Published on September 23, 2014
1. SIGNAL CONDITIONING AEAV 311 Lecture 25-26
2. SIGNAL CONDITIONING • AC-DC signal conditioning – Process, Categories – Amplification: Op Amp : Instrumentation Amplifier • Noise – Signal to noise ration – Sources of noise – Noise figure & noise factor – Numerical • AD-DA Conversion Techniques – Resolution – Quantization – Sampling, S/H Circuits – Numerical
3. INSTRUMENTATION OR MEASUREMENT SYSTEM • A device or a system which is designed to maintain a functional relationship between a prescribed property of a substance and physical variable and communicates to human observer. Medium
4. SIGNAL CONDITIONING • A transducer detects a measurand and converts it into electrical signal. If signal is not strong enough to be displayed by the display devices (which is mostly the case), the signal needs to be conditioned before it can be transmitted and displayed at final stage. Medium
5. SIGNAL CONDITIONING • Signal conditioning element in many cases is simply excitation & amplification system for passive transducers (PT). • Excitation is needed for PT as they do not generate their own voltage or current. • Excitation is not needed for active transducers-thermocouple, inductive pr switch etc as they produce their own voltages by application of external physical quantity. However, they need amplification. • Categories of Signal Conditioning – Linear Processes: Amplification (op-amp instrumental amplifier), math operations viz addition, subtraction, integration , differentiation etc – Non-Linear Processes: Modulation, Demodulation, sampling, filtering, clipping/ clamping , ADC, DAC etc
6. SIGNAL CONDITIONING I/P Stage O/P Stage • Wheatstone bridge & amplifier is excited by external source. • It is balanced using a pot meter . • Bridge is calibrated to indicate unbalanced condition. • Main challenge thermal noise, noise amplification • O/P stage : DC network and LPF before it is ADC for display • For ease of amplification, application of integrated circuits rather than discrete components. Application of Op Amps- low power, low cost, small size, ease of ADC.
7. OPERATIONAL AMPLIFIER • Characteristics & configuration of Op-Amp CMRR, amplification, freq characteristics-slew rate etc. IN, NI configurations Math applications: Summing, Averaging, Scaling, Integrating, differentiating, Subtractor, Comparator – Instrumentation Amplifier
8. INSTRUMENTATION AMPLIFIER • Dedicated differential amplifier with very high input impedance • Stable gain with low temperature coefficient • Low output impedance and low Dc offset Output voltage V out = (V4 –V3) DIFFERENCE FROM ORDINARY OP-AMP • Packaged assembly • Internally wired with accurate and stable F/B resisters. • Gain can be fixed to precise value by adjusting single external resister R1 . • Drift normalized by manufacturer hence no external circuit required. R2 R1 R4 R3 R5 R6 R7 1 2 3
9. INSTRUMENTATION AMPLIFIER Applying superposition theorem to Op-Amp 1 R4 (a) Keeping terminal 2 grounded. R2 Op-Amp reduces to Non-inv config. R1 V= ( 1+ R/R) V---------------1 32 11 R3 R5 R6 R7 1 2 3 (b) Keeping V1 grounded, input is potential available at junction 2. V2 and jn 2 will be approx at same potential due to virtual ground. Hence, Op-amp 1 is in Inv. config V3= – (R2 / R1) V2 ---------------2 (c) On applying superposition theorem to op-amp- 1, using eqn 1 & 2 V3= (1+ R2 /R1) V1 - (R2 / R1) V2 (d) Similarly at Op-Amp-2 V4= (1+ R 3/R 1) V2 – (R3/ R 1) V1 Let R2=R3 & R4=R5=R6=R7= R Op-amp 3 is in diff. ampli config V out = -(V3 –V4) V out = [1+2 (R2/ R1)] (V2 –V1) • Here, all resistors except R1 are internal to Inst Ampli. R1 is adjustable gain resistor
10. PRACTICAL CONSIDERATION IA & BRIDGE RECTIFIER
12. NOISE • Spurious current or voltage extraneous to the signal in a circuit. • It does not convey any useful information. • Source can be both internal or external to the system. • Example: Hum in a radio broadcast, static cracking in communication channel, snow or confetti in digital television broadcast. 09/23/14 16
