Imperial College London – dept EEE 1
Autumn 2003
E302-AC4 InstrumentationAutumn 2010G1
Instrumentation
Christos PapavassiliouAutumn 2010
Imperial College London – dept EEE 2
Autumn 2003
E302-AC4 InstrumentationAutumn 2010G1
Course Details• Lecture: Tuesday 2pm, Wednesday 11am (403a)• Lecturer: Christos Papavassiliou,
room 915, email: [email protected]• Coursework (20%)
2 sets, 1 due week 5, one due week 11
• Course Homepage: Link from CP homepageWEBCT
Imperial College London – dept EEE 3
Autumn 2003
E302-AC4 InstrumentationAutumn 2010G1
Course Overview
• Aims:To give an overview of electrical measurement theory and practice, especially at GHz frequencies; provide an understanding of measurement principles, capabilities and limitations.
Objectives:Understand the operation and limitations of various measurement sensors/transducers, and design suitable circuits and procedures to optimise transducer performance. Apply Signal processing techniques to measurement.
Imperial College London – dept EEE 4
Autumn 2003
E302-AC4 InstrumentationAutumn 2010G1
Course Syllabus• Definitions and examples of measurements and their limitations:
– Resolution, accuracy, sensitivity. – DC and AC bridges.
• Noise in electronic systems and Amplifiers• Oscilloscopes: Analogue, Digital and Sampling. • Phase locked loops, Oscillators and synthesisers. Phase noise.• Sampling and Analogue to Digital conversion. Oversampling. • Counters, Timers: Time and Frequency measurement. • Coherent measurements and interferometry. Correlation techniques.• Microwave measurements: Network and spectrum analysers. • (if time permits) Time and Frequency Domain Reflectometry.
Imperial College London – dept EEE 5
Autumn 2003
E302-AC4 InstrumentationAutumn 2010G1
Course Outline
• 1. Sensors/Transducers; Bridges• 2. Noise, system noise, noise matching• 3. Signal Amplification; autozeroing and chopper amplifiers.• 4. Oscillators, synthesizers, integer and fractional PLL,
transfer oscillators.• 5. Frequency and time measurements• 6. Modelling and data fitting; Calibration• 7. Sampling, oversampling, subsampling. Aliasing and anti-
alias filters.• 8. A/D and D/A Converters. Quantisation noise.• 9. Synchronous measurements and Interferometry• 10. Radio frequency measurements
Imperial College London – dept EEE 6
Autumn 2003
E302-AC4 InstrumentationAutumn 2010G1
Metrology• The scientific study of measurement• The science of weights and measures• Metrology is an important part of semiconductor
manufacturing where high-resolution tools are required to measure the tiny elements in a chip that continue to decrease year after year.
• A measurement is a comparison to a standard. -- William Shockley
Imperial College London – dept EEE 7
Autumn 2003
E302-AC4 InstrumentationAutumn 2010G1
Calibration• The process of adjusting an instrument or compiling a deviation
chart so that its reading can be correlated to the actual value being measured.
• determination of the accuracy of an instrument, usually by measurement of its variation from a standard, to ascertain necessary correction factors
• Checking, adjusting and systematically standardizing the graduations of a device.
• is defined as the process of quantitatively defining the system response to known, controlled signal inputs.
• many more, similar, definitions.
Imperial College London – dept EEE 8
Autumn 2003
E302-AC4 InstrumentationAutumn 2010G1
The importance of calibration (1)There is much talk about the risks associated with inadequate calibration, but many people have asked whether the risks are real or just theoretical.
"Looks like someone forgot to calibrate the 'Instrument Landing System', captain."
Imperial College London – dept EEE 9
Autumn 2003
E302-AC4 InstrumentationAutumn 2010G1
The importance of calibration (2)
city police department's radar speed violation tickets were legally invalidated in court after somebody proved the calibration process for the radar guns wasn't traceable to national standards.
"Honestly officer, battery-powered milk floats can't do 75 miles per hour.... even downhill."
Imperial College London – dept EEE 10
Autumn 2003
E302-AC4 InstrumentationAutumn 2010G1
SI Base Units
QuantityUnit
Name Symbollength meter mmass kilogram kg
time seconds selectric current ampere Athermodynamic temperature kelvin Kamount of substance mole molluminous intensity candela cd
Imperial College London – dept EEE 11
Autumn 2003
E302-AC4 InstrumentationAutumn 2010G1
Unit Prefixes
μ
Imperial College London – dept EEE 12
Autumn 2003
E302-AC4 InstrumentationAutumn 2010G1
accuracy of terminology is important in engineering
•1 million microphones = 1 megaphone ?•1 million bicycles = 2 megacycles ?•52 cards = 1 decacards ?•1/2 lavatory = 1 demijohn ?•3-1/3 tridents = 1 decadent ?•10 monologs = 5 dialogues ?•2 monograms = 1 diagram ?•1 millionth of a fish = 1 microfiche ?•10 rations = 1 decoration ?•10 millipedes = 1 centipede ?
