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

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

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

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

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

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

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

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

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

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

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

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

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

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

Meters and bridges Differential mode Common mode

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

Wheatstone Bridge Application

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

Maxwell Bridge

Bridges Configurations

Bridges circuits AC generators Current sources OPAMPs applications

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

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

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

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)

Aliasing

Perceptual aliasing

Image aliasing

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

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

Time offset in fluctuation studies

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.

Other Techniques Pulsed signals Constant Fraction discriminator

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

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