Oscilloscope Fundamentals Probes and Probing Techniques

Oscilloscope Fundamentals – Probes and
Probing Techniques
LeCroy Corporation

LeCroy is an American company founded in
1964 by Walter LeCroy

Origins are high speed digitizers for particle
physics research

LeCroy corporate headquarters and is
located in Chestnut Ridge, NY

LeCroy has the most advanced technology
and widest line of digital oscilloscopes (from
40 MHz to 100 GHz)

LeCroy is the oscilloscope technology leader
and owns the fastest growing market share
within the oscilloscope industry-testing serial
data devices.

In 2004 and 2006, LeCroy became the world
leader in protocol analysis, by purchasing
both the #1 and #2 protocol analyzer
companies (CATC and Catalyst), and
merging them into a protocol analyzer
division based in Santa Clara, CA.
Overview









Basic Setup of an Oscilloscope
Analog and Digital Bandwidth
Sampling
Acquisition Memory
Waveform Display
Trigger
Measurement (Cursor, Parameter)
Math
Probes and Probing Techniques
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Basic Setup of an Oscilloscope
CH 1
ANALOGDIGITALCONVERTER
FAST
ACQUISITION
MEMORY
ANALOGDIGITALCONVERTER
FAST
ACQUISITION
MEMORY
 ACQUISITION
DOCUMENTATION
INT. HARD DRIVE
CH 2
EXT. HARD DRIVE
USB STICK
LAN
EXT.
TRIGGER
TRIGGER LOGIC
TIME BASE
ACQUISITION
CH 4
REALTIME CLOCK
ANALOGDIGITALCONVERTER
FAST
ACQUISITION
MEMORY
ANALOGDIGITALCONVERTER
FAST
ACQUISITION
MEMORY
FRONT PANEL
CONTROLLER
MAIN
PROCESSOR
MATH COPROCESSOR
PROCESSORMEMORY
PROGRAM &
MATH / ZOOM
SIGNAL DATA
DISPLAY
DISPLAY PROCESSOR
 DISPLAY
RS-232C
GPIB
CH 3
Input Amplifier;
A/D Converter
Acquisition Memory
Trigger Logic
Time Base
MEASUREMENT &
ANALYSIS
Display Processor
Raster Display
 MEASURE/ANALYZE
Processor
Math Co-Processor
Processor Memory
 DOCUMENTATION
Hard Drives
USB-Stick
Interfaces
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Basic Setup - Acquisition
ANALOG
Input
Track &
Hold
DIGITAL
ADC
Analog/Dig.
Converter
Acquisition
Memory
Amplifier
TDC –
Time-toDigital
Converter
External
Trigger
Sampling
Clock
Generator
Trigger
Processor
External
Clock
 Amplification of Input Data to Match the ADC-Input Range
 'Track & Hold' According to the Time Base
 ADC: Conversion into Digital Values to be Stored in the Acquisition
Memory
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Analog and Digital Bandwidth
 Analog Bandwidth
 Bandwidth of the analog
Amplifiers
 Classification of the Instrument
 '100 MHz'
 Digital Bandwidth
 Depending on Sample Rate
 Frequency Response of
Analog/Digital Converters
 Memory Size
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Analog Bandwidth
 Bandwidth of the Analog Amplifiers
 Higher Frequencies Do Not Reach the ADCs
 Wrong Signal Display
 No Signal Display
 Bandwidth of the Probes + Oscilloscope !
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Analog Bandwidth
 Frequency Response of Voltage Vout/Vin
Active Probe
1GHz
Passive Probe
500 MHz
Oscilloscope
600MHz
0.7071
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Digital Bandwidth
 Depending on the Sample Rate (e.g. 1 GS/s)
 Specification of the Instrument
 Influence of the User !
 Influence of the Memory Size
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Sampling
 Real Time Sampling
 Standard Mode for Oscilloscopes
 Transient Signals
 Depending on the Time Base and Number of Channels (Interleaved
Sampling)
 Problem of Aliasing
 Random Interleave Sampling RIS
 Multiple Sampling
 Also Called: Equivalent Time Sampling Mode
 Only for Repeating Signals
 High Effective Sample Rates Possible
 Roll Mode
 Slow Signals
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Sampling (Real Time)
 Sine Wave:
 Narrow Bandwidth
 Sample Rate > 2 x Max.
Frequency
 'Nyquist-Theorem'
 Pulse, Square Wave
 Wide Bandwidth
 Sample Rate > 10 x Max.
Frequency
 Realistically:
 10-Fold Oversampling
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Sampling (Aliasing)
 Reason: Sample Rate to Slow
 Result: too Low Frequency, Wrong Waveform, no Stable Trigger
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Signal Sampling (Aliasing)
 Example of a Square Wave with Aliasing
fC2 = 9.91 MHz
fM2 = 491.1 MHz
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Signal Sampling (Random Interleave Sampling)
 Multiple Sampling of a Signal  High Effective Sampling Rate
 Only for Periodic Signals
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Sampling (Roll Mode)
