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Fundamentals of Designing with Sensors

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Sensors help to capture what is going on in the real world. This deck explains how real-world information, such as pressure, temperature, speed is converted into a useful electronics format that can …

Sensors help to capture what is going on in the real world. This deck explains how real-world information, such as pressure, temperature, speed is converted into a useful electronics format that can be analyzed, manipulated, stored or sent to another device.

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  • As you probably know, our first challenge is that light, sound, weight, speed, temperature, even smell are analog, or continuous wave forms. That’s okay for us analog human beings, but a lousy format for electronic devices.It gets worse, because those real world analog signals aren’t electronic, either.That means we have two challenges: One is to capture the physical attributes and signals from the real world – these analog signals, and Two, convert them to electronic signals that we can manipulate…
  • How we do that is what we’ll be covering in this 12-part course. In the coming months we’ll go through each stage of the basic signal chain, from amplifiers to data converters – those components that convert the analog electronic signal into a digital stream – then to the heart of many modern circuits, the digital processor. We’ll also cover what is needed to power today’s circuits, how to make them portable, and how to lay them out .So let’s go back to the beginning of the circuit and address the first task of turning that analog, non electrical signal into an analog electrical one. How do we do that?
  • By employing sensors. Sensors are devices that respond to changes in these analog, non-electronic signals – temperature, speed, weight, pressure, and so on and turn them into electronic analog signals. Those signals can be in many different forms.
  • As you can see, for the measurement of different kinds of analog real-world signals, different sensors provide different outputs. In all cases, we have achieved goal number one – capturing that signal so we can move it down the signal chain.
  • Let’s look at one example of a sensor. The Thermocouple is a device made of dissimilar metals welded at one end, which generate a voltage that increases with temperature. The output, as we just saw, is a varying voltage. Like any electronic device and like any sensor, it has its benefits and its problems, most important to our goal is that the output of this device is very low level – which means the signal can be below the ambient noise level of the circuit – which in turn means we cannot discriminate it from the noise.
  • Which means we need to take that low-level, fragile signal and perform Sensor Signal Conditioning. In this critical step we will:Amplify the signal to a noise-resistant levelLower the source impedanceLinearize (sometimes but not always)FilterProtectionThe result is a signal clean enough to be sent to the converter for digitizing. Without this critical stage data conversion would be inaccurate, rendering the entire circuit useless. It’s that important.
  • A series of thermocouple amplifiers is available from Analog Devices so you can find the right accuracy and operating temperature range for your needs. These products use very little power, less than 1mW, and are in a small MSOP package. Two of the important things to look at when using a thermocouple amplifier are the initial accuracy and the temperature range where the amplifier is accurate. This is the temperature of the board that the amplifier is on, not the temperature of the thermocouple itself.
  • Before Dave takes any questions, I want to remind you that every month Analog Devices presents a webcast on a current Hot Topic in designing with Semiconductors. A week from today at 3pm, on January 19th, we’ll present a webcast on software simulation for data converters. Next month we’ll be presenting a webcast on the use of RF Detectors and in March on Embedded Design. Registration will be available shortly for both at www.analog.com slash webcast, where you can also access our library of archived webcasts that you can view anytime, on demand.
  • Transcript

