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 A 1 and A 2 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 resistorR G 3.Effectively, the inverting erminal of Op-Amp A1 is fed a voltage V l through R G and the inverting terminal of Op-Amp A 2 is fed by a voltage V 2 throughR G . 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 A 1 (Vo΄)and A 2 (Vo΄΄) is g iven bel ow 2 G 2 1 G V R R V R R V 1 ' O . . . (5.1) 1 G 2 2 G 2 V R R V R R V 1 ' ' O . . . (5.2) The output of two op-amps (A 1 and A 2 ) are applie d to the input of differential amplifie r. Therefore, the final output of the instrumentation amplifier is writt en as follows
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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
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
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
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.
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
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
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.
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
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.
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
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
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
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