Useful Signal
When converting a signal to a quantity, it
is only useful if its representation is kept
unchanged within a knew error
Signal conditioning and its transmission is
very important in applied physics
Electronic
Instrumentation
European PhD – 2009/2011
Transducers and Signal Conditioning
Horácio Fernandes
Electronic Instrumentation, PhD 2009/2011
Sensors and Transducers
Signal paths
Preserve signal quality
DAS less demanding
Preserve and adjust dynamic bandwidth
Resize operational limits
Device
capable of changing one form of energy into
another
Offset
Amplitude
Bandwith
Transducers
Active – External power supply
Passive – Internal source (self-generating)
Sensors
Changing
of a characteristic in an electric circuit (R;
L, C);
Generate an output signal proportional to the stimulus
Linearization
Galvanic isolation
Buffering
Electronic Instrumentation, PhD 2009/2011
Electronic Instrumentation, PhD 2009/2011
Sensors
Transducer Principles
Sensores
Related energy
Example
Comment
Mechanic
Flow-meter
Strain gauge
Pulse counter
Momentum transfer
Thermal
Thermocouple
Thermal radiation
Junction voltage
Infrared sensor
Electromagnetic
Antenna
Space electromagnetic power converted to
electric signals
Magnetic
Hall sensor
MDH Probes
Voltage derived from Hall Effect
Induced EMF
Chemic
pH sensor
Ionic concentration
Nuclear
Ionization chamber
Scintillators
Current generation induced by free charges
Indirect light proportional to brehmstrallung
Electronic Instrumentation, PhD 2009/2011
Resistive
Strain
gauges: Force measurements (W.
Bridges)
Temperature: RTDs, termistors
Light: photoelectric cells and photodiodes
Position: potentiometers as dividers, grids
Electronic Instrumentation, PhD 2009/2011
Sensors
Transducer Principles
Capacitive
Sensors Selection
Scale: limiting extremes (Worst Case)
Threshold
Behavior
multiples
Movement
Dielectric constant
Geometric configuration
Cell
chargers
Inductive
Least
sensors for scale spanning
detected variation (resolution)
Temporal
response
response
Accuracy and resolution
Stress (consistency)
Reproducibility and hysteresis
LVDT
–Differential
Transformer
Hall Effect
Motors as generator
Dynamic
Price
Electronic Instrumentation, PhD 2009/2011
Sensors Operation
Environment
Electronic Instrumentation, PhD 2009/2011
Sensors Operation
Human use
Dirty
Radiation
Pollution
Corrosion/Chemicals
Extreme
Immersion
Temperatures
Water presence and moist
Chemical corrosion: solvers, acids e bases
Environmental protection
Susceptibility: electric/explosion/crash
Erosion/Vibrations
Explosion
Electric
Interference (EMI- high impedance,
low current)
Electronic Instrumentation, PhD 2009/2011
Sensors Operation
Power
manipulation
Electronic Instrumentation, PhD 2009/2011
Calibration
Measurement Error – Comparison
standard should be more exact than
sensor resolution
Calibration table – Calibration curve
Circuit
Charger (photocell)
Excitation source (noise)
Signal Conditioning
Physics size
Electronic Instrumentation, PhD 2009/2011
Physic
model
and dynamic calibration
Bandwidth
Impulsive response
Static
Electronic Instrumentation, PhD 2009/2011
Linearization
Buffering
Transfer function
errors
Impedance
Non-linearity
adaptation
Maximum feed power
Voltage signal
Sensor
Electronics
Signal path
Compensation
Non-linear
Source/Input isolation
Transducer output
Preserve
electronic
signal
Next stage charge circuit
circuit
Piecewise
interpolation
Electronic Instrumentation, PhD 2009/2011
Meters and bridges
Electronic Instrumentation, PhD 2009/2011
Wheatstone Bridge
Differential mode
Common mode
Potentiometer divider
Zero Measurement
CMR>100 dB
Sensibility
Thermal immunity
Rx =
Ra ⋅ Rb
Rc
Electronic Instrumentation, PhD 2009/2011
Wheatstone Bridge
Application
Electronic Instrumentation, PhD 2009/2011
Kelvin bridge
Very low resistors
(<1R)
Double terminals
Electronic Instrumentation, PhD 2009/2011
Electronic Instrumentation, PhD 2009/2011
Maxwell Bridge
Bridges Configurations
Electronic Instrumentation, PhD 2009/2011
Electronic Instrumentation, PhD 2009/2011
Bridges circuits
Bridge noise immunity
AC generators
Current sources
OPAMPs applications
Pick-up noise
Cable resistance
Signal Bandwidth
3-wire connection
Electronic Instrumentation, PhD 2009/2011
Noise reduction
…If noise blocking fails in the origin…
…Nightmare begins!