 Display of Slow Processes/Signals
 Signal Moves from Right to Left on the Display
 Time Base Activates the Roll Mode (e.g. t  0.5 s/div)
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Waveform Display
 Measured Signals
 Direct Display of Signals
 Zoomed Display of Signals
 Calculation (Mathematical Functions on One or More Signals)
 Measurements
 Manual Cursor Measurements
 Automatic Parameter Measurements
 Statistics
 Control Elements (Buttons) / Setup Information
 Horizontal / Vertical Scale
 Trigger Setup
 Channel Menu
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Waveform Display
 Screen Layout
 Vertical Resolution 8 Bit
 8 Divisions 'V/div'
Sampling Interval
 Horizontal Resolution:
 10 Divisions 'Time/div'
Time/div.
 Sampling Interval
 Defined by the Sample Rate
 e.g. 100 MS/s  10 ns
 Acquisition Time
(m)V/div.
Acquisition Time
 Defined by the Horizontal
Resolution
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Practical Exercises / Measurements
First Steps
 Connection of Probes
 Load Default Setup
 Auto Setup Function
 Time Base
 Vertical Resolution
 Positioning of Channels
 Zoom Function
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Waveform Display
 Interpolation of Sample Points
 Sample Points
 Points, Not Connected
 Connected Sample Points
 Linear or Sin x/x
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Waveform Display
 Averaging of Signals
 Raw Measured Sample Data
 Display of all Sample Points
 Averaged Display
 'Clean' Signal (?)
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Waveform Display (Zoom)
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Waveform Display (Measurements / Statistics)
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Waveform Display (Cursor Measurements)
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Memory
 Acquisition Memory for Signals
 Size in Points ('pts')  e.g. '20 kpts' = 20'000 Sample Points
 Memory per Channel or Interleaved
 Combination of Memory When Less Channels are Used
 12.5 Mpts/Ch. (for 4 Channels) / 25 Mpts/Ch. (for 2 Channels)
 Memory Size Defines Acquisition Time
 1 GS/s Sample Rate at 20 kpts Memory Size
 20'000 Sample Points / 1'000'000'000 Samples/s = 0.00002 s or 20 µs
 Memory Sequencing
 For Fast Signals with Large Signal Gaps
 Optimization of Memory Use
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Memory Sequencing
 Efficient & Intelligent Use
of the Memory
 Memory Sequencing up to
10'000 Segments
 Up to 1.25 Million
Acquisitions per Second
 800ns Trigger Rearm
Time
 Acquisition of Fast Signals
with Large Signal Pauses
 Time Marks for Each
Segment
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Memory Sequencing
Display of Memory Segments
 Display of All
Segments in One Grid
 Display of a Zoomed
Section
 Selection of the Zoom
Factor
 Selection of the First
Segment to be
Displayed
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Trigger
 Example for an Edge Trigger
Trigger
Level
Pre Trigger
Post Trigger
Trigger
Point
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Trigger Marks
 Marks for Time and Vertical Position
 Pre / Post Trigger: Time Before and After the Trigger Point
 Trigger Level: Vertical Level of an Edge Trigger
Trigger
Level
Trigger Point
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Trigger – Overview
 Start of Acquisition for Transient/Single Events
 Synchronization Acquisitions  Stable Display
 Trigger Modes
 AUTO Trigger:
Continuous Triggering on Internal Time Base
 NORMAL Trigger:
Continuous Triggering on an Event
(Edge, Pulse Width, Logic, Smart, TV, …)
 SINGLE-Trigger:
Single Triggering on an Event
(Edge, Pulse Width, Logic, Smart, TV, …)
 Trigger Sources:
 Analog Channels 1-4
 Digital Channels
 External Trigger Input
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Trigger – Options
Standard Trigger Functions
Trigger
Description
'Edge', Trigger on Positive, Negative or Both Signal Edges
Trigger on Positive Edge
Trigger on Negative Edge
Trigger on Alternative Edges
Trigger When Signal Outside of Defined Window
'Width', Trigger on Pos./Neg. Pulse, Defined Width/Height
'TV Trigger' (PAL, SECOM, NTSC, HDTV 1080i/1080p/720p)
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Trigger – Options
Advanced Trigger Functions
Trigger
Description
'Qualified Trigger': Trigger on A in Relation to B
'Digital Pattern': High/Low of CH1, CH2, CH3, CH4 and EXT

