frequency  Ramp Rate  Duty Cycle  Serial Output  Logic Output Analog Output Thermocouples, RTDs and thermistors can be inter- faced to an amplifier circuit to provide a voltage which is proportional to temperature. Analog output silicon sensors are available that integrate the sensor and the signal conditioning circuit. Figure 1 shows a block diagram of a typical analog interface circuit. FIGURE 1: Block Diagram of an Analog Output Sensor. Author: Jim Lepkowski Microchip Technology Inc. Temperature Sensor EMI / ESD Filter* Amplifier Anti- ADCAliasing Filter PICmicro® MCU *Required only for remote sensors Temperature Measurement Circuits for Embedded Applications

 2004 Microchip Technology Inc. DS00929A-page 9 AN929 FIGURE 15: Remote Sensor Fault Detection Circuits. Amplifier Selection Criteria Two key op amp specifications in a sensor amplifier are VOS and CMRR. V OS is important in sensor applica- tions when the input signal is of the same magnitude as V OS , while the CMRR ratio is critical in reducing the noise signal induced on the long wires of a remote sensor. Other op amp specifications, such as the AC frequency characteristics are less important because the frequency content of a temperature sensor is typically less than 100 Hz. The op amps used in oscillators are relatively immune to DC specifications such as V OS . The important parameters for oscillators are the amplifier’s frequency response and the Gain Bandwidth Product (GBWP). The op amp’s GBWP should be at least a factor of 100 times the maximum oscillation frequency. SENSOR SELECTION CRITERIA Table 1 provides a summary of the attributes of thermocouples, RTDs, thermistors and silicon IC sensors. TABLE 1: ATTRIBUTES OF THERMOCOUPLES, RTDS, THERMISTORS AND SILICON IC SENSORS R 2 R 1 R 2 >> R1 Instrumentation Amplifier VREF RTD HC14 Sensor BIT Thermocouple Thermocouple Dual Power Supply AmplifierSingle Power Supply Amplifier Attribute Thermocouple (type K) RTD Thermistor Silicon IC Range -184°C to 1260 °C -200°C to 850 °C -55°C to 150 °C -55°C to 125 °C Temperature (t) Accuracy Greater of ±2.2°C or ±0.75% Class B = ± [0.012 + Various, ±0.5°C to 5°C Various, ±0.5°C to 4°C Output Signal 40 µV /°C ≈ 0.00385 Ω /Ω/°C ≈ 4% ∆R /°C for 0°C ≤ t ≤ 70°C Analog, Serial, Logic, Duty Cycle Linearity Fair Excellent Poor Good Precision Fair Excellent Poor Fair Durability Good at lower temp., Poor at high temp, Open- circuit vibration failures Good, Wire wound prone to open-circuit vibration failures Good, Power derated with temperature Excellent Thermal Response Time Fast (function of probe material) Fast (function of probe material) Moderate Slow Cost Low Wire wound – High, Thin-film – Moderate Low Moderate Interface Issues Cold junction compensation, Small ∆V Small ∆R/°C Non-linear resistance Sensor located on PCB

DS00929A-page 12  2004 Microchip Technology Inc. The microcontroller computes the actual temperature by subtracting the cold junction temperature from the temperature determined from the thermocouple amplifier. The cold junction temperature is measured with a TC1047A silicon IC analog output sensor that is located on the PCB. The actual cold junction occurs where the thermocouple wires meet the copper wires, which is typically at a connector. Placing the TC1047A adjacent to the connector can minimize the cold junction error (T CJC_Location) that results by placing the temperature sensor on the PCB. The voltage output of the TC1047A is listed below. The TC1047A provides an output voltage of 10 mV/°C with an offset of 500 mV. The accuracy of the thermocouple amplifier and cold junction circuits were estimated to be ±5.4°C by using the root-squared-sum (RSS) equation. The common mode voltage noise signal entering the circuit from the thermocouple leads was assumed to be equal to 10V. It is important to use tight tolerance resistors for the differential amplifier in order to achieve a high CMRR. 0.1% resistors were chosen for R 3, R4, R5 and R6. The analysis shows that the majority of the measurement error is due to the error of the cold junction compensation.V OUT = [(Temp. (°C)) x (10 mV/°C)] + 500 mV CMRR Resistors 1 Amplifier Gain+ Error CMRR Resistors V CM NOISE– CMRR Amplifier (TC913A) 116db typ.()= Noise Attenuation 1 Error CMRR Amplifier (TC913A) V CM NOISE– TCJC_Sensor (TC1047A) error 3 °C (max.)= V OS error V OS (TC913A) Total errorWorst Case CMRR RESISTORS CMRR AMPLIFIER (TC913A) V OS TCJC_Sensor TCJC_Location++ + += TCJC_Locationerror 2 °C (est.)=

