EMI Basics
EMI Basics
A. TRANSDUCERA measuring device which measures and converts
nonelectrical variable into electrical variable is known as
transducer. Transducers are classified into five types. They are,
Classification on the basis of transduction principle used. Active
and passive transducers Analog and digital transducers Primary and
secondary transducers Transducers and inverse
transducers.Classification on the Basis of Transduction Principle
UsedThis classification is done depending on the transduction
principle i.e., how the input variable is being converted into
capacitance, resistance and inductance values. (These are named as
capacitive transducer, resistive transducer and inductive
transducer respectively).Examples of Capacitive
TransducerApplications
1. Dielectric gaugeIt is used to measure: Thickness - Liquid
Level
2. Capacitor MicrophoneIt is used to measure: Noise -Speech and
music
Examples of Resistive TransducerApplications
1. Resistance ThermometerUsed to measure Temperature and Radiant
Heat
2. Potentiometer DeviceUsed in Displacement and Pressure
Measurement
Examples of Inductive TransducerApplications
1. Reluctance PickupMeasures Pressure, Viberations, Position and
Displacement
2. Magnetostriction GaugeMeasures Sound, Force and Pressure
Active and Passive TransducersThe transducer which does not
requires any external excitation to provide their outputs are
referred as active transducer. The transducer which requires an
external excitation to provide their output is referred as passive
transducer.Examples of Active transducerApplications
1. Photo Voltaic CellUsed in Light Meters and Solar Cells
2. ThermocoupleUsed to measure Temperature, Radiation and
Heatflow
Examples of Passive transducerApplications
1. Capacitive TransducerMeasures Liquid Level, Noise, thickness
etc
2. Resistive TransducerMeasures Temperatue, pressure,
displacement
3. Inductive TransducerMeasures Pressure, Viberation,
displacement etc
Analog and Digital TransducersThe transducer which produces
their outputs in analog form or a form which is a continuous
function of time is referred as analog transducer. The transducer
which produces their outputs in digital form or a form of pulses is
referred as digital transducers.Examples of Analog
TransducerApplications
1. Strain GaugeMeasures Displacement, Force, Torque
2. ThermistorMeasures Temperature and Flow
Examples of Digital TransducerApplications
1. Turbine meterUsed in flow measurement
Primary and Secondary TransducersThe transducer which sends the
measurement and converts them into another variables (like
displacement, strain etc.) and whose output forms the input of
another transducer is called as primary transducer. The transducer
which converts the output of first transducer into an electrical
output called secondary transducer.Examples of Primary
TransducerApplications
1. Bourdon tubeUsed in pressure
2. Strain gaugeUsed in measurements
Examples of Secondary TransducerApplications
1. LVDTUsed to measure Displacement, Force, Pressure and
Position
Transducers and Inverse TransducersA measuring device which
measures and converts nonelectrical variable into electrical
variable is known as transducer. A measuring device which measures
and converts an electrical variable into nonelectrical variable is
known as inverse transducer.Example of TransducerApplications
1. ThermocoupleUsed to measure Temperature Radiation and Heat
flow
Example of Inverse TransducersApplications
1. Piezo-electric crystalUsed to measure Pressure, Vibration and
acceleration
B. PARAMETERS CONSIDERED IN SELECTING A TRANSDUCERParameters to
be considered in the selection of a transducer for a particular
application are.1. Operating Principle: Basically the transducers
are selected based on their operating principle. Examples of
operating principles used by the transducers are resistive,
capacitive, piezoelectric, inductive, up to electronic principle
etc.2. Operating Range: This factor is considered so that the
transducer should be able to function within the specified range
with good resolution. Every transducer should be provided with some
rating within which there will be breakdown in its function.3.
Accuracy: It is one of the most desired characteristic of any
transducer. If the transducer doesn't needs frequent calibration,
it must have high degree of accuracy and repeatability. Because
errors may occur due to the sensitivity of the transducer to other
stimulations.4. Sensitivity: It is also a desired characteristic of
a transducer. Every transducer should be sufficiently sensitive to
provide some output that can be sufficient and detectable.5.