'Interval': Trigger on Distance of Two Same Slope Edges

'Dropout': Trigger on Missing Trigger Event for a Defined Time

'Runt': Trigger on Reduced Positive/Negative Pulses

'Slope': Trigger When Slop is Outside of Defined Value

'Glitch Trigger' for Defined Pulse Width, Height Independent
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Trigger – Options
Serial Bus Trigger Functions
Trigger
Description

Serial Trigger SPI

Serial Trigger I²C

Serial Trigger RS-232

Serial Trigger UART

Serial Trigger MIL-1553-STD

Serial Trigger CAN

Serial Trigger LIN

Serial Trigger FlexRay

Serial Trigger I²S (Audio)
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Measurements
 Two Measurement Modes:
 Manual Cursor Measurements
 Automatic Parameter
Measurements
 Cursor Measurements
 Fast Measurement of Visible
Sample Points / Curve
 Only Display Resolution
 Parameter Measurements
 Selection of Parameter /
Measurement Window
 Full Internal Resolution
 Statistics
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Measurements – Automatic Parameters
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Measurements – Parameter Definition
 Horizontal Parameters
Delay
Pulse Width
Pulse Width
Pulse Width
Average
(50% Amplitude)
Trigger
Point
First Point.
Cycle
Cycle
Frequency = 1/Cycle
Frequency = 1/Cycle
Last Point
2 Full Cycles
Sample Area for Calculation of Area, Points, etc.
Cursor Left
Cursor Right
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Measurements – Parameter Overview
 Horizontal Parameters
Parameter Description
Delay Trigger to 50% Edge
Duty Cycle
Rise / Fall Time
Frequency
Cycle Time
Phase Relation
Skew
Pulse Width Negative / Positive
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Measurements – Parameter Definition
 Vertical Parameters
Maximum
Top
Upper Limit
(90% Amplitude)
Peak/Peak
Average
(50% Amplitude)
Amplitude
Lower Limit
(10%) Amplitude
Base
Minimum
Rise Time
Fall Time
Pulse Width
Cursor Left
Cursor Right
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Measurements – Parameter Overview
 Vertical Parameters
Parameter Description
Amplitude
Area Between Waveform and Time Axis
Base / Top
Maximum / Minimum
Average
Undershoot
Overshoot
Peak-to-Peak
RMS Value
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Measurements - Parameters & Statistics
 Automatic Online Measurements
 Usage of the Complete Acquisition Memory for Online Parameter
Measurements
 Display of Statistical Values
 'Histicons' for Visualization of the Statistics
 Parameters can be Used with WaveScan
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Math
 Calculation of
Single Channels
 e.g. Invert, ReScale, etc.
 Mathematical
Combination of
Channels
 e.g. 'Ch 1 + Ch 2'
 Combined Math:
f(g(x))
 FFT (Fast Fourier
Transformation)
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Math – Functions Overview
 Standard Math Functions
Function
Description
Addition / Sum
Subtraction / Difference
Multiplication / Product
Dividing / Ratio
FFT (Fast Fourier Transformation 25kpts, 'Hann' / Rect. Window)
Zoom Function
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Math – Functions Overview
 Advanced Math Functions I
Function
Description
Absolute Value
Average Value (Cumulated or Continuous)
Derivation
Envelope (Shows Maxima/Minima with Actual Values)
Filter (Enhanced Vertical Resolution Through Virtual Bits)
Advanced FFT (1Mpts, Advanced Windows: Hamming, BlackmanHarris, Hanning)