    • 1. The World Leader in High Performance Signal Processing SolutionsFUNDAMENTALS OF DESIGN Class 1 Introduction Presented by David Kress
    • 2. The Goal Capture what is going on in the real world Convert into a useful electronic format Analyze, Manipulate, Store, and Send Return to the real world
    • 3. The real world is NOT digital
    • 4. Analog to Electronic signal processing Sensor Amp Converter Digital Processor (INPUT) Actuator Amp Converter (OUTPUT)
    • 5. The SensorAnalog, but Analog Sensor NOT (INPUT) AND Amp Converter Digital Processor electronic electronic Actuator Amp Converter (OUTPUT)
    • 6. Popular sensors Sensor Type Output Thermocouple Voltage Photodiode Current Strain Gauge Resistance Microphone Capacitance Touch Button Charge Output Antenna Inductance
    • 7. Thermocouple Very low level (µV/ºC) Non-linear Difficult to handle Wires need insulation Susceptible to noise Fragile
    • 8. Sensor Signal Conditioning Sensor Amp Analog, Analog, electronic, electronic, but “dirty” and “clean” •Amplify the signal to a noise-resistant level •Lower the source impedance •Linearize (sometimes but not always) •Filter •Protect
    • 9. 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
    • 10. 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
    • 11. Thermocouple Output Voltagesfor Type J, K and S Thermocouples THERMOCOUPLE OUTPUT VOLTAGE (mV) 60 50 TYPE K 40 TYPE J 30 20 TYPE S 10 0 -10 -250 0 250 500 750 1000 1250 1500 1750 TEMPERATURE (°C)
    • 12. Thermocouple Seebeck Coefficient vs.Temperature 70 60 TYPE J SEEBECK COEFFICIENT - µV/ °C 50 TYPE K 40 30 20 TYPE S 10 0 -250 0 250 500 750 1000 1250 1500 1750 TEMPERATURE (°C)
    • 13. Thermocouple Basics A. THERMOELECTRIC VOLTAGE C. THERMOCOUPLE MEASUREMENT Metal A Metal A V1 – V2 Metal AV1 T1 Thermoelectric V1 T1 T2 V2 EMF Metal B Metal B B. THERMOCOUPLE D. THERMOCOUPLE MEASUREMENT Copper Copper Metal A R Metal A V Metal A Metal A I T3 T4V1 T1 T2 V2 V1 T1 T2 V2 Metal B Metal B R = Total Circuit Resistance I = (V1 – V2) / R V = V1 – V2, If T3 = T4
    • 14. Using a Temperature Sensor for Cold-Junction Compensations V(OUT) TEMPERATURE V(COMP) COMPENSATION CIRCUIT COPPER COPPER METAL A SAME METAL A 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)
    • 15. AD594/AD595 Monolithic ThermocoupleAmplifier with Cold-Junction Compensation +5V 0.1µF BROKEN 4.7k THERMOCOUPLE VOUT ALARM 10mV/°C OVERLOAD TYPE J: AD594 DETECT TYPE K: AD595 THERMOCOUPLE AD594/AD595 +A – – –TC ICE G + G POINT + + COMP +TC
    • 16. Basic Relationships For SemiconductorTemperature Sensors IC IC N TRANSISTORS ONE TRANSISTOR VBE VN kT  IC  kT  IC  VBE  ln  VN  ln  q  IS  q  N  IS  kT VBE  VBE  VN  ln(N) q INDEPENDENT OF IC, IS
    • 17. Classic Bandgap Temperature Sensor +VIN R R "BROKAW CELL" + VBANDGAP = 1.205V I2 @ I1 Q2 Q1 NA A VN VBE kTVBE  VBE  VN  ln(N) R2 (Q1) q R1 kT VPTAT = 2 ln(N) R2 q R1
    • 18. Analog Temperature Sensors Product Accuracy Max Accuracy Operating Supply Max Interface Package (Max) Range Temp Range Current Range ± 0.5°C 25°C -55°C to TO-52,2-ld FP, AD590 4 to 30V 298uA Current Out ± 1.0°C -25°C to 105°C 150°C SOIC, Die ± 0.5°C 25°C -25°C to 298uA AD592 4 to 30V Current Out TO-92 ± 1.0°C -55°C to 150°C 105°C 0°C to 85°C -55°C to TO-92, SOT23, TMP35 ± 2.0°C 2.7 to 5.5V 50uA Voltage Out -25°C to 100°C 150°C SOIC TO-92, SOT23, -40°C to 125°C -55°C to 50uA TMP36 ± 3.0°C 2.7V to 5.5V Voltage Out SOIC 150°C ± 2.0°C -50°C to 150°C -50°C to AD22100 4 to 6.5V 650uA Voltage Out TO-92, SOIC, Die 150°C ± 2.5°C 0°C to 100°C 0°C to 100°C AD22103 2.7 to 3.6V 600uA Voltage Out TO-92, SOIC
    • 19. Digital Temperature SensorsComprehensive Portfolio of Accuracy Options Product Accuracy (Max) Max Accuracy Interface Package Range ± 0.2°C -10°C to 85°C ADT7420/7320 I2C/SPI LFCSP ± 0.25°C -20°C to 105°C ADT7410/7310 ± 0.5°C -40°C to 105°C I2C/SPI SOIC ± 1°C (B grade) 0°C to 85°C ADT75 I2C MSOP, SOIC ± 2°C (A grade) -25°C to 100°C ± 1°C 0°C to 70°C ADT7301 SPI SOT23, MSOP ± 1°C 0°C to 70°C TMP05/6 PWM SC70, SOT23 ± 1.5°C -40°C to 70°C AD7414/5 I2C SOT23,MSOP ADT7302 ± 2°C 0°C to 70°C SPI SOT23,MSOP ± 4°C TMP03/4 -20°C to 100°C PWM TO-92,SOIC,TSSOP21
    • 20. Position and Motion Sensors Linear Position: Linear Variable Differential Transformers (LVDT) Hall Effect Sensors  Proximity Detectors  Linear Output (Magnetic Field Strength) Rotational Position:  Optical Rotational Encoders  Synchros and Resolvers  Inductosyns (Linear and Rotational Position)  Motor Control Applications Acceleration and Tilt: Accelerometers Gyroscopes
