Measurements and Instrumentations Engineering/823...Measurements and Instrumentations Lecture 3: Digital Voltmeters and Frequency Meters Dr. Haitham El-Hussieny Electronics and Communications

Measurements and InstrumentationsLecture 3: Digital Voltmeters and Frequency Meters

Dr. Haitham El-Hussieny

Electronics and Communications EngineeringFaculty of Engineering (Shoubra)

Benha University

November 2018

Dr. Haitham El-Hussieny Measurements and Instrumentations 1 / 27

Lecture Outline:

1 Introduction.

2 Ramp Type Digital Voltmeters.

3 Dual Slope Digital Voltmeters.

4 DVM Range Changing.

5 Digital Voltmeter Accuracy.

6 Types of Digital Multi-meters.

7 Basic Digital Frequency Meters (DFM).


Table of Contents

1 Introduction.








Introduction:

Two types will be covered: Ramp-type andDual slope Integrator DVMs.

Digital voltmeters (DVM) are essentiallyanalog-to-digital converters with digitaldisplays to indicate the measured voltage.

Digital Voltmeter Basic Block Diagram


Table of Contents

1 Introduction.








Ramp Type Digital Voltmeters:

A ramp signal is generated.

the comparator compares theinput Vi with the ramp Vr.

V1 =

{1, if Vi ≥ Vr0, if Vi < Vr

}

If the comparator output V1 ishigh, the counting circuit willcount the pulses from clockgenerator.

If the output V1 is low, thecounting will stop.

Npulses ∝ Vi .The value of Vi will be displayedby the end of the ramp signal.

Ramp type DVM block



Ramp type DVM block DVM waveform



The use of the Latch:The latch isolates the display from the counting circuit during the t1.

It will connect the display to the counting circuit at the rising edge of the comparator output.

Ramp type DVM block


Table of Contents

1 Introduction.








Dual Slope Digital Voltmeters:

Dual Slope DVM block

Limitations of Ramp type DVM

The ramp type DVM requiresprecise ramp voltage andprecise time periods. (Notaccurate)

The Dual-slope-integrator DVMeliminates this requirement.




An integrator (e.g. capacitor) iseither charged negatively from Vi ordischarged at a constant rateaccording to the control signal.




The control signal is derived from theclock generator and a frequencydivider.

During the charging time t1, theintegrator is charged to Vo thatdepends on Vi .

During the discharging, the integratoris discharged in constant rate induration t2 that depends on Vo andhence on Vi .

A voltage comparator is used aszero-crossing-detector to output highif integrator voltage is lower thanzero.



Dual Slope DVM blockDVM waveform


Dual Slope Digital Voltmeters:How the Dual slope integrator DVM eliminates the need for accurate timing ?

(1) During charging:

Vo = −Vit1

(2) During discharging:

Vo = Kt2 K is constant

So,

Vi = −Kt2t1

Thus the input voltage measurement is notdependent on the clock frequency, but depends on

the ratiot1t2

.DVM waveform


Table of Contents

1 Introduction.








DVM Range Changing:

The attenuation circuit is used toselect the range of input voltage:

if Vin ≤ 1.999 V , the input isapplied directly on thecomparator.

if 1.999 V < Vin ≤ 19.99 V , theinput is attenuated and thedecimal point is changed.

and so on for19.99 V < Vin ≤ 199.9 V

DVM Range ChangingDr. Haitham El-Hussieny Measurements and Instrumentations 16 / 27

Table of Contents

1 Introduction.








Digital Voltmeter Accuracy:

Accuracy in DVMs:

Digital voltmeter accuracy is usually stated as:

±(0.5% rdg + 1digit)

where 1 digit refers to the extreme right (least significant digit) that depends on the range.

Example

If the accuracy is ±(0.5% rdg + 1 digit)What is the maximum error of reading1.800 V on:(1) the 2 V scale.(2) the 20 V scale.

Solution:(1) error =±[0.5%× 1.8V + 0.001] = ±0.01V(2) error =±[0.5%× 1.8V + 0.01] = ±0.019V


Table of Contents

1 Introduction.








Types of Digital Multi-meters:

Hand-held Multimeter Bench-type Multimeter


Types of Digital Multi-meters:Clamp Meters:

Advantage:

It is an electrical device having two jawswhich open to allow clamping around anelectrical conductor. This allows tomeasure electric current throughconductor, without having to makephysical contact with it, or to disconnectit for insertion through the probe.


Types of Digital Multi-meters:How Do Clamp Meters Operate ?:

A current transformer(CT) used to pick up magnetic flux generated as a result of currentpassing through a conductor.A secondary winding generates a current by electromagnetic induction that is proportionalto the primary current.


Table of Contents

1 Introduction.








Introduction:

1 Frequency of a periodic signal is defined as: the number of occurrences of a repeatingevent per unit time = Number of signal’s cycles per one second.

Principle of Frequency Meters:

To measure the frequency of a certain periodicsignal, the waveform of that signal is used totoggle a counter for a certain fixed time.The number of counted cycles per unit timeindicates the signal frequency.


Basic Digital Frequency Meters (DFM):

Register

BCD-to-seven-

segment drivers

AND

gate

Amplifier/

attenuator

Display

Time

base

Wave

shaping

circuit

Register reset

Latch

Flip flop

Q

Latch trigger

Amplified/

attenuated input

Shaped waveform

_Q

1 s 1 sTime

base

output

_Q output

Q output

Register reset

Counting

No

counting

Latch triggered

To correct display

Input

Basic Digital Freq. Meter

The basic DFM consists of:

Accurate timing source(time base) withfrequency of 1 Hz.

Digital counting circuitto count the inputwaveform cycles.

Amplifier/Attenuationcircuit to amplify orattenuate the inputsignal.

Waveform shapingcircuit to convert theinput signal to squarewave.


Basic Digital Frequency Meters (DFM):

The operation of basic DFM:

1 The input signal is amplified or attenuated as necessary.

2 The input signal is converted to a square wave and is fed to one terminal of the AND gate.

3 The time base signal with 1 Hz. freq. is fed to a flip-flop.

4 The flip-flop changes its state at each falling-edge of the time base. It divides thefrequency by 2 giving a high on the Q terminal for 1 s and a low for another 1 s. Theterminal Q̄ is an inverted version of Q.

5 One terminal of the AND gate is fed from the flip-flop Q output and the other terminal isfed from the shaped input signal. So, the counter circuit will count the input pulses forthe duration of 1 s. (Frequency).

6 The counter will reset to zero at each negative (falling) edge of the Q̄.

7 The latch will isolate the counting from the display during the first 1 s and will updatethe display on the rising edge of Q̄ output.


End of Lecture

Best Wishes

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Introduction.Ramp Type Digital Voltmeters.Dual Slope Digital Voltmeters.DVM Range Changing.Digital Voltmeter Accuracy.Types of Digital Multi-meters.Basic Digital Frequency Meters (DFM).

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Measurements and Instrumentations Engineering/823...Measurements and Instrumentations Lecture 3: Digital Voltmeters and Frequency Meters Dr. Haitham El-Hussieny Electronics and Communications