Base (Shows Minima with Actual Values)
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Math – Functions Overview
 Advanced Math Functions II
Function
Description
Integral
Negative Value
Inverse
Linear Re-Scale
'Roof' (Shows Maxima with Actual Values)
Square
Square Root
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Math – FFT
 FFT Display
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Probes
 Passive Probes
 Standard Accessory
 Analog Bandwidth 350-500 MHz
 Active Probes
 High Analog Bandwidth > 500 MHz
 Requires Power Supply  LeCroy ProBus System
 Differential Probes
 Ext. Creation of Differential Signals  Only 1 Scope Channel Required
 Measurement of Serial Data (CAN, FlexRay)
 High Voltage Probes up to 20 kV
 Current Probes up to 500 A
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What is an Oscilloscope Probe?
 A basic probe consists of some type of connector (BNC), a
length of wire and a probe head
 The probe head is the connection point of the probe to the
circuit
 A probe head typically consists of some type of insulated
handle to hold the probe and the actual probe tip or
conductor
 The physical connection of the probe to the circuit and the
electrical characteristics of the probe are why “knowing
your probe” is an important part of any measurement
 The tradeoffs between different types of probes and how
the electrical characteristics of the probe affect a circuit or
device under test are important to understand to ensure
accurate and reliable measurements
Why Worry About Scope Probes?
When you attach a probe to your circuit one of the three things can happen:
1.
You can transfer the true shape of your waveform to the screen of the
scope.
2.
The probe can change the shape of the waveform and you will observe
the differently shaped signal on the scope.
3.
You can change the operation of the device under test (a good device
might start working improperly or vice-versa)
Why Worry About Scope Probes?
 When a probe is connected to a
circuit it takes energy from the
circuit and sends it to the
oscilloscope.
 The probe is an additional “load”
that must be driven by the signal
source.
 This load on the circuit can
change the shape of a signal and
change how the device under test
behaves
 Probes are designed with high
resistance in the probe tip to
reduce the energy drawn from the
signal and reduce the loading
 Maintaining high impedance
across all frequencies of interest
is the only way to guarantee
accurate, reliable and repeatable
measurements
Input Resistance Does Not Tell the Whole Story
 High input resistance is important but only matters at DC or
low frequency AC, at higher frequencies the capacitance of a
probe dominates the overall impedance
 The resulting probe impedance (Resistance and
Capacitance) can affect the signal displayed on the
oscilloscope and the signal transmitted in your device
The Resistance and Capacitance
create an RC network that can
filter the signal and load the
circuit changing the shape
seen on the oscilloscope
Effects of Capacitance on Probe Impedance
 A typical passive probe is physically and electrically
robust, features a high 10 MΩ input resistance
bandwidth up to 500 MHz and about 10 pF input
capacitance
 This means passive probes are great at DC or low
frequency AC but have some limitations at higher
frequencies
 Probe Impedance can be calculated as frequency
increases