 2004 Microchip Technology Inc. DS00929A-page 13 AN929 SINGLE POWER SUPPLY CIRCUIT Figure 19 provides a low-cost, single power supply thermocouple amplifier circuit using a quad op amp. The buffered input differential amplifier topology is similar to an instrumentation amplifier and offers the feature of equal and high input impedance at the ampli- fier inputs. An instrumentation amplifier with integrated gain resistors can also be used to implement this cir- cuit. The gain of the amplifier was selected to be 249, providing a temperature coefficient of 10 mV/°C. The thermocouple inputs are biased to V DD /2 through

10 MΩ resistors, providing the ability to detect a failed

open-circuit thermocouple. FIGURE 19: Schematic of Single Supply Voltage Thermocouple Amplifier. RTD Oscillator Circuits Oscillator circuits can be used to provide an accurate temperature measurement with an RTD sensor. The state variable oscillator provides an output frequency that is proportional to the square root of the product of two temperature-sensing resistors and is a good circuit for precision applications. The astable multi-vibrator or relaxation oscillator provides a square wave output with a single amplifier and is a good alternative for cost-sensitive applications. The components must be chosen carefully so that the change in the oscillation frequency results primarily from the RTD and not from variation due to the compo- nent tolerance, temperature coefficient and drift rate. Metal film resistors, metal foil resistors and NPO porcelain capacitors are recommended to minimize the component error. Capacitors are relatively poor in performance when compared to resistors. Typically, the capacitor limits the accuracy of the oscillator. Further- more, precision capacitors are only available in relatively small values. The state variable oscillator requires two 100 nF capacitors, while the relaxation oscillator uses a 0.68 µF capacitor to produce a nomi- nal oscillation of 1 kHz. The state variable and relax- ation circuits have an uncalibrated measurement accuracy of approximately 1°C and 3°C, respectively. The difference is primarily due to the capacitor error. An application that requires an accuracy of better than ±1°C may require a temperature calibration and burn- in procedure. A temperature compensation algorithm can easily be implemented using the E 2 non-volatile memory of a microcontroller to store temperature correction data in a look-up table. The temperature coefficients are obtained by calibrating the circuit over temperature and comparing the measured temperature against the actual temperature. A burn-in or tempera- ture-cycling procedure can significantly reduce the drift of the resistors and capacitors. Burn-in procedures are useful because the majority of the change in magnitude of resistors and capacitors occurs within the 500 hours of a life test. EMI Filter EMI Filter Connector Shielded Cable TC1047A ADC IN_1 IN_2 +5V R 1 = R2 = 1 MΩ R 1 R 5 R 6 R 8 R 7 C 3 C 1 MCP619 Cold Junction Compensation R 2 C 2 R 3 C 1 = C2 = 1 nF R 7 = R8 = 249 kΩ R 5 = R6 = 1 kΩ R 3 = R4 = 10 MΩ C 3 = 0.1 µF R 4 +5V +5VType K Thermocouple VIN1 VIN2 U 1A U 1B V OUT V IN2 V IN1–() R 7 R 5  V REF+ V∆ IN() 249k == Temp. Coef. 249 40 µ V °C⁄× 10mV °C⁄≅= U 1C