Imperial College London – dept EEE 13
Autumn 2003
E302-AC4 InstrumentationAutumn 2010G1
Introduction
• Instrumentation:– Physics (sensors-actuators)– Electronics (interfacing)– Signal Processing (both A + D)– Control (Feedback)– Modelling
• Metrology– The science of “correct” measurement
• Account for interactions, artifacts, make standards
Imperial College London – dept EEE 14
Autumn 2003
E302-AC4 InstrumentationAutumn 2010G1
Definitions
• Transducers (Energy converters)– Sensors (Physical Electrical)
• Active (own power)– Photovoltaic, Thermovoltaic
• Passive (change impedance)– Thermistor, Photodiode
– Actuators (Electrical Physical)• Servo motors, Loudspeakers• Feedback!
Imperial College London – dept EEE 15
Autumn 2003
E302-AC4 InstrumentationAutumn 2010G1
Sensors / Instruments
• Sensor + Electronics is an instrument• Most discussion applies to eitherA lot of the work of instrumentation is about modelling:
– Sensor is typically a Thevenin or Norton Circuit– V/I source may be dependent on stimulus– Impedance may be dependent on stimulus– Need to maximise power from stimulus to circuit– Need to minimise input noise power.– Sensor may be sensitive to other stimuli (cross-sensitivity)– Can exploit cross-sensitivity to increase SNR, by modulating
secondary stimuli.
Imperial College London – dept EEE 16
Autumn 2003
E302-AC4 InstrumentationAutumn 2010G1
Typical Instrument
Sensor/ Transdu cer
Input Functions
Signal Conditioning and Processing
Ou tput Functions
Real Tim e Display
Data Storage
Transdu cers
Feed back
Physical Signals
Noise
Imperial College London – dept EEE 17
Autumn 2003
E302-AC4 InstrumentationAutumn 2010G1
Ideal sensors
• An ideal sensor:– Does not disturb the system it measures– Does not absorb any energy from the system it measures– Can present an ideal source (V or I) at its output– It is perfectly linear: Output = constant * Input– Has no offset: Zero input zero output– Can respond to any stimulus no matter how small– Does not add any noise of its own to the measurement– Is monotonic (e.g. bigger input bigger output)– Only responds to the intended stimulus
Imperial College London – dept EEE 18
Autumn 2003
E302-AC4 InstrumentationAutumn 2010G1
Real sensors
• A real sensor is a transducer which:– Disturbs the system it measures– Absorbs energy from the system it measures– Present a finite impedance (Thevenin or Norton) at its output– It is never linear: Output = f(Input)– Has some zero offset: finite output for zero input– There is a minimum magnitude stimulus it can respond to– It adds noise of its own to the measurement– May not be always monotonic: increasing in decreasing out– responds to other stimuli (e.g. temperature dependence of a
pressure sensor)
Imperial College London – dept EEE 19
Autumn 2003
E302-AC4 InstrumentationAutumn 2010G1
Sensor description
We describe a sensor as a Taylor series response w/ noise:
• Sensitivity (Gain) – output /input – linear Taylor term• Threshold or detectivity – Constant Taylor term• Zero offset• Non-linearity• Memory:
– Linear– Non-linear
• Hysteresis
Imperial College London – dept EEE 20
Autumn 2003
E302-AC4 InstrumentationAutumn 2010G1
Illustration of a sensor
Detectivity
Input (x)
Output (y)
Noise Floor
Zero Error
Gain Error
Dynamic Range
Resolution
Imperial College London – dept EEE 21
Autumn 2003
E302-AC4 InstrumentationAutumn 2010G1
Illustration of linearity
Best Fit"Ideal" Linear responseO
utput
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Autumn 2003
E302-AC4 InstrumentationAutumn 2010G1
Illustration of hysteresis
Output
Imperial College London – dept EEE 23
Autumn 2003
E302-AC4 InstrumentationAutumn 2010G1
More on sensors• Cross-sensitivity• Resolution• Dynamic range• Non-Monotonicity• Accuracy