Stability and Reliability: The transducer should have high degree
of stability during its function and also storage life. It should
also have a high degree of reliability.6. Usage and Ruggedness: The
ruggedness, size and weight of a transducer should be chosen
depending on the application in which it is used.7. Transient
Response and Frequency Response: The transducer should have
required time domain specifications such as, settling time, rise
time, peak over shoot and small dynamic error etc.8. Loading
Effects: The transducers should undergo minimum loading effect so
that if can provide accurate measurement. The parameters of a
transducer are that, which characterize the loading effect is its
input and output impedances. lt is considered in order to get
minimum loading effects (Which can be neglected). For minimum
loading effect the transducer should have low output impedance and
high input impedance.9. Electrical Parameters: The type and length
of cable required, signal to noise ratio in case the transducer is
used with amplifiers and frequency response limitations should also
be considered.10. Ability to be insensitive to unwanted signals 11.
Environmental compatibility.12. Static Characteristics: The
selected transducer should have low hysteresis, high linearity and
high resolution.C. WORKING OF SEMICONDUCTOR STRAIN GAUGE AND ITS
ADVANTAGESA typical semiconductor strain gauge is formed by the
semiconductor technology i.e., the semiconducting wafers or
filaments of length varying from 2 mm to 10 mm and thickness of
0.05 mm are bonded on suitable insulating substrates (for example
Teflon). The gold leads are usually employed for making electrical
contacts. The electrodes are formed by vapour deposition. The
strain sensitive elements used by the semiconductor strain gauge
are the semiconductor materials such as silicon and germanium. When
the strain is applied to the semiconductor element a large of
change in resistance occur which can be measured with the help of a
wheatstone bridge. The strain can be measured with high degree of
accuracy due to relatively high change in resistance. A temperature
compensated semiconductor strain gauge can be used to measure small
strains of the order of 10-6 i.e., micro-strain. This type of gauge
will have a gauge factor of 130 10% for a semiconductor material of
dimension 1 x 0.5 x 0.005 inch having the resistance of 350 .
Advantages of Semiconductor Strain Gauge The gauge factor of
semiconductor strain gauge is very high, about 130. They are useful
in measurement of very small strains of the order of 0.01
micro-strains due to their high gauge factor. Semiconductor strain
gauge exhibits very low hysteresis i.e., less than 0.05%. The
semiconductor strain gauge has much higher output, but it is as
stable as a metallic strain gauge. It possesses a high frequency
response of 1012 Hz. It has a large fatigue life i.e., 10 x 106
operations can be performed. They can be manufactured in very small
sizes, their lengths ranging from 0.7 to 7.0 mm.
D. RESISTIVE TRANSDUCERResistance of an electrical conductor is
given by, R=l/A where , R = Resistance in , = Resistivity of the
conductor ( - cm), l = Length of the conductor in cm. A =
Cross-sectional area of the metal conductor in cm2. It is clear
from the equation (1) that, the electrical resistance can be varied
by varying, Length, Cross-sectional area and Resistivity or
combination of these.Principle A change in resistance of a circuit
due to the displacement of an object is the measure of displacement
of that object Method of changing the resistance and the resulting
devices are summarized in the following table.Method of changing
resistanceResulting deviceUse
Length Resistance can be changed varying the length of the
conductor, (linear and rotary).Resistance potentiometers or sliding
contact devices displacementsUsed for the measurement ofLinear and
angular.
Dimensions When a metal conductor is subjected to Mechanical
strain, change indimensions of the conductor occurs, that changes
the resistance of the conductor.Electrical resistance strain
gauges.Used for the measurement ofmechanical strain.
Resistivity - When a metal conductor is subjected to a change in
temperature and change in resistivity occurs which changes
resistance of the conductorThermistor and RTD.Used for the
temperaturemeasurement.