    • 21. + THREADED CORE VA ~ VOUT = VA – VB AC SOURCE VB 1.75" _ VOUT VOUTSCHAEVITZ _ POSITION + _ POSITION + E100 LVDT – Linear Variable Differential Transformer
    • 22. AD698EXCITATION AMP ~ REFERENCE OSCILLATOR B VB + A VOUT FILTER AMP B A VA A, B = ABSOLUTE VALUE + FILTER _ 4-WIRE LVDT AD698 LVDT Signal Conditioner
    • 23. Hall Effect Sensors T CONDUCTOR OR SEMICONDUCTORI I VH I = CURRENT B B = MAGNETIC FIELD T = THICKNESS VH = HALL VOLTAGE
    • 24. AD22151 Linear Output MagneticField Sensor V / 2 V = +5V CC CC VCC / 2 R2 + R1 TEMP REF _ R3 _ AD22151 VOUT + OUTPUT AMP CHOPPER AMP VOUT = 1 + R3 0.4mV Gauss NONLINEARITY = 0.1% FS R2
    • 25. Accelerometer Applications Tilt or Inclination  Car Alarms  Patient Monitors  Cell phones  Video games Inertial Forces  Laptop Computer Disc Drive Protection  Airbag Crash Sensors  Car Navigation systems  Elevator Controls Shock or Vibration  Machine Monitoring  Control of Shaker Tables ADI Accelerometer Fullscale g-Range: ± 2g to ± 100g ADI Accelerometer Frequency Range: DC to 10kHz
    • 26. ADXL-family Micro-machined Accelerometers AT REST CS1 CS2 APPLIED ACCELERATIONCENTER PLATE TETHER BEAM CS1 CS1 = CS2 < CS2 FIXED OUTER PLATES DENOTES ANCHOR
    • 27. Using an Accelerometer to Measure Tilt +90° X X 1g  Acceleration 0° –90° +1g Acceleration = 1g × sin  0g  –90° 0° +90° –1g
    • 28. Gyro Axes of Rotational Sensitivity
    • 29. Coriolis acceleration example.
    • 30. Displacement due to the Coriolis Effect
    • 31. Photograph of mechanical sensor.
    • 32. High Impedance Sensors Photodiodes Piezoelectric Sensors  Accelerometers  Hydrophones Humidity Monitors pH Monitors Chemical Sensors Smoke Detectors Charge Coupled Devices and Contact Image Sensors for Imaging
    • 33. Photodiode Equivalent CircuitINCIDENT LIGHT PHOTO RSH(T) CURRENT CJ IDEAL DIODE 100k - 100G NOTE: RSH HALVES EVERY 10 C TEMPERATURE RISE
    • 34. Current-to-voltage Converter (Simplified) R = 1000M ISC = 30pA (0.001 fc) _ VOUT = 30mV + Sensitivity: 1mV / pA
    • 35. Preamplifier DC Offset Errors 1000M R2 IB ~ _ OFFSET VOS RTO R1 IB + R3 DC NOISE GAIN = 1 + R2 R1 IB DOUBLES EVERY 10 C TEMPERATURE RISE R1 = 1000M @ 25 C (DIODE SHUNT RESISTANCE) R1 HALVES EVERY 10 C TEMPERATURE RISE R3 CANCELLATION RESISTOR NOT EFFECTIVE
    • 36. Sensor Resistances Used In BridgeCircuits Span A Wide Dynamic Range  Strain Gages 120, 350, 3500  Weigh-Scale Load Cells 350 - 3500  Pressure Sensors 350 - 3500  Relative Humidity 100k - 10M  Resistance Temperature Devices (RTDs) 100 , 1000  Thermistors 100 - 10M
    • 37. Wheatstone Bridge Produces An Output NullWhen The Ratios Of Sidearm Resistances Match VB THE WHEATSTONE BRIDGE: R4 R3  R1 R2  VO  VB     R1 + R4 R2 + R3  VO AT BALANCE, R1 R2 VO = 0 if  R4 R3 R1 R2
    • 38. Output Voltage Sensitivity And Linearity Of Constant Current DriveBridge Configurations Differs According To The Number Of ActiveElements IB IB IB IB R R R+R R R RR R+R RR VO VO VO VO R R+R R R+R R R+R RR R+R R VO: IBR R IB R IB R IB R 4 R + 2 2 4Linearity 0.25%/% 0 0 0Error: (A) Single-Element (B) Two-Element (C) Two-Element (D) All-Element Varying Varying (1) Varying (2) Varying
    • 39. A Generally Preferred Method Of Bridge Amplification EmploysAn Instrumentation Amplifier For Stable Gain And High CMR VB OPTIONAL RATIOMETRIC OUTPUT VREF = VB +VS R R R  VB VOUT = R GAIN 4 R + RG 2 IN AMP REF VOUT + R R+R -VS* * SEE TEXT REGARDING SINGLE-SUPPLY OPERATION
    • 40. Upcoming webcasts Converter Simulation: Beyond the Eval Board  January 19th at 3:00 p.m. (ET) RF Detectors  February 16th at Noon (ET) Challenges in Embedded Design for real-time systems  March 16th at Noon (ET) www.analog.com/webcast
    • 41. Fundamentals Webcasts 2011 January Introduction and Fundamentals of Sensors February The Op Amp March Beyond the Op Amp April Converters, Part 1, Understanding Sampled Data Systems May Converters, Part 2, Digital-to-Analog Converters June Converters, Part 3, Analog-to-Digital Converters July Powering your circuit August RF: Making your circuit mobile September Fundamentals of DSP/Embedded System design October Challenges in Industrial Design November Tips and Tricks for laying out your PC board December Final Exam, Ask Analog Devices www.analog.com/webcast

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