Imperial College London – dept EEE 24
Autumn 2003
E302-AC4 InstrumentationAutumn 2010G1
Some sensors
• Active (derive power from signal)– Photovoltaic– Piezoelectric– Thermoelectric– Electromagnetic
Model as a Thevenin or Norton controlled source– Source a function of stimulus– Internal Impedance important, may also be a function of the
stimulus!– Need to do impedance matching to maximise SNR
Imperial College London – dept EEE 25
Autumn 2003
E302-AC4 InstrumentationAutumn 2010G1
Photovoltaic
IV Characteristic of PN junction
I
V
dark
Illuminated
Dark current
Open circuit Voltage
Imperial College London – dept EEE 26
Autumn 2003
E302-AC4 InstrumentationAutumn 2010G1
Piezoelectric
• Polarized Materials• Develop voltage proportional to strain
– Strain is fractional elongation• Applications
– Displacement transducers • BOTH sensors and actuators
Imperial College London – dept EEE 27
Autumn 2003
E302-AC4 InstrumentationAutumn 2010G1
Thermoelectric
• Two different types of material in contact– Metal– Semiconductor
• Sensors – Thermocouples
• Actuators– Heaters (forward bias)– Coolers (Reverse bias)
Imperial College London – dept EEE 28
Autumn 2003
E302-AC4 InstrumentationAutumn 2010G1
Electromagnetic
• Coils• Sensors
– Field sensors– Motion sensors
• Actuators– Motors
Imperial College London – dept EEE 29
Autumn 2003
E302-AC4 InstrumentationAutumn 2010G1
Sensor Types (2)• Passive (derive power from circuit)
– Variable “resistance”• Resistive temperature detectors (RTD, thermistors)• Magnetoresistance (Hall, GMR)• Photoconductors • Strain gauges
– Variable Reactance • Capacitance • Inductance
Model as a variable Impedance– Impedance a function of stimulus– Need to do impedance matching to maximise SNR
Imperial College London – dept EEE 30
Autumn 2003
E302-AC4 InstrumentationAutumn 2010G1
Temperature detectors
• Metals have positive R tempco: 2 3
0R R aT bT cT= + + + +… • Semiconductors have negative R tempco.
/geV kTI e−∝ • PN junctions have V at I const. prop to T
/eV kT Ts
kTVI I e rI eI
∂⇒ = =
∂
• BUT: IS is strong function of T =>need 2 PNs
Imperial College London – dept EEE 31
Autumn 2003
E302-AC4 InstrumentationAutumn 2010G1
Photoconductors
• PN junctions• Metal Semiconductor Metal• PIN• Avalanche PD• Photomultipliers
Imperial College London – dept EEE 32
Autumn 2003
E302-AC4 InstrumentationAutumn 2010G1
Photomultiplier
At each dynode the current is multiplied by an average factor m. The totalgain after n dynodes is G=mn . i.e. for each photoelectrons mn electrodes reach the anode. This number is noisy by mn/2, so that SNR=mn/2
Imperial College London – dept EEE 33
Autumn 2003
E302-AC4 InstrumentationAutumn 2010G1
Photomultipliers
Imperial College London – dept EEE 34
Autumn 2003
E302-AC4 InstrumentationAutumn 2010G1
Strain Gauge
• A thin wire will alter its cross/section will alter its resistance when under tension.
• To a first approximation R proportional to elongation dL/L
• by Hooke’s law, elongation is proportional to the force.
Imperial College London – dept EEE 35
Autumn 2003
E302-AC4 InstrumentationAutumn 2010G1
Hall sensor• Sensitive Magnetic Field Sensor • Use Semiconductor (low N)
HBV I R INe
= =
V+
V-
I
xx xyL L
yx yyH H
R RV IR RV I⎡ ⎤⎡ ⎤ ⎡ ⎤
= ⎢ ⎥⎢ ⎥ ⎢ ⎥⎣ ⎦ ⎣ ⎦⎣ ⎦
, , XX YY YX XY HL WR R R R RW L
ρ ρ= = = − =
Imperial College London – dept EEE 36
Autumn 2003
E302-AC4 InstrumentationAutumn 2010G1
Variable capacitance
• Fluid sensors: – C=C0h/H
fluidh
H
• Pressure sensors
• C=C0+a(P-P0)
P P0
Imperial College London – dept EEE 37
Autumn 2003
E302-AC4 InstrumentationAutumn 2010G1
Variable Inductance
• Position Displacement sensors:– Self inductance
• Move core or coil (microphone, motor)– Mutual Inductance
• Move two coils relative to each other (phono cartr.)