E. BOURDON TUBESThe bourdon tubes are available in different
shapes like spiral, helical, twisted and C shaped but all the tubes
have non-circular cross-section. Also the materials used and
working of all these types are same. The materials used in the
construction of bourdon tubes are brass, steel and rubber. The
working principle of bourdon tube is same as that of diaphragms and
bellows i.e., the applied pressure is converted into mechanical
displacement. The displacement generated by the above force summing
devices can be converted into electrical form by transmitting it to
LVDT. The output voltage generated by LVDT is proportional to
displacement and hence applied pressure.F. LINEAR VARIABLE
DIFFRENTIAL TRANSFORMER (LVDT)The operating principle of LVDT
depends on mutual inductance. When the primary winding is supplied
with A.C. supply voltage, it generates alternating magnetic field.
Due to this magnetic field an alternating voltage will be induced
in the two secondary windings. Merits LVDT has good linearity i.e..
it produces linear output voltages. It can measure displacements of
very high range usually from 1.25mm to 250mm. It has high
sensitivity. Since it produces high output, it does not require
amplifier devices. It has low hysteresis. It consume less power
(about < 1w)Demerits It is sensitive to stray magnetic fields.
Performance of LVDT is affected by variations in temperature. It
has limited dynamic response. To provide high differential output,
it requires large displacements.
G. PRESSURE IS MEASURED USING PIEZOELECTRIC TRANSDUCERMerits
Provides electrical output. This transducer does not require any
external power supply. Size in small. Rugged construction.Demerits
It cannot be used for static pressure measurements. The response
will get affected by the variations in temperature. In some cases
it requires signal conditioning circuitry which is complex. Cost is
high.
H. Mention any two types of electronic voltmeter.1. Direct
coupled electronic DC voltmeter.2. Chopper stabilized electronic DC
voltmeter.
I. Different methods available for measuring RF power?i. Voltage
and current methodii. photometric comparison methodiii.
Bolometeriv. Calorimetric measurementv. Directional wattmeter
J. What are the difference between AC and DC electronic
voltmeter? Electronic AC voltmeters differ from their DC
counterpart only in that Ac voltage must be converted to DC before
being applied to the meter movement. Meter scale in the AC
voltmeter is calibrated in terms of the RMS value of a waveform
instead of the average value. DC voltmeter has high sensitivity
than AC voltmeter.
K. Advantages of electronic voltmeter?a. Negligible loading
effect.b. High accuracyc. Cheap and ruggedd. Can measure very low
voltages
L. How are the digital voltmeter classified? Ramp type DVM
Integrating DVM Continuous-balance DVM Successive-approximation
DVM
M. A 3 digit voltmeter is used to measuring voltage and DVM uses
SAR ADC
N. Give the various types of digital voltmeter. a. Ramp type
digital voltmeterb. Dual ramp digital voltmeterc. Integrated type
digital voltmeterd. Analog to Digital converter voltmeter.
O. Enumerate the advantage of digital meter over the analog
meter. The advantage of digital meter over analog meter is that the
output is in digital form so easy to process. Also less power
consumption than analog instruments. Readings are clearly indicated
in decimal number. The resolution of digital instrument is more. P.
Blood pH is regulated to stay within the narrow range of 7.35to
7.45, making it slightly basic.
Q. ADC and DACTypes of ADCsFlash ADCFlash analog-to-digital
converters, also known as parallel ADCs, are the fastest way to
convert an analog signal to a digital signal. They are suitable for
applications requiring very large bandwidths. However, flash
converters consume a lot of power, have relatively low resolution,
and can be quite expensive. This limits them to high frequency
applications that typically cannot be addressed any other way.
Examples include data acquisition, satellite communication, radar
processing, sampling oscilloscopes, and high density disk drives.
parallel A/D Uses a series of comparators Each comparator compares
Vin to a different reference voltage, starting w/ Vref = 1/2
lsb
Sigma-Delta ADCSigma Delta analog-to-digital converters (ADCs)
are used predominately in lower speed applications requiring a
trade off of speed for resolution by oversampling, followed by
filtering to reduce noise. 24 bit Sigma Delta converters are common
in Audio designs, instrumentation and Sonar. Bandwidths are
typically less than 1MHz with a range of 12 to 18 true bits.