DS00929A-page 14  2004 Microchip Technology Inc. STATE VARIABLE OSCILLATOR The state variable oscillator shown in Figure 20 con- sists of integrators A1, A2 and inverter circuit A3. Each integrator provides a phase shift of 90°, while the inverter adds an additional 180° phase shift. The total phase shift of the three amplifiers is equal to 360° and an oscillation is produced when the output of the third amplifier is fed back to the first amplifier. The addition of capacitor C4 helps ensure oscillation start-up. The dual element RTD represented by R1 and R2 is used to increase the difference in the oscillation frequency from the minimum to the maximum sensed temperature. FIGURE 20: State Variable RTD Oscillator. The state variable circuit offers the advantage that a limit circuit is not required if rail-to-rail input/output (RRIO) amplifiers are used and the gain of the inverter stage A 3 is equal to one (i.e., R3 = R4). In contrast, most oscillators require a limit or clamping circuit to prevent the amplifiers from saturating. Amplifier A 4 is used to provide the mid-supply reference voltage (VDD /2) required for the single-supply voltage circuit. Resistors R 5 and R6 form a voltage divider, while capacitor C5 is used to provide additional noise filtering. A comparator circuit A5 is used to convert the sinewave output to a square wave digital signal. The comparator functions as a zero-crossing detector with a switching threshold that is equal to VDD /2. Resistor R8 is used to provide additional hysteresis (VHYS ) to the comparator. RELAXATION OSCILLATOR The relaxation oscillator shown in Figure 21 provides a simple circuit to interface a RTD to a microcontroller. This circuit requires only a comparator, capacitor and a few resistors to generate a frequency output that is proportional to the RTD resistance. The accuracy of this circuit is limited by the poor tolerance and large temperature coefficient available with the required, relatively large, capacitor C The relaxation oscillator functions as a comparator. Resistors R2, R3 and R4 form a voltage divider that sets the hysteresis and voltage trip levels. Resistor R1 and capacitor C1 form the RC time constant that determines the charge and discharge rate or oscillation frequency. If V OUT equals VDD , C1 charges from the comparator’s low threshold (VTL) to the high threshold (VTH ), causing VOUT t o t o g g l e t o VSS . If VOUT equals VSS , C1 discharges from VTH to VTL and VOUT switches to VDD . The voltage-switching process then repeats, which generates the oscillation. The accuracy of the relaxation oscillator can be improved by using a comparator rather than an op amp for the amplifier. A comparator offers several advan- tages over an op amp in a non-linear switching circuit, such as a square wave oscillator. An op amp is intended to operate as a linear amplifier, while the com- parator is designed to function as a fast switch. The switching specifications (such as propagation delay and rise/fall time) of a comparator are typically much better than an op amp’s specifications. Also, the switching characteristics of an op amp typically consist of only a slew rate specification. The accuracy of the relaxation oscillator can be improved by using a higher C 1 VDD VDD /2 R 1 = RTDA C 2 VDD /2 R 2 = RTDB R 4 VDD /2 R 3 R 8 VDD /2 R 7 VDD /2 R 5 R 6 VOUT C 5 C 4 A3 A5 VDD R 1 = R2 = RTD C 5 = 1 µF C 4 = 20 pF C 1 = C2 = 100 nF R 8 = 1 MΩ R 3 = R4 = R5 = R6 = R7 = 1 kΩfO R 1R 2C 1C 2 then fO If R1 = R2 = R, C1 = C2 = C, and R3 = R4 Design Procedure: Set R1 = R2 = R, C1 = C2 = C, R3 = R4 and RO is the RTD resistance at 0°C. 1. Select a desired nominal oscillation frequency. 2. C = 1 / (2πR ofo). where: R o = RTD resistance @ 0°C. 3. Select an op amp with a GBWP ≥ 100 x fmax where: fmax = 1 / (2πR minC) and Rmin = RTD resistance at coldest sensing temperature. 4. Select R3 = R4 equal to 1 to 10 times Ro. 5. Select C4 using the following equations: f-3dB = 1 / (2πR 4C 4) C 4 ≈ 1 / (2πR 4f-3dB) where: f-3dB ≅ op amp’s GBWP

