Lecture 5:
Digital multi-meter & Oscilloscope
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Digital Multi-meter (DMM)
• DMM are used for the measurement of
DC & AC Voltages – current - resistance BJT (beta test) - diode test - short circuit
test.
• The dial sets the function and the Range.
The range determines the position of the
decimal point on the LCD and this
determines how refined, or precise, your
reading is. This is called resolution.
• DMMs have high input impedance
(~10MΩ), which will not load down
sensitive circuits.
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Digital Multi-meter (DMM)
• In its basic mode, DMM is only able to measure DC voltage.
To measure current or resistance, they are converted into
voltage.
• Also to measure AC voltage, AC-DC conversion is required.
• A major component of DMM is the circuitry that converts
the analog voltage being measured into a digital quantity. A
simple and cheap circuit for this purpose is the Ramp-type
digital voltmeter system shown next.
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Ramp-type Analog-to-digital conversion
within an DMM
3.5 digit
display
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Ramp-type Digital Voltmeter System
• When the voltage to be measured, Vin, is applied to the input terminals of the
DMM, both a ramp waveform and a clock are generated.
• The ramp is compared with Vin. As long as it is less than Vin, the comparator
output is HIGH enabling the AND gate and disabling the latch circuit.
• When the ramp equals Vin, the comparator output will change from HIGH to
LOW, disabling the AND gate and hence the counter will stop. The number of
counts made by the counter will be proportional to the value of the input
voltage being measured. The negative going edge of the comparator output
triggers the latch to connect the drivers/decoders to the counting circuit so that
the counter output is decoded and displayed.
• The role of the latch circuit is to isolate the display from the counting circuit
during the time when counting is in progress. Otherwise, the display would be
virtually unreadable.
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Some notes on
measurements with a DMM
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AC Voltage Measurement
• Using the AC voltage function, the DMM can
be used to measure the RMS value of a pure
sine wave signal.
• Remember: the RMS of an AC signal is the
equivalent DC voltage that dissipates the same
heat (energy) as the AC signal.
T
• Most DMM are average responding. That is,
they truly measure the average value of the
signal. For sine wave, the RMS value is
calculated according to the relation:
RMS = 1.1 x Average
• Therefore, measuring complex waveforms
(other than pure sine wave) will give wrong
RMS value. A True RMS meter is needed in
this case!
Average or Mean
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  v(t )dt
T0
T
Mean Square
1 2
  v (t )dt
T0
T
Root Mean Square
1 2
  v (t )dt
T0
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Current measurements
• Current measurement is the most potentially hazardous measurement made
with a multimeter because the meter now is a part of the circuit.
• To measure current with a DMM, turn the power off. Break or open the
circuit, connect the meter in series with the circuit, and re-establish power.
• Although most multimeters have a maximum current capability of 10A,
practically, only small currents are measured with a multimeter, such as 4-20
mA control loops found in most process control systems.
• One common mistake is to measure voltage with the test leads in the current
input jacks. The input impedance of the current inputs jacks is 0.1Ω ~ 8Ω,
depending on the manufacturer. This low impedance is like a short circuit.
Most multimeters current input jacks are fused for protection. You will blow
the fuse if you test in this manner.
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Clamp-on meters
• A safe method for measuring current is the
Clamp meter which avoids the requirement of
breaking the circuit being measured.
• The meter clamps onto a current-carrying
conductor, and the output reading is obtained
by transformer action.
• The principle of operation is that the clamp-on
jaws of the instrument act as a transformer
core and the current-carrying conductor acts
as a primary winding. Current induced in the
secondary winding is rectified and applied to a
meter.
• Although it is a very convenient instrument to
use, the clamp-on meter has low sensitivity
and the minimum current measurable is
usually about 1A.
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Resistance measurements
• To measure resistance, disconnect one lead of the
resistor from the circuit or turn the circuit off.
Otherwise the circuit may damage the meter.
• Place the test leads on each side of the resistor.
• When you first place the meter in the (Ω) function
the meter will give a display of “OL” or “1 ___”
indicating an infinite reading. Therefore, you
should choose a suitable range with the dial.
• Note that other components in parallel with the
resistor being measured will have an effect on the
measurement.
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Continuity measurement
• Continuity is a great test to see if a
circuit is complete, and for switches
and fuses.
• An audible alarm was added to aid
in making fast go-no-go testing
without taking your eyes of your
work.
• Most meters will indicate continuity
from 0 to 50Ω.
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Diode test with a DMM
• To test a diode with a DMM, the diode function applies an
appropriate voltage and then measures the voltage drop across
the diode.
• In forward direction, the voltage drop should measure around
0.5±0.2V while in the reverse direction, you should see an OL on
the display.
• Some meters display the maximum reverse voltage applied to the
diode. In this case, you would see a reading of around 3 volts.
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The Oscilloscope
• The scope graphically displays a time varying voltage
waveform.
• The vertical (Y) axis represents voltage and the horizontal (X)
axis represents time.
• This simple graph can tell you many things about a signal
waveform such as: amplitude, frequency, period, phase, DC
and AC components, noise, shape, etc.
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The digital storage oscilloscope (DSO)
• DSO acquires the waveform as a series of samples and uses an analogto-digital converter (ADC) to convert these samples into digital words.
These points are stored in memory and then displayed on the screen,
using interpolation to smooth the waveform shape between data
points.
• DSOs allow you to capture and view events that may happen only once,
known as transients, even when the signal disappears.