 1 
 R  
 
2fc  


Z
 1 
 R  
 

 2fc  

Effects of Frequency on Probe Impedance
Input Resistance
Dominated
10 MW
At different frequencies, different
characteristics of the probe become important
Input Capacitance
Dominated
At DC or low frequencies the High Input
Resistance dominates the overall impedance
Input
Impedance
W
As frequency increases the capacitance
dominates the impedance and dramatically
lowers the overall impedance
0
DC
Frequency
2 GHz
At 1 Hz the impedance of a passive probe is 10 MΩ
At 100 Hz the impedance of a passive probe is 9.5 MΩ
At 100 kHz the impedance of a passive probe is 174
kΩ
At 1 MHz the impedance of a passive probe is 17.4 kΩ
At 10 MHz the impedance of a passive probe is 1.74
kΩ
At 100 MHz the impedance of a passive probe is 174
Ω
The result of the high probe capacitance
shows up in the signal shape seen on the
oscilloscope
What about Inductance?
 The final piece to the electrical characteristics of a probe is grounding
 Any lead added to the probe tip or probe ground adds inductance to
the circuit
 Inductance from leads can add overshoot and ringing to the signal
seen on the oscilloscope and misrepresent the signal being probed
 A lead can also act as an antenna and pick up electrical noise from the
anvironment
Effects of Inductance on Probing
 Inductance from extender
leads and ground leads can
act as an antenna which
picks up electrical noise
form the environment
 The noise picked up by the
probe from the gets added
to the signal. This is
another good reason to
keep both the signal lead
and ground lead as short as
possible.
Ground lead length and Inductance Affect Pulse Shape
The same pulse is
captured several
times using the same
channel of the
oscilloscope.
Ground leads that
are longer and have
more inductance
cause larger
overshoot (which is
not really present in
the true shape of the
signal).
Basic Types of Probes




Passive Probe

This is the standard oscilloscope probe supplied by all scope manufacturers

No active devices, only passive parts

Physically and electrically robust – rugged mechanical design with the ability to measure several hundred volts

Maximum bandwidth is 500 MHz but at the higher frequencies probe loading becomes an issue
Active Probe

Usually an optional probe that is powered by the oscilloscope through a connector on the front panel

Based off of an active device such as a transistor or FET

Not as robust as passive probe but much wider bandwidths and much lower capcitance

The ideal probe for high frequency measurements
Differential Probe

Active probe that is based off a differential amplifier

Measures the difference between two signals when there is no ground reference

Comes in two flavors

High voltage for floating measurements in a power supply, lighting ballast, motor drive, etc,

High bandwidth for differential serial data streams
Current Probe

Active device that measures the current in a signal rather than the voltage

3 main types, transformer based, Hall effect devices or combination trnasformer/Hall effect

Most modern clamp on current probes are combination transformer/Hall effect
Passive Probe Basics
 Passive probes make an attenuator circuit with the probe impedance
and scope impedance
Probe
9 MΩ
Attenuation ratio
1 M Ω /(9 MΩ +1MΩ)
= 1/10
Scope
1 MΩ
Importance of Oscilloscope Coupling
 Signal is attenuated too much if the coupling is set incorrectly
Attenuation ratio
50ohm/(9Mohm+50ohm) Probe
9 Mohm
= 1/180,001
Most modern passive probes have a probe
sense pin that mates with a probe sense ring
on the oscilloscope – this automatically sets
the correct coupling and attenuation factor
Scope
50 ohm
Scope/Probe Impedance Matching
 As with passive probes , the inpur resistance of a scope does not tell
the whole story
 Every scope input has resistance and capacitance
Scope/Probe Impedance Matching
 Passive probes have an adjustment that allows the
user to tweak the impedance of the passive probe
to match the impedance of the scope it is connected
to, this low frequency adjustment results in
improved pulse shapes on the oscilloscope display
Passive Probe Adjustment
 All oscilloscopes have a “Cal Out” which provides a clean square wave
for passive probe adjustment and compensation
 Adjusting the capacitor in the probe allows the probe to be tuned for
that scope and resulting in the good pulse shape shown below
Good
Active Probe Basics
 Active probes can be based on a transistor or a FET
 Typically Active FET probes can provide higher overall
impedance – high resistance for DC and low frequencies,
low capacitance for high frequencies
Active probes have high
resistance at the probe tip
but terminate in to the 50 Ω
coupling of the scope
High Resistance
Low capacitance
50 ohm
Active or Passive?
 Both passive probes and active probes have




