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.

3 2/25/2018

ELC_314 Grading Policy

• Semester Work 10

• Mid-term Exam 10

• Practical Exam 20

• Final Exam 60

• Total 100

4 2/25/2018

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)

2/25/2018

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.

6 2/25/2018

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.

7 2/25/2018

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.

•

8 2/25/2018

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 .

9 2/25/2018

• 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.

10 2/25/2018

• 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 .

11 2/25/2018

ANALOG ELECTRONIC

MULTIMETERS

(Ch4)

12 2/25/2018

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

13 2/25/2018

# 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 )

14 2/25/2018

Basic (Simple) Emitter Follower Voltmeter

15 2/25/2018

• 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

16 2/25/2018

• 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

17 2/25/2018

• 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 .

18 2/25/2018

Practical Emitter Follower Voltmeter

19 2/25/2018

• 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 .

20 2/25/2018

FET-input Voltmeter

21 2/25/2018

• 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) .

22 2/25/2018

• 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) .

23 2/25/2018

# Difference Amplifier Voltmeter

24 2/25/2018

• 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.

25 2/25/2018

2/25/2018 26

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).

27 2/25/2018

• 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

28 2/25/2018

Op-Amp Voltage-Follower Voltmeter

29 2/25/2018

• 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.

30 2/25/2018

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.

2/25/2018 31

Op-Amp Amplifier Voltmeter

32 2/25/2018

• 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)

33 2/25/2018

• 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)

34 2/25/2018

# 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.

35 2/25/2018

• 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.

36 2/25/2018

37 2/25/2018

• 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.

38 2/25/2018

39 2/25/2018

• 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 .

40 2/25/2018

41 2/25/2018

• 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

42 2/25/2018

43 2/25/2018

# 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.

44 2/25/2018

SERIES OHM-METER

45 2/25/2018

• 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.

46 2/25/2018

SHUNT OHM-METER

47 2/25/2018

• 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

48 2/25/2018

• 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.

49 2/25/2018