Electronic Instrumentation, PhD 2009/2011
Electronic Instrumentation, PhD 2009/2011
What can we filter?
Signal sampling: analog goes digital at
what rate?
Nyquist criteria: fs>2fmax
Low-pass filters (cutoff -40 dB)
Guard-band
Sampling band: [fs-fmax, fs+fmax]
Electronic Instrumentation, PhD 2009/2011
Useful Storage Bandwith
Aliasing
Pratical figures
USB=fs/2.5
Sin Interpolation
USB=fs/10
Linear Interpolation
USB=fs/25
No Interpolation
USB=fs/4.6
Digital correction (factor 1.6) x (1/0.35)
Electronic Instrumentation, PhD 2009/2011
Perceptual aliasing
Electronic Instrumentation, PhD 2009/2011
Image aliasing
Electronic Instrumentation, PhD 2009/2011
Moiré patterns
Electronic Instrumentation, PhD 2009/2011
Filters
Pass-band
Cut-off
Stop-band
Ripple
Order
Phase and amplitude
characteristics
Electronic Instrumentation, PhD 2009/2011
Electronic Instrumentation, PhD 2009/2011
W 1: TENSILE.1.STRAINRATE
W 2: Linear Regression
-1.5
Common Filters
4
-2
0
-2.5
-4
-3
-8
Lowpass
Highpass
Passband
Notch
Digital filtering (made possible with fast ADCs)
-3.5
-12
-4
0
20
40
60
80
100 120 140 160 180 200 220 240 260 280
20
40
60
80
High order, cutoff frequency, complex
transforms
Signal correlation
W 4: Both Fits and Data
4
0
-2
0
-4
-4
-6
-8
-8
-10
-12
-12
0
20
40
60
80
100 120 140 160 180 200 220 240 260 280
0
20
40
60
80
6
6
4
4
2
2
0
0
-2
-2
-4
-4
-8
-6
0
20
40
60
80
100 120 140 160 180 200 220 240 260 280
Electronic Instrumentation, PhD 2009/2011
Other Techniques
Periodic signals
0
20
40
60
80
100 120 140 160 180 200 220 240 260 280
Electronic Instrumentation, PhD 2009/2011
Other Techniques
amplifiers
Pulsed signals
Constant Fraction
discriminator
Cross correlation
Boxcar
100 120 140 160 180 200 220 240 260 280
W6: Polynomial Fit Residuals
8
-6
Lock-in
100 120 140 160 180 200 220 240 260 280
2
W5: Linear Fit Residuals
Advantages:
0
W3: 13th Order Polynomial Fit
integration
mean
Multichannel
Overlap of periodic signals
S/N~N1/2
Electronic Instrumentation, PhD 2009/2011
Transimpedance amplifier
Allow very low current
sources detection, ex:
photodiodes
Spectrometers
Line
I(V) probes
Current detection
Ground
loop
- galvanic isolation
Sweep waveforms – capacitive coupling and
distortion
Fast sweeping – plasma limit operation
Safety
Tomography
Electronic Instrumentation, PhD 2009/2011
radiation filters
Charge measurements, ex:
ion beam
High bandwidth
Electronic Instrumentation, PhD 2009/2011
Electronic Instrumentation, PhD 2009/2011
DAC SinC
Sin(x)/x correction
Signal should be <
fnyquist
Images present at
high frequency
Electronic Instrumentation, PhD 2009/2011
Summary
Highlights for a “orientated” search
Adequate front-end electronics with
sensors
Specified a sensor/diagnostic
Avoid noise – ground loops
Earth vs. ground (signal frequency &
impedance)
Electronic Instrumentation, PhD 2009/2011
Electronic Instrumentation, PhD 2009/2011