strengths and weaknesses
Knowing when to use which probe will help make
accurate and reliable measurements
Passive probes are great for low frequency
measurements where high voltages may be
probed
Active transistor probes are better for
measurements which require the highest
bandwidth possible, typically 2 GHz or greater
Active FET probes are a great general purpose
probe for all frequencies from 10 kHz to 2 GHz but
are not designed for signals above 8 – 12 V
Active vs. Passive – Probe Impedance
• Passive probes provide the highest impedance below 20 kHz
• High input capacitance causes circuit loading at high frequencies or with
low frequency signals containing high frequency content
HFP
ZS Probe
10 MΩ
PP007
• Active FET probes provide high
impedance from DC – 20 kHz and
maintain that impedance out to 1.5
GHz due to low .9 pF capacitance
• FET probes are truly general
purpose and the right probe to use
at any frequency
1 MΩ
Impedance (W )
100 kΩ
10 kΩ
1 kΩ
100 Ω
• Transistor based active probes
are great for high frequency
• Low input resistance causes
circuit loading at low frequencies
10 Ω
Frequency (Hz)
Active Probes Solve Difficult Probing Challenges
 Circuit boards pose a variety of different probing problems
Active probes
with their small
form factor, light
weight and wide
range of
probing
accessories are
equipped to
address all the
probing
scenarios of
today's circuit
boards
IC Legs
Vias
SMD
Probing a small geometry IC
 The method shown here will minimize the overshoot and
ringing caused by the ground loop. A flat, “spade shape”
ground lead has less inductance than a circular “wire shape”
ground lead with the same length. Less ground lead
inductance means less induced overshoot from the probe.
An Insulated IC Tip will
prevent shorting
between legs (close
up view follows on a
later slide).
Use the Ground Blade
and Copper Pad for
the shortest possible
ground loop.
Courtesy LeCroy
Corporation
The copper pad can be soldered to the ground
pins of the IC to emulate the internal ground
plane.
Probing When Signal and Ground very close
together (BGA)
 Probing a test point and a ground point that are extremely
close together can be a difficult geometry.
Use the Sharp Tip for
the signal input to
probe small test
points.
Use the Offset Ground
to wrap around the
probe head and
connect to ground.
Probing Surface Mount Devices Using
 Probing SMD components can be difficult due to their small size.
Use the Discrete SMD
Tip to make a reliable
connection to an
SMD component
Use a variety of ground
leads like the Short
Lead to reach a
ground point
Probe several different test points that have a
common ground
Use the Straight Tip to
probe small test
points
Use the Solder-In
Ground to easily use
the same ground
point over and over
again
Probe a via where it is difficult to keep the probe
connected
Use the Bent Sharp Tip
to reach inside a via
when a traditional
straight tip will not
work
Use the Short Lead with
Sprung Hook or the
Solder-in Ground to
reach the ground
point
Probing Directly on Square Pins
Use the Right Angle
Connector for a
hands free method to
probe a square pin
Use the Long Right
Angle Lead with
Sprung Hook to reach
the second square
pin
Why Differential?
 General purpose oscilloscopes can only measure
“Ground Referenced” voltages however not all
measurements are ground referenced
 Consider power supply measurements where to test
points are referenced to each other and there is no
ground
Upper V measurement required
GS
between point “A” and “B”
+175 Volts
“A”
“B”
Output
?
- 175 Volts
Ground Referenced Need for Differential
 Ground referenced measurements upset by alternate ground
path – low amplitude signals
 Currents in the ground distribution system result in “Ground
is not ground” syndrome
 Common problem when using coaxial shunts
 Noise in the system can be >> signal of interest
V+
V+
I
+
I
v
-
I
Differential Voltage Measurements
 Methods for making differential measurements
 Float the scope
 Channel A minus Channel B
 Isolators
 True Differential
Floating the Scope
 Floating the scope is a bad idea
 Floating the scope can…
 Damage the DUT
 Damage the scope
 Result in poor measurement accuracy
Floating the Scope
HAZARDOUS
VOLTAGE
EXPOSED !!!
Probe Ground Lead
Effects Corrupt Measurement
Scope Transformer or
Power Supply Stressed
CIRCUIT UNDER TEST
vSIGNAL
A
Z CIRCUIT
vA - B
POWER
SUPPLY
SCOPE PROBE
vC - G
B
Z GROUND LEAD
OSCILLOSCOPE
POWER CORD
Z COMMON
iCOMMON
Z SCOPE GROUND
vCOMMON
Z POWER TO GROUND
IDEAL EARTH GROUND
ILL-Defined Load Placed on Circuit
“A” Minus “B” Method
Both A & B must be on screen.
This determines the maximum sensitivity the
oscilloscope can be set at.
Limited channel accuracy matching severely
limits the ability to “Reject” (Subtract) the
signal that is “Common” to both A & B.
Line
Voltage
1W
Shunt
Scope remains
safely grounded
Load Circuit
Channel “A” - Channel “B” is not adequate when:
The Common Mode Voltage >> Voltage Being Measured
“A” – “B” limitations
 This technique will not work when the signal of
interest is much smaller than the common mode
 Scope may not be able to obtain a stable trigger
 Must trigger on either Ch A or Ch B, not the difference.
 Poor high frequency CMRR restricts its use to
rejecting common mode signals at line frequency or
lower.
 Channel gain much be carefully calibrated to match
probe attenuation
 Standard probes and oscilloscope attenuators lack
provision to precisely match AC attenuation.
Isolators
Double
Insulated
Probe
Conventional Scope
“Front End”
Transformer or
optical isolation
+
Line
Voltage
1W
Shunt
Load Circuit

