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MAS836 – Sensor Technologies for Interactive Environments
Second Nature – Sensor Conditioning Electronics
2/04
JAP
Reference Sources
• Jacob Fraden
– AIP Handbook of Modern Sensors, >2’nd Edition
• Ramon Pallas-Areny and John G. Webster
– Sensors and Signal Conditioning, 2’nd Edition
• Thomas Petruzzellis (getting old…)
– The Alarm, Sensor, & Security Cookbook
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Auxilary References (signals)
• Ramon Pallas-Areny & John G. Webster
– Analog Signal Processing
• Paul Horowitz & Winifield Hill
– The Art of Electronics
• Don Lancaster (online sources)
– Active Filter Cookbook
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Magazines
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Sensors Magazine - Free!
Circuit Cellar - Best EE-hacker magazine out
NASA Tech Briefs - Free! (still there?)
Test and Measurement - Free!
IEEE Sensors Journal
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Websites
• http://www.sensorsportal.com/
– References, hints, sources
• http://www.sensorsmag.com/
– Sensors Magazine site
• Buyers guide, Archive articles
• http://www.cs.cmu.edu/~chuck/robotpg/robofaq/10.html
– Robotics sites often list sensor vendors, hints
• http://www.billbuxton.com/InputSources.html
– Bill Buxton’s encyclopedia on input devices
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Some Classic Sensor Module Sources
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http://www.parallax.com/
http://www.sparkfun.com/
http://www.ramseyelectronics.com/
http://www.adafruit.com/
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Basic Sources for Electronics
Digikey - www.digikey.com
Mouser - www.mouser.com
Newark
Allied
Hosfelt Electronics
JameCo
Mat Electronics
JDR
All Electronics
Radio Shack (mainly online now)
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Trading Modality
• Sensor modes are intrinsically synaesthetic
• Use physics and constraints to couple a
measured quantity into an unknown
– Temperature can infer wind velocity (heat loss)
– Displacement can infer:
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Pressure (with an elastomer or spring: F = kx)
Volume of fluid in a tank (V = Ah)
Velocity (2 measurements at different times: v = dx/dt)
Temperature (thermometer level)
Angle from vertical (displacement of a bubble)
– Measurements are used with a mathematical model
to derive other parameters
• Estimation and Kalman Filtering, etc.
– Not covered here...
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Signal Conditioning
Zo
i
i
Wants
High Zi
Zi
Zo
Io
Zi
Vo
Wants
Low Zi
• Sensors produce different kinds of signals
– Voltage output or current output
– Can’t necessarily take sensor output and put right
into microprocessor ADC or logic input
– Signal may need:
• High-to-low impedance buffer, current-to-voltage
conversion, gain, detection, filtering, discrimination...
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The Comparator
• Makes an analog signal into a 1-bit digital signal
– Directly drives logic pin on microprocessor
• Detects when signal is above threshold
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The Schmidt Trigger
Deadband
• Suppresses jitter and spurious triggering from noisy signals
• Deadband thresholds, V+ and V-, can be calculated via superposition
• Ground VIN, and with Rf Tand Ri asT a voltage divider on Vout , calculate the
voltage at the OpAmp’s noninverting pin
• Note that this assumes a low-impedance VIN (source impedance sums with Ri)
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Negative Feedback
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Transimpedance Amplifier
Voltage Follower
Non-Inverting Amplifier
Inverting Amplifier
Inverting Summer
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The Voltage Follower
• A unity-gain buffer to enable high-impedance
sources to drive low-impedance loads
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The Non-Inverting Amplifier
• Like voltage follower, but gives voltage gain
– Gain can be adjusted from unity upward via resistor ratio
– High-Z input is good for conditioning High-Z sensors
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The Transimpedance Amplifier
• Converts a current into a voltage
– Generates a proportional (w. Rf) voltage from an input current
– Produces a low-impedance output that can drive a
microcomputer’s A-D converter, for example
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The Inverting Amplifier
• Inverts signal, voltage gain varies from zero upward with the ratio of two resistors
– Extension to summer is trivial with additional Ri’s
– Input impedance is not infinite: Zin = Ri
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The Summing Amplifier
• No crosstalk between inputs because of virtual ground
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Biasing
• AC Coupling
• Biasing noninverting input
• Biasing at inverting input
Buffer the voltage divider’s output and use it everywhere...
