SDIC 11marks (Q & a) [Unit 5]

8/8/2019 SDIC 11marks (Q & a) [Unit 5]

http://slidepdf.com/reader/full/sdic-11marks-q-a-unit-5 1/21

1

UNIT 5

1. Draw the circuit and explain instrumentation amplifier. Explain with relevant

diagrams any two multiplier Applications.

Instrumentation Amplifiers

Instrumentation Amplifier constructed using three Op-Amps is shown in diagram

1. Op-Amps A1 and A2 are connected basically, in non-inverting amplifier

configuration.

2. The only change is that instead of grounding inverting terminals of both Op-Amps

as in non-inverting configuration), they are connected to resistor RG

3. Effectively, the inverting erminal of Op-Amp A1 is fed a voltage V l through RG

and the inverting terminal of Op-Amp A2 is fed by a voltage V2 through RG. This

is obvious by virtual ground concept.

Fig 5.1. Basic instrumentation amplifier

Derivation for Output Voltage

As per the superposition theorem, the output of A1 (Vo΄)and A2 (Vo΄΄) is given below

2

G

2

1

G

2V

R

RV

R

RV

1'O . . . (5.1)

1

G

2

2

G

2V

R

RV

R

RV

1''

O. . . (5.2)

The output of two op-amps (A1 and A2) are applied to the input of differential amplifier.

Therefore, the final output of the instrumentation amplifier is written as follows



2

Output '''OOO

VR

RV

1

f . . . (5.3)

Substituting the equations (5.2) and (5.1) in equation (5.3)

2

G

2

1

G

2

1

G

2

2

G

2

1

f

oV

R

RV

R

RV

R

RV

R

R

R

RV 11

1

G

2

1

G

2

1

fV

R

RV

R

R1

R

R

G

2

G

2

1

1

f

R

R

R

R1

R

R

. . . (5.4)

The gain may be adjusted by varying resistance RG

Features of Instrumentation Amplifier

1. High gain accuracy2. High CMRR3. High gain stability with low temperature coefficient

4. Low DC offset5. Low output impedance

Applications of Instrumentation Amplifier

1) Data acquisition from low output transducers;

2) Medical instrumentation;

3) current/voltage monitoring;4) Audio applications involving weak audio signals or noisy environments;5) High-speed signal conditioning for video data acquisition and imaging

Applications of multiplier:

The AD633 is well suited for such applications as modulation and demodulation,automatic gain control, power measurement, voltage-controlled amplifiers, and frequency

doublers.

G

2

1

1

f

R

2R1

R

R

OV



3

Multiplier connectionsFigure 11 shows the basic connections for multiplication. The X and Y inputs

normally have their negative nodes grounded, but they are fully differential, and in many

applications, the grounded inputs may be reversed (to facilitate interfacing with signals of a

particular polarity while achieving some desired output polarity), or both may be driven.

Squaring and frequency doubling

As shown in Figure 12, squaring of an input signal E is achieved simply by

connecting the X and Y inputs in parallel to produce an output of E2 /10 V. The input can

have either polarity, but the output is positive. However, the output polarity can be reversed

by interchanging the X or Y inputs. The Z input can be used to add a further signal to theoutput.



4

When the input is a sine wave E sin ωt, this squarer behaves as a frequency doubler, because

Equation 2 shows a dc term at the output that varies strongly with the amplitude of the input,E. This can be avoided using the connections shown in Figure 13, where an RC network is

used to generate two signals whose product has no dc term. It uses the identity

At ωo = 1/CR, the X input leads the input signal by 45° (and is attenuated by √2), and the Yinput lags the X input by 45° (and is also attenuated by √2). Becausee the X and Y inputs are90° out of phase, the response of the circuit is (satisfying Equation 3)

which has no dc component. Resistors R1 and R2 are included to restore the output amplitudeto 10 V for an input amplitude of 10 V.

The amplitude of the output is only a weak function of frequency; the output amplitude is

0.5% too low at ω = 0.9 ω0 and ω0 = 1.1 ω0.

