DIGITAL INSTRUMENTS
Chapter 8
Chapter Contents
Digital Voltmeter
Types of DVM
Digital Multimeter
LCR Meter
Electronic Counters
Digital IC Tester
Signal Tracer
X-Y Recorders
Types of Digital Test Instruments
Digital Voltmeter
Digital Multimeter
Frequency Counter
Digital Timer
Digital IC Tester
LCR Meter
Arbitrary Waveform Generator
Digital Oscilloscope
Digital Spectrum Analyzer
Types of Digital Voltmeter
Digital Voltmeter have been classified on the basis of
A/D converter used for digital Volts display.
Ramp Type DVM (ADC)
Staircase Ramp Type DVM
Dual Slop DVM
Voltage to Frequency Converter DVM
Successive Approximation DVM
Flash Type A/D Converter
Ramp Type DVM
Ramp A/D converter
also termed as single
slope converter
Basic sections are ramp
generator , voltage
comparator , clock and
digital counter with
Latch and Display
Ramp Type DVM
Ramp generators are based on idealized
constant current source charging capacitor
circuit , producing a linear voltage ramp
across the capacitor
As long as ramp voltage is less than Vin
being measured the output of comparator is
high, enabling the AND gate , which passes
the clock pulses to counter .
When the output of ramp gen exceeds the
value of Vin , comparator output goes low ,
clock to counter is stopped and conversion
cycle is ended , Value of counter is latched
on the display which is proportional to the
value of Vin
It is simple circuit and its accuracy is limited
by both the linearity and slope of the ramp
and frequency stability of clock
Ramp Type DVM
Staircase Ramp A/D converter (DVM)
Similar to Ramp type , but Ramp
generator is replaced by D/A converter
The output of the D/A converter is a
staircase
At the start , counter is reset to zero , the
output D/A is zero , the output of
comparator is high and clock pulses go
through the AND gate to counter
As the count increases , the staircase
waveform from D/A converter proceeds
in increments towards Vin .
When the staircase voltage level exceeds
Vin the output of comparator goes low,
and the count stops .
The count is converted to decimal form
and is used to updated the display
It is more accurate , economical and good
noise imunity
Dual-Slope DVM (A/D Converter)
A dual slop ADC is common in
DVM
Ramp gen output is switched
between Vin and a negative
reference voltage (VREF)
It provide high accuracy ,
economy and good immunity
to noise
However tend to be slower
than many other types due to
their nature, which makes them
well suited for digital
voltmeter where high speed
not required
Dual-Slope DVM (A/D Converter)
Dual Slop DVM (A/D Converter)
At the start counter is zero , output of ramp gen is zero and the count is zero
Now a positive voltage Vin is applied to the input through the Switch SW as selected by
control
The Capacitor C will charge linearly as a result negative going linear ramp volts at the output
of A1 .
When the counter reaches at specific count , it will reset and control logic will switch the
negative reference voltage –VREF to input of A1 and capacitor discharges linearly , which
produces positive going ramp at the output of A1 , starting at –V , having a constant slop
(angle) that is independent of charge voltage , the counter advances from its reset state
The time it takes capacitor to discharge to zero depends on initial volts –V (which is
proportional to Vin) because the discharge slop is constant
When A1 output reach zero , the comparator switches to low , stops the counter , the count is
latched , the value of count is proportional to input voltage
A large Vin produces large slope and small Vin produces small slope.
Since t1(charge Time) is constant the peak value of ramp is proportional to the value of Vin.
The slop of positive going ramp is constant , meaning that larger value of peak ramp volts
take proportionately longer than smaller values to return to zero .
Thus t2 (Discharge time) is a linear function of Vin, and the number of pulses counted during
time t2 is proportional to analog input (Vin)
Comparison of 3 Types DVM
Ramp Type Staircase Ramp Type Dual-Slope Type
It is simplest Its simple circuit
It consists of single slope Ramp generator is It consists of dual-slope
ramp generator replaced by DAC ramp generator
Its accuracy is limited by Its accuracy better than It provides high accuracy ,
both the linearity and slope Ramp type and depends on economy and good noise
of the ramp and frequency DAC immunity
stability of clock
Its speed is slow Its speed is better than Its speed is slowest of all
slope type converter
Successive Approximation ADC (DVM)
It has much faster conversion time than
dual-slope and ramp type ADCs
It takes microseconds rather than several
milliseconds.
It is similar to staircase ramp ADC with a
little different in control logic
Instead of allowing staircase to climb from
zero to Vin , it first tries the step equal to
MSB
If this step does not exceed Vin it is stored
and retained otherwise set to zero.
The next largest bit is tried , and so on
until all bits have been tried in order of
decreasing size.