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Math and Measurement Operations
• Digital oscilloscopes offer many math operations that can be
done on displayed waveforms such as addition, multiplication,
division, integration, Fast Fourier Transform, and more.
• DSO may have other functions such as:
- Measurement cursors
- Interfaces to connect the oscilloscope to a computer.
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Oscilloscope front panel
• An oscilloscope’s front panel includes a display screen and the
knobs, buttons, and switches used to control signal acquisition and
display.
• The front panel also includes input connectors to which the probes
are connected. One strong merit of the oscilloscope is its high input
impedance, typically 1MΩ, which means that the instrument has a
negligible loading effect in most measurement situations.
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The display graticule
• The grid markings on
oscilloscope screen create the
graticule. The graticule usually
consists of 8-by-10 major
divisions. There are also tick
marks called minor divisions.
• To display a waveform, try to
make it vertically occupies most
of display in order to get more
accurate measurement.
• Also, best accuracy seems to
display at least two cycles
horizontally.
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Main Oscilloscope Components
• Vertical display controls
Scales the input voltage to set the size and position of the waveform.
• Horizontal display controls
Sets the “sweep rate” (time / division) and adds a horizontal position
control.
• Trigger System and controls
The trigger stabilizes the waveform by controlling where, on a waveform’s
voltage and slope, the display trace begins each time.
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Waveform measurements
Voltage measurements
• Vp-p = (VOLTS/DIV)*(Number of divisions from peak to peak)
Period and Frequency measurements
• T = (TIME/DIV)*(Number of divisions/cycle)
• f = 1/T Frequency and Period
Phase
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Example
TIME/DIV = 0.5ms/DIV
VOLTS/DIV = 100mV/DIV = 0.1V/DIV
Waveform A:
Vp-p = 0.1*4.6 = 0.46V
T= 0.5*(8.8/2) = 2.2 ms
f = 1/T = 545.6 Hz.
Waveform B:
Vp-p = 0.1*2 = 0.2V
T= 0.5*(8.8/6) = 0.733 ms
f = 1/T = 1.36 kHz.
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Trigger System
• Imagine the jumble on the
screen that would result if each
sweep started at a different
place on the signal.
• The trigger makes repetitive
waveforms appear static on the
oscilloscope display by starting
each sweep at the correct point
of the signal.
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Trigger Level and Slope
• The trigger point is determined by the level and slope.
• The level control determines the voltage value of the trigger
point.
• The slope control determines whether the trigger point is on
the rising (positive) or the falling (negative) edge of a signal.
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Trigger Modes: Auto vs. Normal
• In normal mode the oscilloscope only sweeps if the input
signal reaches the trigger point; otherwise the display
will be frozen on the last acquired waveform.
• In Auto mode the oscilloscope sweeps, even without a
trigger. This ensures that the display will not be frozen if
the signal does not cause a trigger.
• In practice, you will probably start with auto mode
because it requires less adjustment and then use normal
mode because it lets you see just the signal of interest.
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Trigger Hold-off
• This feature is useful when you are displaying complex waveform
shapes so that the oscilloscope only triggers on an eligible trigger
point.
• Trigger hold-off is an adjustable period of time after a valid trigger
during which the oscilloscope cannot trigger.
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Input Coupling (DC, AC, or
ground)
• DC coupling shows all of an input signal.
• AC coupling blocks the DC component of a signal so that you see the
waveform centered around zero volts.
• AC coupling is useful when the entire signal (AC+DC) is too large for
the volts/div setting.
• The ground setting disconnects the input signal from the vertical
system, which lets you see a horizontal line on the screen that
represents zero volts.
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The bandwidth
• The most important specifications of an oscilloscope are its bandwidth, its rise
time and its accuracy.
• The bandwidth is defined as the maximum frequency over which the
oscilloscope amplifier gain is within -3dB of its peak value. The -3dB point is
where the gain is 0.707 times its maximum value.
• Therefore, when applied to signal-amplitude measurement, the oscilloscope is
only usable at frequencies up to about 0.3 times its specified bandwidth.
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The rise time
• The rise time is the transit time between the 10% and 90%
levels of the response when a step input is applied to the
oscilloscope.
• Oscilloscopes are normally designed such that:
Bandwidth x Rise time = 0.35
• Thus, for a bandwidth of 100 MHz,
Rise time = 0.35/108 = 3.5 ns.
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Pulse Measurements
• In many applications, the details of a pulse shape are
important.
• Pulses can become distorted and cause a digital circuit to
malfunction, and the timing of pulses in a pulse train is often
significant.
• Standard pulse measurements are pulse width and pulse rise
time.
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Pulse width and rise time
Measurements
• Rise time is the time a pulse takes to go from a low to high
voltage. By convention, the rise time is measured from 10% to
90% of the full voltage of the pulse. This eliminates any
irregularities at the pulse’s transition corners.
• Pulse width is the time the pulse takes to go from low to high
and back to low again. By convention, the pulse width is
measured at 50% of full voltage.
• Pulse measurements often require fine-tuning the triggering. To
become an expert at capturing pulses, you should learn how to
use trigger hold-off to display complex signals and time/div to
see fine details of a fast pulse.
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References
• The DMM material is taken from the following source: “The
Basics of Digital Multimeters A guide to help you
understand the basic Features and Functions of a Digital
Multimeter.” by Patrick C Elliott, IDEAL Industries, Inc. ,
January 2010, Version 1.
• The material on Oscilloscopes is taken from the tutorial "XYZs
of Oscilloscopes" by Tektronix Inc.
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