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