2.Modulators and demodulators

Balanced Modulator Principle

Multiplier is used for arithmetic applications, much emphasis is placed on linear

operation with respect to both inputs.



5

But, modulators or mixers require linear operation for only one of the inputs. In suchapplications, one input is referred to as the carrier input and the other is referred to as

the modulation input.

A linear response is only required for die modulating input, since the carrier is usually

a constant amplitude ac signal.

If the modulating input is t V vmmm

cos and the carrier input is t V vccc

cos . Then, the

balanced modulator output,o

v

t V KV vcmcmo

coscos

t t V KV

mcmc

mc coscos

2 (5.4)

Above consists of only two sidebands and neither carrier nor modulating frequency

appears in it. This is the basic requirements of balanced modulator.

Applications of Balanced Modulator

The balanced modulator is used in number of applications such as AM signal

generation ; frequency multiplication, phase detection, synchronous AM and FM

demodulation, frequency discrimination, and automatic gain control.

In most of these applications, the carrier input is normally driven with a high levelsignal, such that the modumlator circuit functions as a set of synchronous switches,

and effectively "chops" the modulating signal.It is expressed as

)()()( 1 t St vK t V cmO

(5.2)

where Kl is the modulator gain. As given by Eq. (5.1), Sc(t). can be expressed as an infinite

sum of discrete frequencies at the integral multiplies of carrier frequency

In most applications, the higher order hannonics of Sc(t),can be filtered out by means of a

low-pass filter at the output of the modulator as shown in Fig 5.4

Fig. 5.4 . Use of low-pass filter to eliminate higher order harmonics in

modulator output



6

Thus the modulated output will contain the frequency of components mc

and

mc

Since there is no component of the output at the carrier frequency, this type of

modulation is known as double sideband-suppressed carrier (DSB-SC) modulation and it

comes about from the basic null suppression property of a balanced modulator

3.Ics used in radio receivers

The majority of linear ICs being produced today is in the field of Op-Amps,comparators, and regulators.

This is due to the fact that these devices can take advantage of the well matched

characteristics of monolithic components.

Now, monolithic ICs also find their applications in communication systems.

.Amplitude Modulation (AM) Radio Receiver Sub-Systems

The ICs available for radio receivers contain either RF/IF amplifier units or complete

system upto and including the detector, and audio section.

RF/IF amplifier units such as CA 3002, CA 3004, consist of cascaded RF amplifiers

and IF amplifiers only.

They can provide power gains of upto 12 dB at AM (amplitude modulation)frequencies with noise-figures less than 8 dB.

Once sufficient amplification has been obtained and the RF frequency has been

reduced to the IF frequency, 455 kHz ± 5 kHz,

The IF amplifier need only be capable of providing gain at this frequency and at thisnarrow bandwidth some thing that many Op-Amps.

Another type of ICs contain a complete system upto and including the detector, and

even a pre-annplifier for the audio.

Fig. 5.5 shows an AM radio system IC block diagram using type LM 3820 device.

In the figure, the device block diagram is enclosed by dashed lines.

This IC provides all of the RF and IF signal processing. In Fig. 14.60, the varioussubsections of this IC are shown together with the necessary external circuit

components.

The AGC voltage that is generated on the chip is supplied to the base of QZ to adjustthe quiescent current level of the RF amplifier.



7

Fig. 5.5 . AM radio receiver block diagram.

4. Explain the operation of PRBS generator with truth table. How this can be converted

into Gaussian noise source

PRBS Generator

Introduction to PRBS

PRBS is stands for Pseudorandom binary sequence, it can be useful in many

applications.

A shift register and a XOR gate is used to construct a PRBS generator and is useful to

generate the PRBS waveform.

The inputs to the feedback network (input to the XOR gate is given from any two flip-flop output and output of XOR is given to the input of shift register), which have to be

linear and follow combinational logic, are the outputs at selected stages of the shiftregister.

The maximum length of the PRBS waveform is (2" - 1) bits, where n is the number of

stages in the shift register.

