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1
MODERN ACADEMY FOR
ENGINEERING & TECHNOLOGY
IN MAADI
2/25/2018
2
ELECTRONIC
MEASUREMENTS
ELC_314
2/25/2018
Text Books
• David A. Bell, A. Foster Chin, “Electronic
Instrumentation & Measurements”, 2nd Ed.,
Prentice-Hall Inc., 1997
• Larry D. Jones, A. Foster Chin, “Electronic
Instrumentation & Measurements”, 2nd Ed.,
Prentice-Hall Inc., 1991.
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ELC_314 Grading Policy
• Semester Work 10
• Mid-term Exam 10
• Practical Exam 20
• Final Exam 60
• Total 100
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5
Course Contents
• Introduction
• Analog and Digital Measurements (Ch4)
• Cathode Ray Tube Oscilloscope (Ch3)
• Waveform Analysis (Ch5)
• Physical Quantities Measurements (Ch1)
• Data Acquisition Systems (Ch2)
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INTRODUCTION
Life in the 21st century relies heavily on
precision measurement, it is at the heart of
many critical experiences like:
• Medical and food industry where
elementary components ordered from
different suppliers and interact together .
• Satellite navigation systems that depend
on ultra stable clocks, as any small error in
timing can throw navigation a long way off
course.
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Principles for Good Measurements
1. Right tools
Measurements should be made using
equipment and methods that have been
demonstrated to be fit for purpose
2. Right people
Measurement staff should be competent,
properly qualified, and well informed.
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3. Right procedures
Well-defined procedures consistent with
national or international standards should
be in place for all measurements
4. Regular review
There should be both internal and
independent assessment of the technical
performance of all measurement facilities
and procedures.
•
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Measurements Quality
When talking about measurement quality, it is
important to understand the following
concepts:-
• Precision is about how close measurements
are to one another. Thus precision is
represented by a cluster of consistent
measurements, with no guarantee that they
are accurate .
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• Accuracy
is about how close measurements are to the
„true value‟. In reality, it is not possible to
know the „true value‟ and so we introduce
the concept of uncertainty to help quantify
how wrong our value might be.
• Uncertainty
is the quantification of the doubt about the
measurement results and tells use insight
about quality.
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• Error
is the difference between the measured
value and the true value of the variable
being measured
• True value
is the value that would be obtained by
theoretically perfect measurements .
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ANALOG ELECTRONIC
MULTIMETERS
(Ch4)
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Electronic Voltmeter
• Electronic voltmeters differ from ordinary PMMC*
electromechanical voltmeters in a way that they
offer a high input resistance, and amplify low
voltages to measurable levels.
• Electronic voltmeters can be analog, in which the
measurement is indicated by a pointer moving
over a calibrated scale, or digital which display the
measurement in numerical form.
* PMMC: Permanent Magnet Moving Coil
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# EMITTER FOLLOWER VOLTMETER
• An emitter follower voltmeter offers a high input
resistance to voltages being measured & provides
a low output resistance to derive current through
the coil of deflection meter.
• The basic (simple) emitter follower voltmeter
circuit, illustrated in next slide, shows a PMMC
instrument with a multiplier resistance Rs connected
in series to the meter coil resistance Rm to
increases the voltmeter range.
• The PMMC is connected such that the emitter
current passes through the meter coil, thus:
( Im = IE )
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Basic (Simple) Emitter Follower Voltmeter
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• In the basic (simple) emitter follower voltmeter
circuit, the voltage to be measured E is
connected to the transistor base, thus the circuit
input resistance :
Ri = E / IB
• Since the transistor base current IB is much
lower than the emitter (meter) current Im
IB = Im / hFE *
• Thus Ri is much larger than the meter circuit
resistance (Rs+Rm).
* hFE : the transistor current gain
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• To illustrate the effect of the transistor on the
voltmeter input resistance Ri, assume that:
E =10V, hFE =100, Rs+Rm=9.3 kΩ, Im=1 mA (FSD)*
Meter Voltage&Current
o VE = E - VBE = 10 – 0.7 = 9.3 V
o Im = VE / ( Rs+Rm) = 9.3 V / 9.3 KΩ = 1 mA
Input Resistance
o Without transistor : Ri = Rs + Rm = 9.3 kΩ
o With transistor : IB = Im / hFE = 1 mA / 100 = 10 μA
Ri = E / IB = 10V / 10 μA = 1 MΩ
*FSD : Full Scale Deflection
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• In the basic (simple) emitter-follower voltmeter, the
transistor base-emitter voltage drop (VBE)
introduces an error.
