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PRACTICAL DESIGN TECHNIQUES FOR SENSOR SIGNAL CONDITIONING 1 Introduction 2 Bridge Circuits 3 Amplifiers for Signal Conditioning 4 Strain, Force, Pressure, and Flow Measurements 5 High Impedance Sensors 6 Position and Motion Sensors I 7

  1. PRACTICAL DESIGN TECHNIQUES FOR SENSOR SIGNAL CONDITIONING 1 Introduction 2 Bridge Circuits 3 Amplifiers for Signal Conditioning 4 Strain, Force, Pressure, and Flow Measurements 5 High Impedance Sensors 6 Position and Motion Sensors I 7 Temperature Sensors 8 ADCs for Signal Conditioning 9 Smart Sensors 10 Hardware Design Techniques 7.0 a

  2. APPLICATIONS OF TEMPERATURE SENSORS I Monitoring N Portable Equipment N CPU Temperature N Battery Temperature N Ambient Temperature I Compensation N Oscillator Drift in Cellular Phones N Thermocouple Cold-Junction Compensation I Control N Battery Charging N Process Control 7.1 a

  3. TYPES OF TEMPERATURE SENSORS THERMOCOUPLE RTD THERMISTOR SEMICONDUCTOR Widest Range: Range: Range: Range: –184ºC to +2300ºC –200ºC to +850ºC 0ºC to +100ºC –55ºC to +150ºC High Accuracy and Fair Linearity Poor Linearity Linearity: 1ºC Repeatability Accuracy: 1ºC Needs Cold Junction Requires Requires Requires Excitation Compensation Excitation Excitation Low-Voltage Output Low Cost High Sensitivity 10mV/K, 20mV/K, or 1µA/K Typical Output 7.2 a

  4. COMMON THERMOCOUPLES TYPICAL NOMINAL ANSI JUNCTION MATERIALS USEFUL SENSITIVITY DESIGNATION RANGE (ºC) (µV/ºC) Platinum (6%)/ Rhodium- 38 to 1800 7.7 B Platinum (30%)/Rhodium Tungsten (5%)/Rhenium - 0 to 2300 16 C Tungsten (26%)/Rhenium Chromel - Constantan 0 to 982 76 E Iron - Constantan 0 to 760 55 J Chromel - Alumel –184 to 1260 39 K Platinum (13%)/Rhodium- 0 to 1593 11.7 R Platinum Platinum (10%)/Rhodium- 0 to 1538 10.4 S Platinum Copper-Constantan –184 to 400 45 T 7.3 a

  5. THERMOCOUPLE OUTPUT VOLTAGES FOR TYPE J, K, AND S THERMOCOUPLES 60 THERMOCOUPLE OUTPUT VOLTAGE (mV) 50 TYPE K 40 TYPE J 30 20 TYPE S 10 0 -10 1000 -250 0 250 500 750 1250 1500 1750 TEMPERATURE (°C) 7.4 a

  6. THERMOCOUPLE SEEBECK COEFFICIENT VERSUS TEMPERATURE 70 60 TYPE J SEEBECK COEFFICIENT - µV/ °C 50 TYPE K 40 30 20 TYPE S 10 0 1000 -250 0 250 500 750 1250 1500 1750 TEMPERATURE (°C) 7.5 a

  7. THERMOCOUPLE BASICS A. THERMOELECTRIC VOLTAGE C. THERMOCOUPLE MEASUREMENT Metal A Metal A V1 – V2 Metal A Thermoelectric V2 V1 T1 V1 T1 T2 EMF Metal B Metal B D. THERMOCOUPLE MEASUREMENT B. THERMOCOUPLE Copper Copper V Metal A R Metal A Metal A Metal A T3 T4 I V2 V1 T1 T2 V2 V1 T1 T2 Metal B Metal B R = Total Circuit Resistance V = V1 – V2, If T3 = T4 I = (V1 – V2) / R 7.6 a

  8. CLASSICAL COLD-JUNCTION COMPENSATION USING AN ICE-POINT (0°C) REFERENCE JUNCTION METAL A METAL A V1 – V(0°C) T1 V1 METAL B V(0°C) ICE T2 BATH 0°C 7.7 a

  9. USING A TEMPERATURE SENSOR FOR COLD-JUNCTION COMPENSATION V(OUT) TEMPERATURE COMPENSATION V(COMP) CIRCUIT COPPER COPPER METAL A METAL A SAME TEMP TEMP SENSOR T1 V(T1) V(T2) T2 METAL B V(COMP) = f(T2) ISOTHERMAL BLOCK V(OUT) = V(T1) – V(T2) + V(COMP) IF V(COMP) = V(T2) – V(0°C), THEN V(OUT) = V(T1) – V(0°C) 7.8 a