It can be obtained by a proper choice of the tappings for the shift register. The

tappings have been mathematically evaluated and published in tabular form.

The frequency of the PRBS waveform is the same as the clock frequency of the shift

register.

4 Stage PRBS generator

A cicuit which is widely used to generate PN sequence is shown below



8

We have selected type D flip flopand arranged in such a way that each data inputexcept Do is the Q output of the preceding flip flop. The input

to Do is the output of parity generator. A parity generator is generally constructed of an array

of EXCLUSIVE OR logic gates, which satisfies the following conditions

INPUTS OUTPUT

X Y Z

0

0

1

1

0

1

0

1

1

0

0

1

As a matter of fact, the characteristics of PN sequence generated depends on the

number of flip flops employed and on the selection of which flip flop output are connected to

parity generator.

Sequence length: It is

always possible to find a set of connections from flip flop outputs to parity generator whichwill yield a maximal length of sequence

For the cases n=1 to n=15 one logic design for maximal length sequences is given below

N Do for L=2 -1



9

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

Qo

Qo+ Q1

Q1+ Q2

Q2+ Q3

Q3+ Q4

Q4+ Q5

Q5+ Q6

Q1+ Q2+Q3+ Q4

Q4+ Q8

Q6+ Q9

Q8+ Q10

Q1+ Q9 +Q10+ Q11

Q0+ Q10 +Q11+ Q12

Q1+ Q11 +Q12+ Q13

Q13+ Q14

An example sequence for a 3 bit PRBS generator

Clock Designed Sequence Output State

0

1

2

3

4

5

6

7

000

001

101

010

100

110

011

111

Initial State

First State

Second State

Third State

Fourth State

Fifth State

Sixth State

Seventh State



10

8 001 First State

Randomness properties

1. Balanced property:

Good balance requires that in each period of the sequence, the number of binary

ones differ from the number of binary zeros by at most one digit

2. Run property:

A run is defined as a sequence of a single type of binary digit(s) .The appearance

of the alternate digit in a sequence starts a new run. The length of the run is the number of digits in the run

3. Correlation property:

A period of the sequence is compared term by term with any cyclic shift of itself, it

is best if the number of agreements differs from the number of disagreements by not more

than one count.

PRBS

PRBS stands for Pseudo Random Binary Sequence .in general a sequence is the onewhich follows the continuous numbering like table 1

Clock Output

0

1

2

3

4

5

6

000

001

010

011

100

101

110



11

7

8

9

111

000

001

But in cause of random sequence it follows a discontinuous sequence as shown in table 2

Clock Output

0

1

2

3

4

5

6

7

8

9

000

001

101

010

100

110

011

111

001

101

5. Light Emitting Diode - LED

LED Operating Principle

LED converts the electrical energy into optical energy, this phenomenon is known as

electroluminescence.

The pn junction of LED is made from heavily doped material. On forward bias

condition, majority carriers from both sides of the junction cross the potential barrier

and enter the opposite side where they are then minority carrier and cause local

minority carrier population to be larger than normal. This is termed as minority

injection.

These excess minority carrier diffuse away from the junction and recombine with

majority carriers.

In LED, every injected electron takes part in a radiative recombination and hence

gives rise to an emitted photon.

Under reverse bias no carrier injection takes place and consequently no photon (light)

is emitted.



12

Single 7- segment Display drive circuit

Figure 5.7, shows a circuit to display single digit driver circuit

The BCD (Binary Coded Decimal) code is applied to this circuit

The 7447 decoder IC converts a BCD code applied to its inputs to 7 segment

code to display the number represented by the BCD code.

Fig 5.7 Single digit seven segment circuit

The above circuit is used to display the single digit

LIQUID CRYSTAL DISPLAY (LCD)

Liquid Crystal Displays (LCDs) are used for display of numeric and alphanumeric

character in dot matrix and segmental displays.



13

The two liquid crystal materials which are commonly used in display technologyare nematic and cholesteric whose schematic arrangement of molecules is shown in

Fig. 5.8 (a).