• When E = 5 V in the previous example, the meter
should read ½ FSD, that is Im = 0.5 mA
• However a simple calculation indicates that :
Im = (5 – 0.7) V / 9.3 KΩ = 0.46 mA
due to the constant diode drop (VBE = 0.7 V)
• This error can be eliminated by using a potential
divider and an additional emitter-follower, as
illustrated in the practical emitter-follower voltmeter
in the next figure .
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Practical Emitter Follower Voltmeter
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• In the practical emitter follower voltmeter, the
meter circuit is connected between the transistors
emitters.
• When no input applied (E = 0 V), the base voltage
of Q2 is adjusted to give zero meter current.
• This makes Vp = 0, VE1 = VE2 = - 0.7 V, and the
meter circuit voltage V = 0 .
• When an input voltage (E = 5 V) is applied to Q1
base, the meter voltage is:
V = VE1 - VE2
= (E - VBE1) – VE2
= ( 5 V – 0.7 V) – (- 0.7 V) = 5 V .
• Thus the transistor VBE error is eliminated .
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FET-input Voltmeter
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• The FET-input voltmeter further increases the
input resistance of the practical emitter-follower
voltmeter, by including an additional emitter-
follower connected at the base of Q1.
• The additional emitter-follower is a FET source-
follower with its gate input resistance typically in
excess of 1 MΩ, that value is added to the input
resistance of the practical emitter-follower
voltmeter
• Also, in the FET- input voltmeter, an attenuator
circuit stage is used to maintain the maximum
value of the gate voltage EG at 1 V (FSD) .
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• To maintain the n-channel FET in cut off region, its gate-to-source voltage VGS should be kept negative (nearly -5 V).
• To get zero meter deflection when E = 0 V, – the source terminal voltage must be at +5 V, thus Q1
base will be also at +5 V.
– The base of Q2 should be adjusted by the potential divider R5 at +5 V.
• When a voltage to be measured is applied to circuit input (at any meter range 1, 5, 10 25 V), EG will be a maximum of 1 V.
• This cause VS, and the base of Q1 to increase by a maximum of 1 V, and this increase appears across the meter circuit (see example 4.3) .
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# Difference Amplifier Voltmeter
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• The Difference Amplifier Voltmeter, is capable to
measure low level voltage in order of mV.
• Transistors Q1 and Q2 are identical as well as
resistors in both sides, which constitute a
symmetrical differential (emitter coupled) amplifier.
• Initially (when both inputs are zero), VC1 and VC2 are
adjusted differentially by means of R3, and the
meter voltage is set to zero (balanced)
• When the voltage at the base of Q2 is zero, and small
input voltage E is applied to Q1 base, the difference
between the two base voltages (which is E) is
amplified and applied to the meter circuit.
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For balanced circuit, the KVL in Q1 or Q2 input loop:
VBE + IERE - VEE = 0 where IE = (IE1 + IE2)
IE = (VEE - VBE) / RE which is constant value
Note: (IB RB) is very small voltage and is neglected in both sides
• When a small positive voltage is applied to the
base of Q1, IC1 is increased and IC2 is decreased
by the same amount (keeping IE constant)
• This causes VC1 to decrease, and VC2 to increase
• Therefore the voltage across the meter circuit (V =
VC1 - VC2) increases at the RHS and decreases at
the LHS.
• Thus the meter voltage will deflect to right by a
value which is proportional to difference between
the two collector voltages, which is equal to the
amplified difference between the two base
voltages (input voltage).
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• The Op. Amp IC is a perfect choice to be used in
the electronic voltmeters as:
– Voltage follower, comparable to emitter follower
– Differential amplifier, comparable to difference
amplifier
• Ideal Op. Amp has the following characteristics:
Rin = ∞
Ro = 0
Av = ∞
Av is the open loop voltage gain
Operational Amplifier Voltmeter Circuits
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Op-Amp Voltage-Follower Voltmeter
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• The input voltage (EB) is applied to the op-amp non-
inverting, input thus: EB = V+
• The output voltage (Vo) is applied to the op-amp
inverting, thus: Vo = V-
• The very high internal voltage gain of he op-amp,
combined with the negative feedback, tends to keep
the inverting input terminal voltage exactly equal to
that at the non-inverting input terminal, thus:
• Therefore the output voltage exactly follows the
input voltage: Vo = EB
Note: there is no voltage drop across R4 because there
is no current entering the op-amp input terminals.
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V- = V+
• The Op Amp voltage follower voltmeter, has
much higher input resistance, and lower
output resistance than that of the basic
emitter follower voltmeter, also it has no base-
emitter voltage drop error from input to
output.
