1. Electronic Instrumentation
European PhD – 2009
Transducers and Signal Conditioning
Horácio Fernandes

2. 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 transmission is very important in applied physics

3. Signal paths Preserve signal quality DAS less demanding
Preserve and adjust dynamic bandwidth
Resize operational limits
Offset
Amplitude
Bandwith
Linearization
Galvanic isolation
Buffering

4. Sensors and Transducers
Device capable of changing one form of energy into another
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
Basically a sensor is a transducers where the output is electric

5. 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

6. Sensors Transducer Principles
Resistive
Strain gauges: Force measurements (W. Bridges)
Temperature: RTDs, termistors
Light: photoelectric cells and photodiodes
Position: potentiometers as dividers, grids

7. Sensors Transducer Principles
Capacitive
Movement
Dielectric constant
Geometric configuration
Cell chargers
Inductive
LVDT –Differential Transformer
Hall Effect
Motors as generator

8. Sensors Selection Scale: limiting extremes (Worst Case) Threshold
multiples sensors for scale spanning
Threshold
Least detected variation (resolution)
Behavior
Temporal response
Dynamic response
Accuracy and resolution
Stress (consistency)
Reproducibility and hysteresis
Price

9. Sensors Operation Environment Dirty Pollution Extreme Temperatures
Water presence and moist
Chemical corrosion: solvers, acids e bases
Environmental protection
Susceptibility: eletric/explosion/chash

10. Sensors Operation Human use Radiation Corrosion/Chemicals manipulation
Immersion
Erosion/Vibrations
Explosion
Electric Interference (EMI- high impedance, low current)

11. Sensors Operation Power Signal Conditioning Physics size
Circuit Charger (photocell)
Excitation source (noise)
Signal Conditioning
Physics size

12. Calibration
Measurement Error – Comparison standard should be more exact than sensor resolution
Calibration table – Calibration curve
Physic model
Static and dynamic calibration
Bandwidth
Impulsive response

13. Linearization Transfer function errors Compensation Non-linearity
Sensor
Electronics
Signal path
Compensation
Non-linear electronic circuit
Piecewise interpolation

14. Buffering Source/Input isolation Transducer output
Impedance adaptation
Maximum feed power
Voltage signal
Transducer output
Preserve signal
Next stage charge circuit

15. Meters and bridges
Differential mode
Common mode

16. Wheatstone Bridge Potentiometer divider Zero Measurement CMR>100 dB
Sensibility
Thermal immunity

17. Wheatstone Bridge Application

18. Kelvin bridge
Very low resistors (<1R)
Double terminals

19. Maxwell Bridge

20. Bridges Configurations

21. Bridges circuits
AC generators
Current sources
OPAMPs applications

22. Bridge noise immunity Pick-up noise Cable resistance Signal Bandwidth
3-wire connection

23. Noise reduction …If noise blocking fails in the origin…
…Nightmare begins!

24. 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]
guard band is an unused part of the radio spectrum between radio bands

25. Useful Storage Bandwith
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)

26. Aliasing

27. Perceptual aliasing

28. Image aliasing

29. Filters Pass-band Cut-off Stop-band Ripple Order
Phase and amplitude characteristics

30. Common Filters Lowpass Highpass Passband Notch
Digital filtering (made possible with fast ADCs)
Advantages: High order, cutoff frequency, complex transforms
Signal correlation

31. Time offset in fluctuation studies

32. Other Techniques Periodic signals Lock-in amplifiers
Cross correlation
Boxcar integration
Multichannel mean
Overlap of periodic signals
S/N~N1/2
Operation of a lock-in amplifier relies on the orthogonality of sinusoidal functions - locka a fase conhecida a frequencia tornando a media nula do ruido
Boxcar (fig) - signal which has a constant phase relationship to a synchronizing pulse.

33. Other Techniques
Pulsed signals
Constant Fraction
discriminator

34. Transimpedance amplifier
Allow very low current sources detection, ex: photodiodes
Tomography
Spectrometers
Line radiation filters
Charge measurements, ex: ion beam
High bandwidth
~10 pA

35. I(V) probes Current detection Ground loop Safety - galvanic isolation
Sweep waveforms – capacitive coupling and distortion
Fast sweeping – plasma limit operation
I(V) probe