The most popular liquid crystal structure is the Nematic Liquid Crystal (NLC). In

this type, all the molecules align themselves approximately parallel to a unique

axis (director), while retaining the complete translational freedom. The liquid is normally transparent, but if subjected to a strong electric field,

disruption of the well ordered crystal structure takes place causing the liquid to

polarise and turn opaque.

The removal of the applied electric field allows the crystal structure to regain

its original form and the material becomes transparent.

Based on the construction, LCDs are classified into two types.They are

(i) Dynamic scattering type and

(ii) Field effect type.

Fig. 5.8 (a) Schematic arrangement of molecules in liquid crystal, (i) Nematic, (ii)

Cholesteric and (b) Construction of a dynamic scattering LCD

Dynamic scattering type



14

The construction of a dynamic scattering liquid crystal cell is shown in Fig. 5.8(b).

The display consists of two glass plates, each coated with tin oxide (Sn0 2) on

the inside with transparent electrodes separated by a liquid crystal layer, 5 to 50

µm thick.

The oxide coating on the front sheet is etched to produce a single or multi-segment pattern of characters, with each segment properly insulated from each

other.

A weak electric field applied to a liquid crystal tends to align molecules in thedirection of the field.

As soon as the voltage exceeds a certain threshold value, the domain structure

collapses and the appearance is changed.

As the voltage grows further, the flow becomes turbulent and the substanceturns optically inhomogenous. In this disordered state, the liquid crystal scatters

light.

Advantages of LCD

(i) The voltages required are small.

(ii) They have a low power consumption. A seven segment display requires

bout 140 W (20 W per segment), whereas LEDs require about 40 mW

per numeral.

(iii) (iii) They are economical.

Disadvantages of LCD

(i) LCDs are very slow devices.

(ii) When used on d.c., their life span is quite small. Therefore, they are used

with a.c. supplies having a frequency less than 50 Hz.

(iii) They occupy a large area.

LCD Driver Circuit



15

Fig 5.9 7- segment LCD driver circuit

Fig. 5.9 shows a 7-segment LCD being driven by a type 4511 latch and driver.

The ac signal required for LCD can be generated using type 4070 B CMOS EX-ORgates.

The TTL EX-OR gates are not used because they may produce a dc voltage greater

than 50 mV which will tend to shorten LCD life.

The backplane drive signal is connected to a 50 per cent duty cycle clock.

Assume the segment a output of the 4511 is 1 (high). Then, when the backplanevoltage is high the voltage to segment a is low, and vice-versa.

Thus an ac voltage (typically 3 to 5 V) is created between segment a and the

backplane of the LCD, turning the segment on.

Comparison between LED and LCD

S.No LED LCD

1 Require more power Require less power

2It can be operate the temperature range

- 40 to 85C

It can be operate the temperature range -

20 to 60C

3 More lifetime Less life time compare to LED



16

4 operating voltage (1.6 to 5V) operating voltage (3 to 20V) is required

5 Less response time More response time

Biomedical applications heart rate meter & blood pressure meter

ECG waveform

The typical ECG wave is shown. It consists of P wave QRS complex and T wave.The origin, amplitude and duration of the different waves in the electrocardiogram are

given in table below:

Origin Amplitude mV Duration sec.

P wave

R wave (QRS

complex)

T wave

S-T wave

Arial depolarisation or contraction

Repolarisation of the atria and the

depolarisation of the ventricles

Ventricular repolarisation

(Relaxation of myocardium)

Ventricular contraction

Slow repolarisation of the

intraventricular(Purkinje fibers)

system

0.25

1.60

0.1 to 0.5

0.12 to 0.22 (P-R

interval )

0.07 to 0.1

0.05 to 0.15 (S-T

interval)

0.2



17

U wave <0.1 (T interval)

The complete wave form is called electrocardiogram with labels PQRSTU indicating

important diagnostic features. For example if the PR interval is more than 0.22 sec,

the AV block occurs. When the QRS complex duration is more than 0.1 sec the

bundle block occurs.