Imperial College London – dept EEE 38
Autumn 2003
E302-AC4 InstrumentationAutumn 2010G1
Strange things
• Fundamental physics often leads to excellent sensors
• A few examples– Quantum Hall Effect– Josephson Junction– SQUID
Imperial College London – dept EEE 39
Autumn 2003
E302-AC4 InstrumentationAutumn 2010G1
Quantum Hall effect• Strange universal behaviour• SI Standard for resistance!
( ) ( ) ( )2
25.8 , 0HhR n k R n
nne= = Ω =
Imperial College London – dept EEE 40
Autumn 2003
E302-AC4 InstrumentationAutumn 2010G1
Josephson Junction• Sandwich of two superconductors and insulator• Magnetic field detector• Extremely sensitive!
– Josephson made a 10fV voltmeter
Imperial College London – dept EEE 41
Autumn 2003
E302-AC4 InstrumentationAutumn 2010G1
Josephson Junction in RF• IV curve dramatically modified• Allows frequency generation –measurement• Steps V=Nhf !!!
Imperial College London – dept EEE 42
Autumn 2003
E302-AC4 InstrumentationAutumn 2010G1
The SQUID• Superconducting Quantum Interference Device• Ring of two Josephson Junctions• Ultimate magnetic field sensor (MEG!)
• , Φ0=h/2e=2.68 x 10-15 Wb
• Sensitivity: 10fT• Earth’s field: 50uT (Aurora borealis: 1uT)• Heartbeat currents: 50 pT• Brain currents : approx 100 fT
00
sinI I π Φ=
Φ
Imperial College London – dept EEE 43
Autumn 2003
E302-AC4 InstrumentationAutumn 2010G1
The SQUID as RF Detector• Excellent microwave receiver• Tuneable (by B)
Imperial College London – dept EEE 44
Autumn 2003
E302-AC4 InstrumentationAutumn 2010G1
Bridge Circuits• The Wheatstone Bridge• Zi can be R, L, C• Balance condition: Z1Z4=Z2Z3• Can be used for R, C, L, f measurement
R1
R2
R3
R4 Sense
ExcitationVcc
Imperial College London – dept EEE 45
Autumn 2003
E302-AC4 InstrumentationAutumn 2010G1
Bridge circuits (2)
• Can be electronically balanced:
R1
R2
R3
R4
+V
–V
RAVrefVBVA
Call V- =0, and then treat Vref and V+ by superposition.
Imperial College London – dept EEE 46
Autumn 2003
E302-AC4 InstrumentationAutumn 2010G1
Bridge circuits (3)• Example: simple capacitance bridge• R1 var R, R3 fixed R, R4 fixed C, R3 unknown• Cx=CREFRVAR/RREF
R1
R2
R3
R4 Sense
ExcitationVcc
Imperial College London – dept EEE 47
Autumn 2003
E302-AC4 InstrumentationAutumn 2010G1
Bridge Circuits (4)
• Use one branch as a sensor• Self calibration:
– Identical sensor w/o stimulous on adj. branch• Offset null!
– If possible interchange roles• Gain self calibration!
– Or use standards on reference branch• Example: ice water in T sensors
Imperial College London – dept EEE 48
Autumn 2003
E302-AC4 InstrumentationAutumn 2010G1
Bridge Circuits (4)
• Can use L and C together – Make freq. dependent bridge– Refer your measurement to time domain
• Resonant condition• Frequency Deviation from measurement.• MUCH more accurate• MUCH higher resolution
– Extensive lit. on bridges. READ it !(would be half a course by itself)
Imperial College London – dept EEE 49
Autumn 2003
E302-AC4 InstrumentationAutumn 2010G1
Conclusion• Think of the application• Don’t solve a bigger problem than needed• Work out realistic specifications• Be cautious of marketing claims of suppliers• Account for non-idealities:
– Nonlinearity– Offset– Non-monotonicity
• Model sensors as controlled sources: Thevenin, Norton• Include Noise (next topic)• Model other effects: Time response. Complex Z or Y
Imperial College London – dept EEE 50
Autumn 2003
E302-AC4 InstrumentationAutumn 2010G1
Conclusion (2)• Measure by comparison:
– Use Bridges– Use Standards
• Reduce to a time measurement– Easier– More accurate (more later in course)
• For difficult measurements MODULATE or• TALK to an expert to see if better sensor exists :
– A Physicist– A Chemist– A Mechanical Engineer– A Chemical Engineer
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E302-AC4 InstrumentationAutumn 2010G1
CONCLUSION
Think of the system!
Circuit Design solves only
part of the problem!
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