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Biasing an entire circuit with a Buffered Voltage
Bias Buffer
AC Coupling
Capacitor
X11 noninverting
stage
X10 inverting
stage
AC Coupling Capacitor
(decouple accumulated
offset errors)
X10 inverting
stage
A 60 dB (x1100) highimpedance, ACCoupled amplifier with
bias made from a quad
OpAmp
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The Simple Differential Amplifier
• Subtracts two input signals
– Input resistors must be equal, feedback and shunt resistors must be equal
– Provides voltage gain
• The input impedances aren’t equal, however
– The amplifier is unbalanced!
• A high-impedance sensor will produce common-mode errors (e.g., the system will be
sensitive to the common voltage)
• Differential sensors will be more sensitive to induced pickup signals (which tend to be
high impedance)
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The Basic Instrumentation Amplifier
• Buffer each leg of the differential amplifier
by a voltage follower
– Impedance is now extremely high at both inputs
– Impedance can be set by a shunt resistor across inputs
– This is a balanced “instrumentation” amplifier
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The Three-OpAmp Instrumentation Amplifier
• Gain is varied by changing only one resistor, R1
– No need to re-trim other components for a gain change
– Gain at first stages is better for signal/noise
– This is the instrumentation amplifier of choice
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Commercial Instrumentation Amplifiers
INA2321
500 kHz, 94 dB CMRR, R-R, µA sleep
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Analog Devices AD623
Analog Devices AD AMP01
BurrBrown (TI) INA series (INA2321)
TI TLC271
Can be fairly slow, but precise DC properties, low drift, high
gain, well matched
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Passive RC Filters
• Passive LP Filter: RC network: fc = 1/(2πRC)
-3dB = 0.707
• Passive HP filter: RC network: fc = 1/(2πRC)
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Passive RC Filter Rolloff
Bode Plot:
Freq. Response as a log-log plot
Rolloff is 6 dB per Octave (2x)
20 dB per Decade (10x)
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The First-Order Active High Pass Filter
• Low impedance drive
• Voltage gain via Rf/Ri
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The First-Order Active Low Pass Filter
f
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The Band-Select Filter
• Cascaded high and low pass filters
– Always follow high-pass with low-pass (noise)
• Low-Pass cutoff needs to be below high-pass cutoff!
– No Q, first-order rolloffs
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Sallen-Key Filters – Ref. Active Filter Cookbook
VCVS Filters
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Multiple Feedback Bandpass
Single-OpAmp VCVS BP filter
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Fr. Active Filter Cookbook
JAP
Low Pass Filter Responses
Response set by adjusting R’s and C’s
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Or just run an applet…
• Analog Devices, etc.
http://www.analog.com/en/amplifierslinear/products/dt-adisim-design-simtool/filter_wizard/resources/fca.html
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Picking an OpAmp
High-Level Tree (AD)
OLD
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Picking a Particular OpAmp
Low-Level Tree (AD)
OLD
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Picking a Particular OpAmp
Interactive Parametric Search (AD)
CURRENT
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Sampling
• Nyquist: fin < fs/2
• Bandlimited (demodulation) sampling
– Dfin < fs/2
– Loose absolute phase information
• Don’t know whether phase moves forward or backward
– Quadrature sampling
• Bandlimited sampling at t and a quarter-period later
• Form the “Analytic Signal”
– I.E., the Quadrature (complex) Amplitude
• Can also do this with multipliers and quadrature
demodulation
– Synchronous undersampling for periodic signals
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Peak Detector
Vs
t
Vo
Capacitor holds peaks!
Need reset switch to continue tracking
t
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Pulse Stretcher
Vs
C
R
-Resistor continually (and
slowly) bleeds capacitor charge
Vo
e-t/RC
-Automatic “reset”
-Tune time constant to match
signal dynamics (so peaks are
always followed)
-Enables “lazy” sampling to catch transients
t
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Analog Multiplexers
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