ECG Lead configurations

Usually surface electrodes are used with jelly as electrolyte between skin and

electrodes. The potentials generated in the heart are conducted to body surface. The

potential distribution changes in a regular manner during each cardiac cycle. Therfore

to record electrocardiograms, we must choose standardised electrode positions. There

are three types of electrode systems

1)Bipolar limb leads or standard leads

2)Augmented unipolar limb leads

3)Chest leads or precordial leads

4)Frank lead system or corrected orthogonal leads

MEASUREMENT OF HEARTRATE:

Heart rate is derived by amplification of the ECG pulse and measuring either the average or

instantaneous time intervals between two successive R-peaks . The measuring range is 0-250

beats/min.

Limb or chest ECG electrodes are used as sensors.

Average Heart Rate meters: The heart rate meter which forms part of patient monitoring systems

is usually of the average reading type. They work on the basis of converting each R wave of the ECG

into pulses of fixed amplitude and duration and then determining the average current from these

pulses. They incorporate specially designed frequency to voltage convertor circuit to display averageheart rate in terms of beats per minute. The Averaging circuit commonly used for frequency to

voltage conversion for display of average heart rate is the “diode pump” circuit. This circuit is shownbelow.



18

If a capacitor C is fully charged by a pulse of voltage amplitude V, then the charge stored by it

with one pulse would be

q=CV

If there are N pulses in a time interval t such that each adds a charge q on the capacitor, then the total

change would beQ=Nq=NCV

Thus the average current over a period t would be

The equation shows that the average current is directly proportional to the number of pulses per

unit time. Thus , a current meter can be calibrated to give a direct reading of the average heart rate in

beats/min.

When a positive voltage pulse of amplitude v is applied at the input terminal of the circuit,

capacitor C 1 would be charged to C1V through the diode D1, which would conduct and offer

negligible forward resistance. Therefore, the charging of the capacitor would be governed by the timeconstant R1C1 which should be much smaller than the width of the input pulse.

When the input pulse returns to zero, the cathode of D2 is forward biased and is forward

Biased and starts conducting. Capacitor C1 then discharges through Diode D2 , the meter and

resistance R1

And R. Capacitor C2 is used to average the current through the meter and hence it should be much

larger than C1. The circuit is arranged in such a way that capacitor C1 is completely discharged before

the next pulse appears at the input terminal. Another pulse of magnitude V would again contribute a

charge q which is pumped through the meter when the input pulse returns to zero.

If the current I av Passes through the resistance R (resistance of the indicating meter), then

voltage across the capacitor is

e=CVfR

This relation is true only if e is made small proportion of V. The linearity of 0.1 % can be

achieved by using V=150V and e=1V. This is not practicable most of the times in a solid state

circuitry. Therefore , some form of modification is carried out to obtain a voltage output which has alinear relationship with frequency.

The block diagram of a direct reading average heart meter is shown in fig. The ECG pulse

received from the electrodes is amplified in a preamplifier to a level that would operate the Schmitt

trigger circuit. The Schmitt trigger converts each R wave into a rectangular pulse. The rectangular

wave form is then differentiated in the RC differentiator to yield sharp pulses for triggering

monostable multivibrator. The output of this multivibrator which consists of uniform pulses of equalamplitude and variation goes to the integrated (diode pump circuit) which produces the current

directly proportional to input frequency. Burbage(1973) Describes an average heart rate meter which



19

uses multiple feedback LPF with an Op-Amp as an Active element to achieve the desired integration

and converting a train of heart pulses into a corresponding Voltage.

ULTRASONIC BLOOD FOLWMETER:

There are basically two types of ultrasonic blood flow velocity meters. The first typeis the transit time velocity meter and the second, Doppler-shift type. For routine clinical

measurements. The transcutaneous Doppler instrument has ,by far, superseded the transit

time type . Therefore, most of the recent efforts have been concentrated on the development

of Doppler shift instruments , which are now available for measurement of blood velocity ,

volume follow , flow direction l flow profile and to visualize the internal lumen of a blood

vessel.