 2004 Microchip Technology Inc. DS00929A-page 19 AN929 FIGURE 30: Interfacing an Analog Output Silicon Sensor to an ADC. The sensor’s output impedance can affect the ADC. The temperature sensor’s output pin is typically driven by a buffer op amp, while the input of a typical ADC consists of a sample and hold circuit with a switch that connects V OUT to C SAMPLE . ROUT , RSWITCH and C SAMPLE form a time constant which must be less than the sampling rate (TSAMPLE ) of the ADC, as illustrated by the equation in Figure 30. An external capacitor C FILTER can be added to the output pin to provide additional filtering. However, this may impact the time response of the sensor. Enough time must be provided to allow CFILTER to charge sufficiently between ADC conversions. The magnitude of the CFILTER capacitor should be in the range of 1 nF to 100 nF to prevent the sensor amplifier from oscillating. A small resistor of approximately 10Ω to 100Ω can be added between the output pin of the sensor and C FILTER to isolate the sensor’s amplifier from the capacitive load. The output impedance of the sensor R OUT varies as a function of frequency. Thus, a series resistor should be added to the effective ROUT resistance, if CFILTER is intended to serve as the ADC’s anti-aliasing filter. VOUT R SWITCH Sample Hold C SAMPLE R OUT Analog Output Silicon Sensor ADC’s Input Stage R OUT R SWITCH+() C SAMPLE 0.1≤ TSAMPLE×× C FILTER

DS00929A-page 20  2004 Microchip Technology Inc. CONCLUSION Temperature sensors are used in embedded systems for both thermal monitoring and management applica- tions. A designer must evaluate the trade-offs of the sensor, conditioning circuitry and sensor output in order to maximize the measurement accuracy while easing the interface to the microcontroller. In addition, the designer must consider system integration issues such as the location of the sensor, grounding, EMI/ESD protection and shielding in order to provide a robust temperature measurement. A sample of practical circuits and interface techniques has been provided along with design equations. The following sensor guidelines can be used as a start- ing point to select a temperature sensor. If your appli- cation requires a high-temperature measurement, thermocouples are a good choice because of their wide temperature operating range. Thermocouples are typi- cally used as remote sensors and, therefore, the circuit must provide noise immunity by using good grounding and shielding methods. If your application requires pre- cision, RTDs set the standard with their superior repeatability and stability characteristics. For applica- tions such as the temperature measurement on a PCB, either thermistors or silicon IC sensors should be con- sidered. Thermistors are available in more packages, are lower in cost and have a faster thermal response time than silicon sensors. However, thermistors require additional signal-conditioning circuitry, while silicon sensors provide both the sensor and circuitry on a single IC that can be interfaced directly to the microcontroller. The output of the sensor is selected by the available microcontroller hardware and software resources, in addition to the complexity of the sensor circuit. The sensor output can consist of an analog, frequency, ramp rate, duty cycle, serial or logic format that is proportional to temperature. Temperature measure- ment is a popular topic and the designer should review the literature to evaluate the many sensor and circuit options available. REFERENCES 1. AN679, “Temperature Sensing Technologies”, DS00679, Baker, Bonnie, Microchip Technology Inc., 1999. 2. “High-Accuracy CMOS Smart Temperature Sensors”, Bakker, A. and Huijsing, J., Kluwer Academic Publishers, Boston, 2000. 3. AN913, “Interfacing the TC77 Thermal Sensor to a PICmicro ® Microcontroller”, DS00913, Bible, S. and Lepkowski, J., Microchip Technology Inc., 2004. 4. AN897, “Thermistor Temperature Sensing with the MCP6S2X PGA”, DS00897, Blake, K., Microchip Technology Inc., 2004. 5. AN512, “Implementing Ohmmeter/Temperature Sensor”, DS00512, Cox, D., Microchip Technology Inc., 1997. 6. TB052, “Multi-Zone Temperature Monitoring with the TCN75 Thermal Sensor”, DS91052, Dietz, K., Microchip Technology Inc., 2001. 7. AN895, “Oscillator Circuits for RTD Temperature Sensors”, DS00895, Haile, E. and Lepkowski, J., Microchip Technology Inc., 2004. 8. “Section 7: Temperature Sensors, Practical Design Techniques for Sensor Signal Conditioning”, Kester, W., Bryant, W. and Jung, W., Analog Devices, 1999. 9. AN871, “Solving Thermal Measurement Problems Using the TC72 and TC77 Digital Silicon Temperature Sensors”, DS00871, Lepkowski, J., Microchip Technology Inc., 2003. 10. “Silicon Sensors Harness Thermal Management”, Marsh, D., EDN, December 11, 2003. 11. “NTC Thermistor Basics and Principles of Operation”, McGillicuddy, D. Sensors, December, 1993. 12. “The ABCs of RTDs”, McGovern, B., Sensors, November 2003. 13. “Noise Reduction Techniques in Electronic Systems”, Ott, H, John Wiley, N.Y ., 1998. 14. AN571, “Using Analog Temperature Sensors with ADCs”, Maxim Semiconductor, 2001. ACKNOWLEGDEMENT The author appreciates the assistance of Kumen Blake of Microchip Technology. The thermistor circuit design examples are based on his work.