13. SIGNAL TO NOISE RATIO (SNR) • Ratio of desired signal to unwanted noise S/N = (Signal Power watts or dB / Noise Power watts or dB) = ( Signal power in volts)2/ Noise power in volts)2 • Desirable to have SNR as large as possible. • Extent to which noise in acceptable in measurement depends on relative value of noise, signal and purpose of measurement 09/23/14 17
14. SOURCES OF NOISE 1. Generated Noise: When the source of noise are internal or intrinsic to the circuit of operation. Active as well as passive elements- resistors, capacitors, transistors are possible sources of noise. 2. Conducted Noise: Power supply to amplifier could be source of noise since it may have spikes, ripples or random deviations that are conducted to amplifier and amplified. This is extrinsic to the system i.e outside the circuit of operation. 50 Hz power supply has harmonics that are carried along with the signal to the amplifier. 3. Radiated Noise: EM radiations or fields that disturb the circuit operation . Unwanted signals are radiated into the internal to circuit. Base for EW, Multi-color pattern on TV screen by keeping a speaker close to TV, interference in avionics by in-flight use of electronic devices. 09/23/14 18
15. GENERATED NOISE: WHITE NOISE • (a) Thermal Noise/ White Noise/ Johnson’s Noise: Generated by random motion of electrons in resistors due to thermal energy. Atoms contribute conduction electrons due to which current flows. • Atoms are in rapid vibrations due to thermal or temp effect. Its transferred to electrons, thereby producing noise component in current. • Wideband spectrum of noise, independent of freq of input signal. Eg noise in band of 100 to 200 Hz is same as in 1000 to 1100 Hz • Noise has no defined peak amplitude value or peak freq value. • Discovered by John B Johnson, 1926, Bell Labs 09/23/14 19
16. GENERATED NOISE: WHITE NOISE (a) Thermal Noise or Johnson’s Noise: Noise power Pn= kT Δf : k= Boltzmann Constant Thevienien eq. ckt (1.38x10 -23 J/0K) Δf= Noise bandwidth, Hz T= temp, kelvin Rn + En RL 09/23/14 20
17. GENERATED NOISE: SHOT NOISE • (b) Shot Noise: Generated by internally by short time electrical events with in active devices. • In active devices electrons charge across pn junction, therefore move from one energy level to another. • Number of charge carrier define current in pn junction device. The no. of carrier per unit time fluctuate about average value, hence current noise or shot noise. • Discovered in 1918 by Schottky • Shot Noise RMS current Amps, rms e= charge of electron, coulomb Δf= noise bandwidth, Hz IDC= DC current flow, amps, 09/23/14 22
18. GENERATED NOISE: SHOT NOISE • (b) Shot Noise: BJT and semiconductor diodes generate shot noise, but there is not shot noise is FETs as there is no potential difference across the barrier across which carriers flow. • Shot noise Spectral Density: Noise power or noise current per unit of noise bandwidth A 2 / Hz e= charge of electron, coulomb Δf= noise bandwidth, Hz IDC= DC current flow, amps, Ist= Shot noise RMS current, amps 09/23/14 23
19. OTHER GENERATED NOISES • (c) Avalanche Noise: Similar to shot noise. Happens in Zener Diode circuits when they are biased in breakdown region. • Avalanche noise is much greater than shot noise for same IDC, as volume of charge carrier crossing breakdown region are large. • (d) Contact Noise: Happens due to fluctuating resistances, at imperfect contacts between elements at a junction in a circuit. • Occurs at contacts in relays, switches, resistors, diodes, BJTs, FETs. • Also known as Flicker Noise or Low Freq Noise. 09/23/14 24
20. SOURCES OF NOISE Under MPT Rn=RT & max noise power will be delivered to load P 09/23/14 25
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