Doppler shift flow velocity meters

It is a non invasive technique to measure blood velocity in a particular vessel from the

surface of the body. The principle is illustrated in the fig. the incident ultrasound is scattered

by blood cells and the scattered wave is received by the second transducer. The frequency

shift due to the moving scatterers is proportional to the velocity of the scatterers . Alteration

in frequency occurs first as the ultrasound arrives at the „scatterer‟ and second as it leaves thescatterer.If the blood is moving towards the transmitter, the apparent frequency f 1 is given by

F1=f(C-v cos θ)/C

Where f= transmitted frequency

C= velocity of sound in blood

Θ=angle of inclination of the incident wave to the direction of blood flow

V=velocity of blood cells



20

Assuming that the incident and scattered radiations are both inclined at θ to thedirection of flow as shown.

F2=f 1(c/(C+v cos θ))

This relationship forms the basis of monitoring blood velocity. Depending on theapplication, either a signal proportional to the average instantaneous velocity or a signal

proportional to the peak instantaneous velocity may be required. The peak signal is easier to

obtain and has been shown to be of particular importance in localizing and quantifying the

severity of peripheral vascular diseases.

In order to measure absolute velocity , the angle of inclination of the ultrasonic beam

to the direction of flow must be know. Several methods are available for doing so. The first

uses the principle that Doppler shift signal is zero when the ultrasonic beam is at right angles

to the direction of flow . So, by finding the position of zero Doppler signal with the probe

over the vessel and then be moving it through a known angle of inclination, the angle

becomes known. However, due to separation of transmitter and receiver, the Doppler shift

frequencies are not zero(woodcock,1970).In such cases. The position at which the minimum

Doppler shift frequencies are present is taken for the probe to be at right angles . Fahrbach

(1970) used two separate ultrasonic flowmeters in which the probes are connected at right

angles to each other. By measuring the two Doppler signals, the angle of incidence of the

ultrasonic beams can be established, Fox(1978) suggested another technique which employs

multiple cross transmit transducers at different offset frequencies to obtain the velocity vector

of particles.

The early instruments used continuous wave (CW) ultrasonic beam. Satamura (1959)

was the first to demonstrate detection of transcutaneous blood flow. The technique was then

used by Franklin et al.(1961) for blood flow velocity detection in animals whereas

Baker(1964) used this technique for the first time on human beings. Basically , a CW

ultrasonic Doppler technique instrument works by transmitting a beam of high frequency

ultrasound 3-10 Mhz towards the vessel of interest.

A highly loaded lead zirconate titanate transducer is usually used for this purpose. The

transducer size may range form 1 or 2 mm to as large as 2 CM or more. Separate element is

used to detect the ultrasound back scattered from the moving blood. The back scattered signalis Doppler shifted by an amount determined by the velocity of the scatters moving through

the sound filed. Since the velocity varies with the vessel diameter to form a velocity profile,

the returned signal will produce a spectrum corresponding to these velocities. Arts and

RoEVROS (1972) suggest a method to estimate the velocity from the received signal of a

Doppler flow power density spectrum of the received signal.

Flax et al. (1973) discuss the circuit blocks of the Doppler ultrasonic blood flow meter

. The piezoelectric crystal A is electrically excited to generated ultrasonic waves which enter

the blood. Ultrasound scattered from the moving blood cells. The detector produces sum and

difference frequencies at D. the low pas filter selects the difference frequency, resulting in



21

audio frequencies at E, Each time the audio wave crosses the zero axis. A pulse appears at G.

the filter pitfalls are encountered in Doppler ultrasonic blood flowmeters. High frequency

response is usually in adequate which introduces a non linearity in to the input output

calibration curve. Also the low frequency gain is normally too high, resulting in wall motion

artefacts. These authors calculated the maximum Doppler shift at about 15Khz. The wallmotion signal can be significantly reduces by filtering out frequencies below 100hz

Documents

SDIC 11marks (Q & a) [Unit 5]