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
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.
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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.
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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.
18. 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
19. 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
20. 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
21. SOURCES OF NOISE
Under MPT
Rn=RT & max noise power
will be delivered to load
P
09/23/14 25