  10. TERMINATING THERMOCOUPLE LEADS DIRECTLY TO AN ISOTHERMAL BLOCK COPPER V(OUT) = V1 – V(0°C) T2 METAL A COPPER T1 V1 TEMPERATURE COMPENSATION CIRCUIT TEMP SENSOR METAL B COPPER T2 ISOTHERMAL BLOCK 7.9 a

  11. USING A TEMPERATURE SENSOR FOR COLD-JUNCTION COMPENSATION (TMP35) 3.3V TO 5.5V 0.1µF R5* R4* 1.21M Ω Ω 4.99k Ω Ω TMP35 R1* TYPE K 24.9k Ω Ω THERMO P1 COUPLE 50k Ω Ω 0 °C < T < 250 °C – V OUT 0.1 - 2.6V R3* 1.24M Ω Ω OP193 CHROMEL 10mV/°C Cu + R6 + 100k Ω Ω R7* 4.99k Ω Ω COLD 0.1µF JUNCTION FILM – Cu R2* ALUMEL 102 Ω Ω * USE 1% RESISTORS ISOTHERMAL BLOCK 7.10 a

  12. AD594/AD595 MONOLITHIC THERMOCOUPLE AMPLIFIERS WITH COLD-JUNCTION COMPENSATION +5V 0.1µF BROKEN 4.7k Ω Ω V OUT THERMOCOUPLE 10mV/°C ALARM OVERLOAD DETECT TYPE J: AD594 TYPE K: AD595 AD594/AD595 THERMOCOUPLE +A –TC – – ICE G + G POINT + + COMP +TC 7.11 a

  13. AD77XX ADC USED WITH TMP35 TEMPERATURE SENSOR FOR CJC 3V OR 5V (DEPENDING ON ADC) 0.1µF AIN1+ CONTROL REGISTER TMP35 AIN1– Σ∆ Σ∆ OUTPUT MUX THERMO PGA REGISTER ADC COUPLE AIN2+ G=1 TO 128 SERIAL INTERFACE AD77XX SERIES AIN2– (16-22 BITS) TO MICROCONTROLLER 7.12 a

  14. RESISTANCE TEMPERATURE DETECTORs (RTD) I Platinum (Pt) the Most Common I 100 Ω, Ω, 1000 Ω Ω Standard Values I Typical TC = 0.385% / °C, 0.385 Ω / Ω / °C for 100 Ω Ω Pt RTD I Good Linearity - Better than Thermocouple, Easily Compensated 0.400 11.5 RTD TYPE S 100 Ω Ω Pt RTD RESISTANCE THERMOCOUPLE 10.5 0.375 TYPE S TC, ∆Ω ∆Ω / °C SEEBECK THERMOCOUPLE COEFFICIENT, 9.50 0.350 µV / °C 8.50 0.325 7.50 0.300 6.50 0.275 5.50 0 400 800 TEMPERATURE - °C 7.13 a

  15. A 100 Ω Ω Pt RTD WITH 100 FEET OF 30-GAUGE LEAD WIRES R = 10.5 Ω Ω COPPER 100 Ω Ω Pt RTD R = 10.5 Ω Ω COPPER RESISTANCE TC OF COPPER = 0.40%/°C @ 20°C RESISTANCE TC OF Pt RTD = 0.385%/ °C @ 20°C 7.14 a

  16. FOUR-WIRE OR KELVIN CONNECTION TO Pt RTD FOR ACCURATE MEASUREMENTS FORCE SENSE R LEAD LEAD LEAD TO HIGH - Z 100 Ω Ω I IN-AMP OR ADC Pt RTD FORCE SENSE R LEAD LEAD LEAD 7.15 a

  17. INTERFACING A Pt RTD TO A HIGH RESOLUTION ADC 3V OR 5V (DEPENDING ON ADC) +VREF R REF 6.25k Ω Ω –VREF CONTROL REGISTER + AIN1+ 400µA 100 Ω Ω Σ∆ Σ∆ MUX OUTPUT Pt RTD PGA REGISTER ADC – AIN1– G=1 TO 128 SERIAL INTERFACE AD77XX SERIES (16-22 BITS) TO MICROCONTROLLER 7.16 a

  18. CONDITIONING THE PLATINUM RTD USING THE ADT70 +5V 0.1µF 1k Ω Ω Pt RTD ADT70 + 2.5V 1k Ω Ω REF REFERENCE – RES MATCHED 1mA SOURCES SHUT + DOWN INST AMP GND – REF OUT = 5mV/ °C R G = 50k Ω Ω Note: Some Pins Omitted for Clarity -1V TO -5V 7.17 a

  19. RESISTANCE CHARACTERISTICS OF A 10k Ω Ω NTC THERMISTOR 40 ALPHA THERMISTOR, INCORPORATED 30 RESISTANCE/TEMPERATURE CURVE 'A' 10 k Ω Ω THERMISTOR, #13A1002-C3 THERMISTOR RESISTANCE k Ω Ω 20 10 Nominal Value @ 25 °C 0 0 20 40 60 80 100 TEMPERATURE - °C 7.18 a