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Op-Amp Amplifier Voltmeter
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• The Op Amp amplifier voltmeter, shown in the
previous slide, known as a non-inverting amplifier,
because its input voltage E is applied to its non-
inverting terminal.
• The output voltage Vo is divided across resistors
R3 and R4, and VR3 is fed back to the op-amp
inverting terminal.
• The internal voltage gain of the op-amp and the
negative feedback always result in :
VR3 = E
consequently the output voltage:
Vo = E (R3+R4 / R3) = 1+ (R4 / R3)
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• Thus the Op Amp amplifier voltmeter, has an
external voltage gain of:
Av = 1+ (R4 / R3)
• The Op Amp non inverting amplifier voltmeter can
be easily designed by selecting the value of
current I4 through R3 and R4 to be much larger
than the op amp input bias current IB, then the
resistors R3 and R4 are calculated as:
R3 = E / I4
R4 = (Vo – E) / I4
(see Example 4.4)
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# AC Electronic Voltmeter
• The IC op amp voltage follower and voltage
amplifier DC voltmeters, already discussed,
can be modified to measure AC voltages by
adding a rectifier circuit in series with the
meter circuit.
• The meter series resistance must be
calculated to give a deflection proportional to
the rms value of the input sin wave.
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• The next figure indicates an AC electronic
voltmeter using an op amp voltage
follower configuration, and a diode D1
connected to the op amp output.
• The voltage drop VF across the diode is a
source of error equal to 0.7 V in silicon
diodes, and varies with temperature
change.
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• To avoid this error, the voltage follower
feedback connection, to the inverting terminal,
is taken from the cathode of diode D1, instead
of from the amplifier output .
• The result is that the half-wave-rectifier output
precisely follows the positive half cycle of the
input voltage, with no voltage drop error from
input to output.
• This circuit is known as a precision rectifier, as
shown in the next figure.
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• Low level AC voltage should be amplified
before being rectified and applied to the
meter circuit.
• When amplification is combined with half-
wave rectification, the circuit is precision
rectifier amplifier, as shown in the next
figure .
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• AC electronic voltmeter can also be constructed
using voltage to current converter with full-wave
rectification, as illustrated in the next figure.
• During the positive and negative half cycles of the
input voltage, the current flows in one direction (top
– down) through the meter coil .
• The meter peak current is limited to :
Ip = Ep / R3
• The meter average current and rms current in full-
wave rectifier circuit are:
Iav = 0.637 Ip
Irms = 0.707 Ip
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# Ohm Measurements
• Analog electronic voltmeter can be made to
function as Ohmmeter by adding a battery or
regulated power supply, and a potential divider
constituted by precision standard resistors.
• A Series Ohmmeter, shown in the next figure,
uses a 1.5 V battery in series with the standard
resistors, and the unknown resistance Rx is
connected across the voltmeter terminals (A
and B), so that the voltmeter input E is the voltage
drop across Rx.
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SERIES OHM-METER
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• Suppose that the range is set to the 1 kΩ, standard
resistor R1:
– With terminals A and B open-circuit (Rx not
connected), the voltmeter indicates full scale (1.5 V),
indicating Rx = ∞ .
– If terminals A and B are short-circuit, E becomes zero,
and the pointer is at LHS of the scale, indicating Rx = 0 .
– With 0 < RX < ∞ , the battery voltage EB is divided
across R1 and Rx, giving : E = EB (RX / (R1+RX))
– When Rx = R1 = 1 kΩ, E = 1.5 (0.5) = 0.75 V
• The meter scale indicates the ratio Rx/R1, thus it will
always indicate half-scale when Rx = R1, whatever
rang is selected.
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SHUNT OHM-METER
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• In Shunt-type Ohmmeter circuit, shown in the next
figure, the standard resistors are connected in
shunt with a regulated power supply.
– With terminals A and B open-circuit (RX = ∞)
E = EB ( R2 / (R1 + R2))
= 6V (1.33 kΩ / (4 kΩ + 1.33 kΩ) = 1.5 V
Therefore, a 1.5V range gives a FSD when RX = ∞
– With terminals A and B short-circuit (RX = 0), E = 0,
and the pointer is at the LHS of scale.
– At any value of RX:
E = EB (R2ǁRX / (R1 + R2ǁRX))
The meter indicates half-scale when:
RX = R1ǁR2
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• The values of R1 and R2 used in the
previous figure gives the instrument 1 kΩ
rang.
• Resistance values of 10 times larger
would give a 10 kΩ range
• Similarly resistances 10 times smaller
give a 100 Ω range.
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