The presentation describes digital voltmeter systems, Ramp type DVM, Dual slope Integrator DVM, Digital Multi-meter: block diagram and charactersitics, Digital Frequency Meter system

1. Digital voltmeter, multi-meter,
frequency meter
Unit II: Transistor voltmeter circuits, AC electronic voltmeter,
current measurement with electronic instruments, probes Digital
voltmeter systems, digital multi-meters, digital frequency meter
system
4/4/2017 1
NEC 403 Unit I by Dr Naim R Kidwai,
Professor & Dean, JIT Jahangirabad

2. Digital voltmeter systems
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NEC 403 Unit I by Dr Naim R Kidwai,
Professor & Dean, JIT Jahangirabad
Digital voltmeters are basically analog to digital converters with
digital display to indicate voltage
E Vo
Attenuator
or
range
selector
Amplifier
High input/
low output
impendence
Rectifier
Analog
to digital
converter
Digital
Display
BCD to
seven
segment
display

3. Ramp Type DVM
• Ramp type uses comparison
of input voltage with Ramp
waveform.
• Ramp output triggers
counter, output of which is
displayed when counting
stops
• Counting circuit, BCD to
seven segment decoder and
digital readout constitutes a
2000 scale counter display.
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NEC 403 Unit I by Dr Naim R Kidwai,
Professor & Dean, JIT Jahangirabad
+
-
Ramp
generator
Clock
generator
Decade Counter
Latch
BCD to seven
segment drivers
1 9 9 9
V
Vi
Display
Reset

4. Ramp Type DVM
• Ramp starts at t=0, when ramp reaches to
Vi (t=t1), comparator output switches.
• Counting continues in period t1.
• No counting in rest time period of ramp t2
• Latch separates display drivers from
counters during counting (t1)
• At end of t1, latch triggers transfer of
counts to display through drivers.
4/4/2017 4
NEC 403 Unit I by Dr Naim R Kidwai,
Professor & Dean, JIT Jahangirabad
RampVi
Comparator
output
t1 t2
Reset Latch
Latch trigger to correct display
Counting
No Counting
Clock pulse during t1
Accurate measurement in Ramp type DVM requires precise Ramp
voltage and time period.

5. Dual Slope Integrator DVM
• Dual slope Integrator eliminates requirement of precise voltage
and time period, by using a special ramp generator (integrator)
4/4/2017 5
NEC 403 Unit I by Dr Naim R Kidwai,
Professor & Dean, JIT Jahangirabad
Clock
generator
Counters
BCD to seven
segment drivers
1 9 9 9
Vi
Display
Reset
+
-
Integrator
Zero crossing
detector
Frequency
divider
Constant
current
source
• Integrator capacitor is charged
from Vi and then discharged at
constant rate. Discharge time
is proportional to Vi
• During discharge duration,
counter counts.
• Two slopes; charge &
discharge gives its name dual
slope.
• Accuracy of Dual slope
integrator DVM depends on
current source

6. Dual Slope Integrator DVM
• Integrator charges negatively from
Vi. In fixed time duration t1 . The
charge voltage Vo  Vi
• Capacitor discharge at constant rate
till zero crossing detector output
goes negative
• Discharge time t2  Vo  Vi.
• During discharge, counter counts
• Counted result is displayed.
• If clock frequency drifts, both t1 & t2
change, thus not affecting result.
4/4/2017 6
NEC 403 Unit I by Dr Naim R Kidwai,
Professor & Dean, JIT Jahangirabad
Clock pulse
Start
counting
Stop
counting
t1
t2
Integrator
control
waveform
Dual slope
ramp from
integrator
Zero crossing
detector
output

7. Digital Multi-meters (DMM)
• A DMM measures two or more electrical quantities, principally
– AC/DC voltage (volts)
– AC/DC current (amps) and
– resistance (ohms)
• DMM have high accuracy, reliability than analog multi-meters
• DMM also come with special features measurement
– Frequency
– Capacitance
– Temperature
– Continuity testing/ Diode testing
• DMM works on Ohm’s law to measure, voltage/current/ resistance
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NEC 403 Unit I by Dr Naim R Kidwai,
Professor & Dean, JIT Jahangirabad

8. Digital Multi-meters (DMM)
4/4/2017 8
NEC 403 Unit I by Dr Naim R Kidwai,
Professor & Dean, JIT Jahangirabad
Analog to
Digital
Converter
BCD to seven
segment drivers
1 9 9 9
ACV
To resistance
Constant
current
source
Buffer Amplifier
Calibrated
attenuator
Current to Voltage
converter
Current to Voltage
converter
Calibrated
attenuator
ACA
DCA
DCV
Rectifier

9. Digital Multi-meters (DMM)
Resolution, digits and counts
• Resolution refers to smallest change in value that can be measured.
• Digits and Counts are used to describe a multi-meter’s resolution.
–A 3 1⁄2-digit meter can display 3 full digits (ranging from 0 to 9), and one
“half” digit which display only 2 symbols (0 & 1), with maximum value 1.
Thus a 3 1 ⁄2-digit meter can display up to 1,999 (2000 counts ).
–A 4 1 ⁄2-digit meter can display up to 19,999 (20000 counts).
–A 3 3 ⁄4-digit meter can display up to 3999 (4000 counts).
–A 2000/20000-count meter may have volt ranges of 200 mV, 2 V, 20 V,
200 V, 750 V
–As the range increase, resolution decrease
–DMM are built with 3200, 4000, or 6000 counts.
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NEC 403 Unit I by Dr Naim R Kidwai,
Professor & Dean, JIT Jahangirabad

10. Digital Frequency Meter System
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NEC 403 Unit I by Dr Naim R Kidwai,
Professor & Dean, JIT Jahangirabad
Input waveform Amplifier/
Attenuator
Waveform shaping
/ Schmitt trigger
Time
base
Gated
Flip Flop
Counters
BCD to seven
segment drivers
1 9 9 9Display
Latch
Reset
Latch trigger
Latch trigger
to display
input
counting
1 s
Time base
waveform
Register reset
Shaped
waveform
1 s
Q output
Q’ output
No counting
Q
Q'

11. Digital Frequency Meter System
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NEC 403 Unit I by Dr Naim R Kidwai,
Professor & Dean, JIT Jahangirabad
• Digital frequency meter consists of time base circuit, waveform
shaping circuit, circuit for gating the waveform to counter, and
display circuitry.
• Wave shaping circuit converts input wave into square or pulse
waveform of same frequency.
• Shaped waveform is applied to counter through gated control for
the fixed time period (1 s).
• Counter output is frequency of input waveform which is transferred
to display through decoder/ drivers.