Oversampled input signal goes in the integrator Output of
integration is compared to GND Iterates to produce a serial
bitstream Output is serial bit stream with # of 1s proportional to
Vin
Dual Slope converter The sampled signal charges a capacitor for
a fixed amount of time By integrating over time, noise integrates
out of the conversion. Then the ADC discharges the capacitor at a
fixed rate while a counter counts the ADC's output bits. A longer
discharge time results in a higher count.
Successive ApproximationSuccessive-approximation-register (SAR)
analog-to-digital converters (ADCs) are frequently the architecture
of choice for medium-to-high-resolution applications, typically
with sample rates fewer than 5 mega samples per second (Msps). SAR
ADCs most commonly range in resolution from 8 to 16 bits and
provide low power consumption as well as a small form factor. This
combination makes them ideal for a wide variety of applications,
such as portable/battery-powered instruments, pen digitizers,
industrial controls, and data/signal acquisition Sets MSB Converts
MSB to analog using DAC Compares guess to input Set bit Test next
bit
TypeSpeed (relative)Cost (relative)
Dual SlopeSlowMed
FlashVery FastHigh
Successive AppoxMedium FastLow
Sigma-DeltaSlowLow
DACThere are several ways of making a digital to analog
converter. Some of them are given as under.Binary weighted Resistor
DACThe binary-weighted DAC contains one resistor or current source
for each bit of the DAC connected to a summing point. These precise
voltages or currents sum to the correct output value. This suffers
from poor accuracy because of the high precision required for each
individual voltage or current. Such high-precision resistors and
current sources are expensive, so this type of converter is usually
limited to 8-bit resolution or less. It consists of four major
components. n switches one for each bit applied to the input a
weighted resistor ladder network, where the resistance are
inversely proportional to the numerical significance of the
corresponding binary digital a reference voltage Vrefand a summing
amplifier that adds the current flowing in the resistive network to
develop a signal that is proportional to the digital input.The
behavior of the circuit may be analyzed easily by using
"Millman'stheorem". It state that "the voltage appearing at any
node in a resistive network is equal to the summation of the
current entering the node (assuming the node voltage is zero)
divided by the summation of the conductance connected to the
mode".Advantages As only one resistor is used per it in the
resistor network, thus it is an economical and simple
FastDisadvantages / Limitations Resistors used in the network have
a wide range of values, so it is very difficult to ensure the
absolute accuracy and stability of all the resistors. It is very
difficult to match the temperature coefficients of all the
resistors. This factor is specially important in D/A converters
operation over a wide temperature range. When n is so large, the
resistance corresponding to LBS can assume a large value, which may
be comparable with the input resistance of the amplifier. This
results in error. As the switches represent finite impedance that
are connected in series with the weighted resistors and their
magnitudes and variations have to be taken in to account in a D/A
converter design. Needs large range of resistor values (2000:1 for
12-bit) with high precision in low resistor values Needs very small
switch resistancesR-2R Ladder Network In case of weighted resistor
DAC requires a wide range of resistance values and switches for
each bit position if high accuracy conversion is required. A
digital to analog converter with an R-2R ladder network eliminates
these complications at the expense of an additional resistor for
each bit. The operation of R-2R ladder DAC is easily explained
considering the weights of the different bits one at a time. This
can be followed by superposition to construct analog output
corresponding any digital input word. Advantages Only two values of
resistors are used; R and 2R. The actual value used for R is
relatively less important as long as extremely large values, where
stray capacitance enters the picture, are not employsonly ratio of
resistor values is critical. R-2R ladder network are available in
monolithic chips,. These are laser trimmed to be within 0.01% of
the desired ratios. The staircase voltage is more likely to be
monotonic as the effect of the MSBresistor is not many times grater
than that for LSB resistor.Disadvantages Lower conversion speed
than binary weighted DAC.By: Tushar SaxenaPage 11