It takes only N steps to produce an N-bit
output , regardless of the magnitude of
Vin
T = N/f
ADC0808 A/D Converters with 8-Channel Multiplexer
(Successive Approximation)
Basic Digital Multimeter 1/2
(3 Digit)
DMM
Current Measurement
Digital Multimeter
Resistance
Measurement
Safety precautions for use of DMM
Use appropriate leads for measurement and don’t use
damaged leads
Correct knob Selection of range and quantity to be
measured
Never touch voltage terminals when selector knob on
ammeter
Never measure resistance when circuit power is On
Keep your fingers behind the probes to avoid shock
Never use meter on 4800 V (High Voltage)
Replace battery when low
Be careful when measuring 60V DC or 30V AC
Electronic Counters
Frequency & Period Measurement
A Frequency counter is a device that counts selected
input signal transitions for a fixed period of time
and the displays the resultant frequency on
LED/LCD Display.
Basic Frequency Counter
Input Signal applied to signal conditioner circuit where it is amplified and
conditioned (Squared up) to make it compatible with digital circuitry
The conditioned square signal is applied to one input of a count gate.
The other input of AND Gate is controlled by Q output of FF
Pulses to be counted pass trough the AND gate only when FF Q terminal is high
Counting circuit are reset to zero by negative going edge of the Q’ output so that
count always start from zero.
Basic Frequency Counter
The output frequency of 1MHz Time base
oscillator is divided by the time base divider
and applied as Clock Signal to a toggle FF.
The division ratio is selectable in steps of factor
10 , providing timing pulses at frequencies of
1kHz, 100Hz,10Hz,1Hz which corresponds To
the time period of 1ms, 10ms , 100ms and 1s
respectively.
Period Measurement
Period measurement determine the time required for a signal to complete one full cycle of
oscillation (Mathematically it is reciprocal of frequency T=1/f
The count section count pulses from time base section, and the input signal controls the gate FF.
The number of pulses counted is directly proportional to the length of one cycle of the input signal
Example . Time base output is 100 KHz , and gate opened by input signal is 1ms , count will be
100 . 1ms /100=10usec , count will be represented as 100x10us =1.00ms
The Universal Counter / Timer
The UCT combines the frequency and time measurement
function in one instrument at a relatively inexpensive
price
UCT is capable of operating in any one of the five
basic modes:
1. Frequency measurement
2. Period measurement
3. Event Counter
4. Time interval measurement
5. Frequency ration
Universal Counter / Timer (UCT)
UCT scheme used for frequency and time
measurement is similar to that already explained
previously
UCT used as Frequency Measurement UCT used as Period Measurement
Digital IC Tester
LCD Display
Keypad/ Microcontroller ZIF
Control
Regulated
Power Supply
Digital IC Tester
A profile for an i.c. to be tested is first generated as per
device’s datasheet by microcontroller.
Within the profile, an instruction sequence is specified that
applies defined logic levels to the specified input pins, and
records the results generated on the output pins.
The actual results received are compared against those
that are expected, and from this it is possible to
ascertain if the i.c. is functioning correctly.
It should be noted that some ICs require a great many
individual logic operations to test them completely. For
example, the
7430 8-input NAND gate requires 256 separate input
logic level permutations to be tested.
Digital IC Tester
ZIF socket is used to easily place and remove IC to
be tested
Results are displayed on display
Keypad is used to enter IC part No that is to be
checked or search option can be used if no Part no
not available or visible
Recorders
Types of Recorders
There are two types of recorder used as electronic
test equipment.
1. Strip chart recorder
2. X-Y Recorder
Strip Chart Recorder
Strip chart recorder using light source
X-Y Recorder
X-Y recorder uses a single stationary
sheet of chart paper , and records by
moving pen simultaneously in both X
and Y directions.
Pen holder slides along a movable pen
carriage.
A servomotor , idler pulley , and drive
cord control the pen position.
The entire pen carriage and drive
system can be made to slide across the
chart paper under the control of
another servomotor.
Servomotor controlling the position of
the pen carriage uses a crossed drive
cord which is mounted on four idler
pulleys.
The control circuits for X and Y input
are each essentially the same.
Plotting X-Y Characteristics on X-Y Recorder
Signal Tracer & Injector
•Also termed as Audio signal tracer
•Oscilloscope sees the signal or display the signal for visual inspection where as signal
tracer (listen the signal) produces sound when receives audio signal
•Signal tracer is simple and low cost solution to test audio transmission signals
•It consists of an amplifier and a loud speaker , often battery powered and packed
into a small hand test probe
•Diode detector is also available in some tracers to test AM signal
•It is used to locate defective stage of malfunctioning system such as radio/ audio
amplifier or guitar amplifier
Signal Tracer
•The circuit in the diagram is an oscillator and audio amplifier , which
produces audio pulses which can be injected into audio and radio
amplifier stages
•Connecting an ear clip or headphone to J1, the circuit will
automatically change into a two-stage amplifier and any audio signal
coming from the device under test and picked-up by the probe will be
heard through the headphones
