Musical Chairs and Magic Carpets

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MAS836 Sensor Technologies for Interactive
Environments
Second Nature Sensor Conditioning Electronics
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Reference Sources

• Jacob Fraden
• AIP Handbook of Modern Sensors, gt2nd Edition
• Ramon Pallas-Areny and John G. Webster
• Sensors and Signal Conditioning, 2nd 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

• 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.ht
  ml
• Robotics sites often list sensor vendors, hints
• http//www.billbuxton.com/InputSources.html
• Bill Buxtons encyclopedia on input devices

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Some Classic Sensor Module Sources

• 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
• 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
Zi
Zo
Io
Zi
i
Wants Low Zi
Wants High Zi
Vo

• Sensors produce different kinds of signals
• Voltage output or current output
• Cant 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 and Ri as a voltage
  divider on Vout , calculate the voltage at the
  OpAmps noninverting pin
• Note that this assumes a low-impedance VIN
  (source impedance sums with Ri)

T
T
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Negative Feedback

• 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
  microcomputers 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
  Ris
• 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 dividers output and use it
everywhere...
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Biasing an entire circuit with a Buffered Voltage
Bias Buffer
AC Coupling Capacitor
X10 inverting stage
X10 inverting stage
X11 noninverting stage
A 60 dB (x1100) high-impedance, AC-Coupled
amplifier with bias made from a quad OpAmp
AC Coupling Capacitor (decouple accumulated
offset errors)
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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 arent 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

• 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/(2pRC)
• Passive HP filter RC network fc 1/(2pRC)

-3dB 0.707
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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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Low Pass Filter Responses
Fr. Active Filter Cookbook
Response set by adjusting Rs and Cs
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Or just run an applet

• Analog Devices, etc.

http//www.analog.com/en/amplifiers-linear/product
s/dt-adisim-design-sim-tool/filter_wizard/resource
s/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 lt fs/2
• Bandlimited (demodulation) sampling
• Dfin lt fs/2
• Loose absolute phase information
• Dont 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
t
Capacitor holds peaks! Need reset switch to
continue tracking
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Pulse Stretcher
Vs
C
R

• Resistor continually (and slowly) bleeds
  capacitor charge
• Automatic reset
• Tune time constant to match signal dynamics (so
  peaks are always followed)

Vo
e-t/RC
t

• Enables lazy sampling to catch transients

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Analog Multiplexers