This document discusses digital voltmeters (DVMs). It explains that DVMs display voltage measurements as numerical readings rather than using an analog needle gauge. The document covers various types of DVMs including ramp type, dual slope integrating type, successive approximation type, and microprocessor-based versions. It provides block diagrams and explanations of the operating principles for different DVM designs. Advantages of DVMs like accuracy, ease of reading, and versatility are also summarized.

1. BY
DANIEL MANOJ
SMEC

2. Digital Voltmeter (DVM)
 Voltmeter is an electrical measuring instrument used to
measure potential difference between two points. The
voltage to be measured may be AC or DC.
 Two types of voltmeters are available for the purpose of
voltage measurement i.e. analog and digital. Analog
voltmeters generally contain a dial with a needle moving
over it according to the measure and hence displaying the
value of the same.
 Digital voltmeters display the value of AC or DC voltage
being measured directly as discrete numerical instead of a
pointer deflection on a continuous scale as in analog
instruments.

3. Advantages:
 Read out of DVMs is easy as it eliminates
observational errors in measurement committed by
operators.
 Error on account of parallax and approximation is entirely
eliminated.
 Reading can be taken very fast.
 Output can be fed to memory devices for storage and
future computations.
 Versatile and accurate
 Compact and cheap
 Low power requirements
 Portability increased

4. Very high accuracy (5 to 0.05%)
Has very input impedance (2MΩ)
which ensures less loading effect on
the input.
The numeric display of digital meters
provide zero parallax error

5. Block Diagram of DVM

6. Classification of DVM’s :
• Ramp type digital voltmeter
• Dual slope Integrating type voltmeter
• Micro processor based ramp type digital
voltmeters
• Successive approximation type digital
voltmeter
• Continuous balance type digital voltmeter

7. Digital Voltmeter(DC)
The voltage is first
attenuated and
then converted
into a digital word
which is
displayed.

8. Features

Polarity detection
Auto ranging
Auto zeroing
Polarity detection:
The information is given by the ADC.
When the voltage is +ve, the characteristic
curve of the converter is in first quadrant.
The characteristic curve will be in the fourth
quadrant, if the voltage is –ve

9. DVM (AC)

The AC is processed first i.e., phase
compensated attenuation with
rectification and filtering is done.
The dc voltage obtained is converted
into a digital word, which is displayed.
For high frequencies, a precision
diode with small reverse recovery time
is required.

10. RAMP TYPE DVM
 The main building block of ramp type dvm is a ramp
generator. This is generating a waveform which is
representing a ramp. The heart of the circuit is the ramp
generator.
 Therefore it is called ramp type digital voltmeter(DVM).
 The input which should be measured is given at input
voltage.
 This input fed to ranging and attenuator circuit which will
amplify the signal if it is small or attenuates the
signal if it is large.

11.  This is given to an input comparator which will compare two
signals and generate the output.
 One input to the input comparator is from the input voltage and
another input from the ramp.
 This input voltage and ramp signal are compared and output is
given. If the ramp signal is more than input voltage there will be
no output but if the input voltage is greater than the ramp signal
then a is generated which will open the gate.
 Now when the gate get opened, clock oscillator will send clock
pulses which are counted by the counter and displays on the
screen.

12. Operating Principle:
 The operating principle of ramp type digital
voltmeter is to measure the
time that a linear ramp voltage takes to change from
the level of the input voltage to zero
voltage (or vice versa). This time interval is measured
with an electronic time interval counter
and the count is displayed as a number of digits on
electronic indicating tubes of the output
readout of the voltmeter.

13. Timing Diagram:

14. Dual slope Integrating type DVM
 The dual-slope type of A to D conversion is a very
popular method for digital voltmeter applications.
When compared to other types of analog-to-digital
conversion techniques, the dual-slope method is slow
but is quite adequate for a digital voltmeter used for
laboratory measurements.
 For data acquisition applications, where a number of
measurements are required, faster techniques are
recommended.

15. Introduction:
Dual Slope Integrating Type DVM – In
ramp techniques, superimposed noise
can cause large errors.
In the dual ramp technique, noise is
averaged out by the positive and
negative ramps using the process of
integration.

16. Principle of dual slope DVM:
 The input voltage ’ei’ is integrated, with the slope of
the integrator output proportional to the test input
voltage.
 After a fixed time equal to t1, the input voltage is
disconnected and the integrator input is connected to
a negative voltage — er The integrator output will have
a negative slope which is constant and proportional to
the magnitude of the input voltage.

17. BLOCK DIAGRAM:

19. Successive approximation type DVM:
 Successive Approximation type ADC is the
most widely used and popular ADC method.
 The conversion time is maintained constant
in successive approximation type ADC, and
is proportional to the number of bits in the
digital output, unlike the counter and
continuous type A/D converters.

20.  The basic principle of this type of A/D converter is
that the unknown analog input voltage is
approximated against an n-bit digital value by
trying one bit at a time, beginning with the MSB.
 The principle of successive approximation process
for a 4-bit conversion is explained here. This type
of ADC operates by successively dividing the
voltage range by half, as explained in the following
steps.

21.  It consists of a successive approximation register (SAR),
DAC and comparator. The output of SAR is given to n-bit
DAC. The equivalent analog output voltage of DAC, VD is
applied to the non-inverting input of the comparator.
 The second input to the comparator is the unknown analog
input voltage VA.
 The output of the comparator is used to activate the
successive approximation logic of SAR.
When the start command is applied, the SAR sets the MSB
to logic 1 and other bits are made logic 0, so that the trial
code becomes 1000.

23.  Advantages:
1 Conversion time is very small.
2 Conversion time is constant and
independent of the amplitude of the analog
input signal VA.
 Disadvantages:
1 Circuit is complex.
2 The conversion time is more compared to
flash type ADC.

24. Digital Frequency Meter
 Digital frequency meter is a general
purpose instrument that displays the
frequency of a periodic electrical signal to
an accuracy of three decimal places. It
counts the number events occurring within
the oscillations during a given interval of
time.

25.  A frequency meter has a small device which
converts the sinusoidal voltage of the
frequency into a train of unidirectional
pulses.
 The frequency of input signal is the
displayed count, averaged over a suitable
counting interval out of 0.1, 1.0, or 10
seconds. These three intervals repeat
themselves sequentially.

26. Block Diagram:

27.  The range of modern digital frequency meter is
between the range from104 to 109 hertz. The possibility
of relative measurement error ranges between from 10-
9 to 10-11 hertz and a sensitivity of 10-2 volt.
 For testing radio equipment
 Measuring the temperature, pressure, and other
physical values.
 Measuring vibration, strain
 Measuring transducers

28. Direct probes
Isolation Probes
High impedance or 10: 1 Probes
Active Probes
Current Probes
Differential Probes