MODERN ACADEMY FOR ENGINEERING & TECHNOLOGY IN MAADI

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1 MODERN ACADEMY FOR ENGINEERING & TECHNOLOGY IN MAADI 1 2/25/2018

2 ELECTRONIC MEASUREMENTS ELC_ /25/2018

3 Text Books David A. Bell, A. Foster Chin, Electronic Instrumentation & Measurements, 2 nd Ed., Prentice-Hall Inc., 1997 Larry D. Jones, A. Foster Chin, Electronic Instrumentation & Measurements, 2 nd Ed., Prentice-Hall Inc., /25/2018

4 ELC_314 Grading Policy Semester Work 10 Mid-term Exam 10 Practical Exam 20 Final Exam 60 Total /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) 5 2/25/2018

6 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

7 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

8 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

9 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

10 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

11 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

12 ANALOG ELECTRONIC MULTIMETERS (Ch4) 12 2/25/2018

13 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

14 # 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

15 Basic (Simple) Emitter Follower Voltmeter 15 2/25/2018

16 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

17 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 = = 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 *FSD : Full Scale Deflection Ri = E / IB = 10V / 10 μa = 1 MΩ 17 2/25/2018

18 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

19 Practical Emitter Follower Voltmeter 19 2/25/2018

20 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 = 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

21 FET-input Voltmeter 21 2/25/2018

22 The FET-input voltmeter further increases the input resistance of the practical emitter-follower voltmeter, by including an additional emitterfollower connected at the base of Q1. The additional emitter-follower is a FET sourcefollower 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

23 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, 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

24 # Difference Amplifier Voltmeter 24 2/25/2018

25 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

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 26 2/25/2018

27 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

28 Operational Amplifier Voltmeter Circuits 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 28 2/25/2018

29 Op-Amp Voltage-Follower Voltmeter 29 2/25/2018

30 The input voltage (EB) is applied to the op-amp noninverting, 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: V- = V+ 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

31 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 baseemitter voltage drop error from input to output. 31 2/25/2018

32 Op-Amp Amplifier Voltmeter 32 2/25/2018

33 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 noninverting 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

34 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

35 # 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

36 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 37 2/25/2018

38 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 39 2/25/2018

40 Low level AC voltage should be amplified before being rectified and applied to the meter circuit. When amplification is combined with halfwave rectification, the circuit is precision rectifier amplifier, as shown in the next figure. 40 2/25/2018

41 41 2/25/2018

42 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 fullwave rectifier circuit are: Iav = Ip Irms = Ip 42 2/25/2018

43 43 2/25/2018

44 # 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

45 SERIES OHM-METER 45 2/25/2018

46 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

47 SHUNT OHM-METER 47 2/25/2018

48 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 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

49 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

Half-wave Rectifier AC Meters

Note-4 1 Half-wave Rectifier AC Meters Disadvantages: 1. In negative half-cycle, reverse current flows through the circuit reduces average value of current meter reads lower than actual. 2. High peak inverse