 2004 Microchip Technology Inc. DS00929A-page 21 Information contained in this publication regarding device applications and the like is intended through suggestion only and may be superseded by updates. It is your responsibility to ensure that your application meets with your specifications. No representation or warranty is given and no liability is assumed by Microchip Technology Incorporated with respect to the accuracy or use of such information, or infringement of patents or other intellectual property rights arising from such use or otherwise. Use of Microchip’s products as critical components in life support systems is not authorized except with express written approval by Microchip. No licenses are conveyed, implicitly or otherwise, under any intellectual property rights. Trademarks The Microchip name and logo, the Microchip logo, Accuron, dsPIC, KEE LOQ , MPLAB, PIC, PICmicro, PICSTART, PRO MATE, PowerSmart and rfPIC are registered trademarks of Microchip Technology Incorporated in the U.S.A. and other countries. AmpLab, FilterLab, microID, MXDEV, MXLAB, PICMASTER, SEEVAL, SmartShunt and The Embedded Control Solutions Company are registered trademarks of Microchip Technology Incorporated in the U.S.A. Application Maestro, dsPICDEM, dsPICDEM.net, dsPICworks, ECAN, ECONOMONITOR, FanSense, FlexROM, fuzzyLAB, In-Circuit Serial Programming, ICSP , ICEPIC, Migratable Memory, MPASM, MPLIB, MPLINK, MPSIM, PICkit, PICDEM, PICDEM.net, PICtail, PowerCal, PowerInfo, PowerMate, PowerTool, rfLAB, Select Mode, SmartSensor, SmartTel and Total Endurance are trademarks of Microchip Technology Incorporated in the U.S.A. and other countries. SQTP is a service mark of Microchip Technology Incorporated in the U.S.A. All other trademarks mentioned herein are property of their respective companies. © 2004, Microchip Technology Incorporated, Printed in the U.S.A., All Rights Reserved. Printed on recycled paper. Note the following details of the code protection feature on Microchip devices:  Microchip products meet the specification contained in their particular Microchip Data Sheet.  Microchip believes that its family of products is one of the most secure families of its kind on the market today, when used in the intended manner and under normal conditions.  There are dishonest and possibly illegal methods used to breach the code protection feature. All of these methods, to our knowledge, require using the Microchip products in a manner outside the operating specifications contained in Microchip’s Data Sheets. Most likely, the person doing so is engaged in theft of intellectual property.  Microchip is willing to work with the customer who is concerned about the integrity of their code.  Neither Microchip nor any other semiconductor manufacturer can guarantee the security of their code. Code protection does not mean that we are guaranteeing the product as “unbreakable.” Code protection is constantly evolving. We at Microchip are committed to continuously improving the code protection features of our products. Attempts to break Microchip’s code protection feature may be a violation of the Digital Millennium Copyright Act. If such acts allow unauthorized access to your software or other copyrighted work, you may have a right to sue for relief under that Act. Microchip received ISO/TS-16949:2002 quality system certification for its worldwide headquarters, design and wafer fabrication facilities in Chandler and Tempe, Arizona and Mountain View, California in October 2003. The Company’s quality system processes and procedures are for its PICmicro® 8-bit MCUs, KEE LOQ ® code hopping devices, Serial EEPROMs, microperipherals, nonvolatile memory and analog products. In addition, Microchip’s quality system for the design and manufacture of development systems is ISO 9001:2000 certified.

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