  20. TEMPERATURE COEFFICIENT OF 10k Ω Ω NTC THERMISTOR -60000 ALPHA THERMISTOR, INCORPORATED -50000 RESISTANCE/TEMPERATURE CURVE 'A' 10 k Ω Ω THERMISTOR, #13A1002-C3 THERMISTOR TEMPERATURE COEFFICIENT ppm/ °C -40000 -30000 -20000 0 20 40 60 80 100 TEMPERATURE - °C 7.19 a

  21. LINEARIZATION OF NTC THERMISTOR USING A 5.17k Ω Ω SHUNT RESISTOR 40 30 RESISTANCE k Ω Ω 20 THERMISTOR PARALLEL COMBINATION 10 0 0 20 40 60 80 100 TEMPERATURE - °C 7.20 a

  22. LINEARIZED THERMISTOR AMPLIFIER V OUT ≈ ≈ 0.994V @ T = 0°C 226µA V OUT ≈ ≈ 0.294V @ T =70°C ∆ V OUT / ∆ ∆ ∆ T ≈ − ≈ − 10mV/°C AMPLIFIER OR ADC 10k Ω Ω NTC 5.17k Ω Ω THERMISTOR LINEARIZATION RESISTOR LINEARITY ≈ ± ≈ ± 2°C, 0°C TO +70°C 7.21 a

  23. BASIC RELATIONSHIPS FOR SEMICONDUCTOR TEMPERATURE SENSORS I C I C N TRANSISTORS ONE TRANSISTOR V BE V N         kT IC kT IC = = = =         VBE ln VN ln       ⋅ ⋅   q IS q N IS kT ∆ VBE ∆ = = − − = = VBE VN ln( ) N q INDEPENDENT OF I C , I S 7.22 a

  24. CLASSIC BANDGAP TEMPERATURE SENSOR +V IN "BROKAW CELL" R R V BANDGAP = 1.205V + I 2 ≅ ≅ I 1 Q2 Q1 NA A V N V BE kT (Q1) ∆ ∆ VBE = = − − = = VBE VN ln( ) N R2 q R1 kT V PTAT = 2 ln(N) q R2 R1 7.23 a

  25. CURRENT OUTPUT SENSORS: AD592, TMP17 V+ AD592: TO-92 PACKAGE TMP17: SO-8 PACKAGE V– I 1µA/K Scale Factor I Nominal Output Current @ +25°C: 298.2µA I Operation from 4V to 30V I ± ± 0.5°C Max Error @ 25°C, ± ± 1.0°C Error Over Temp, ± ± 0.1°C Typical Nonlinearity (AD592CN) I ± ± 2.5°C Max Error @ 25°C, ± ± 3.5°C Error Over Temp, ± ± 0.5°C Typical Nonlinearity (TMP17F) I AD592 Specified from –25°C to +105°C I TMP17 Specified from –40°C to +105°C 7.24 a

  26. RATIOMETRIC VOLTAGE OUTPUT SENSORS V S = +3.3V 0.1µF REFERENCE I(V S ) ADC + V OUT INPUT – R(T) GND AD22103     VS 28 mV = = × × + + × ×     VOUT 0 25 . V TA     ° ° 3 3 . V C 7.25 a

  27. ABSOLUTE VOLTAGE OUTPUT SENSORS WITH SHUTDOWN +V S = 2.7V TO 5.5V SHUTDOWN ALSO V OUT SO-8 TMP35 OR TO-92 TMP36 TMP37 0.1µF SOT-23-5 I V OUT : N TMP35, 250mV @ 25°C, 10mV/°C (+10°C to +125°C) N TMP36, 750mV @ 25°C, 10mV/°C (–40°C to +125°C) N TMP37, 500mV @ 25°C, 20mV/°C ( +5°C to +100°C) I ± ± 2°C Error Over Temp (Typical), ± ± 0.5°C Non-Linearity (Typical) I Specified –40°C to +125°C I 50µA Quiescent Current, 0.5µA in Shutdown Mode 7.26 a

  28. ADT45/ADT50 ABSOLUTE VOLTAGE OUTPUT SENSORS +V S = 2.7V TO 12V V OUT ADT45 ADT50 0.1µF SOT-23 I V OUT : N ADT45, 250mV @ 25°C, 10mV/°C Scale Factor N ADT50, 750mV @ 25°C, 10mV/°C Scale Factor I ± ± 2°C Error Over Temp (Typical), ± ± 0.5°C Non-Linearity (Typical) I Specified –40°C to +125°C I 60µA Quiescent Current 7.27 a

  29. THERMAL RESPONSE IN FORCED AIR FOR SOT-23-3 35 SOT-23-3 SOLDERED TO 0.338" x 0.307" Cu PCB V+ = 2.7V TO 5V 30 NO LOAD 25 TIME CONSTANT- SECONDS 20 15 10 5 0 500 0 100 200 300 400 600 700 AIR VELOCITY - LFPM 7.28 a

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