An isolator allows the oscilloscope to make safe floating measurements


Parasitic
Capacitance
Scope remains
safely grounded
Earth Ground
Consists of oscilloscope front end protected with insulation which drives a system based on optical or transformer
isolation
Limitations of Isolators

Unbalanced inputs

Parasitic capacitance

Low CMRR at high frequencies
True Differential Measurements
+175 Volts
“A”
“B”
Output
- 175 Volts
Important Characteristics:
• Common Mode Range
• Common Mode Rejection Ratio
• True Balanced Inputs
Load “sees” high Impedance
Lead parasitic effects cancel out !
The Differential Solution
Remaining Lead Parasitic
Effects from CM are
Canceled Out by
Differential Input Stage
Circuit Attachment is SAFELY Made
Using Probes Rated for the Job!
Scope is Not
Overdriven
CIRCUIT UNDER TEST
vSIGNAL A
ZSCOPE PROBE
ZCIRCUIT
vA - B
B
POWER
SUPPLY
DA1855
DIFFERENTIAL
AMPLIFIER
ZSCOPE PROBE
vA - B
OSCILLOSCOPE
POWER CORD
ZCOMMON
iCOMMON
ZSCOPE GND
vCOMMON
IDEAL EARTH GND
Common Mode Voltage
(Line & Ground Loop)
Is Subtracted Out
Circuit Loading is Balanced High Impedance
Scope and
Amplifier
Operate
SAFELY at
Ground
ZPOWER TO GND
High CMRR, Wide Common Mode Range,
Signal Gain, Offset, Low Noise and Fast
Overdrive Recovery Allow Minute Details of
the A - B Signal to be Seen
True Differential Amplifiers – The Best Solution
 Accurate differential measurements while
oscilloscope is safely grounded
 Two high impedance inputs matched inputs
 High CMRR over wide frequency ranges
 Two types of products
 High Voltage Active Differential Probes
 Good performance, low cost but CMRR, Noise and Overdrive
Recovery is sacrificed
 CMRR up to 10,000:1
 Differential Amplifier with probe pair
 Excellent performance, highest CMRR, low noise and excellent
overdrive recovery, cost is higher
 CMRR up to 100,000:1
Common Mode Rejection
 Common Mode Rejection is the ability of the differential
amplifier to eliminate the common mode voltage from the
output.
 Real world differential amplifiers do not remove all of the
common mode signal.
 The measure of how effective the differential amplifier is in
removing common mode is Common Mode Rejection Ratio
(CMRR).
 Why do we care about CMRR?
 Common mode feedthrough sums with the signal of interest and
becomes indistinguishable from the true signal
Common Mode Rejection
Flyback Topology Switchmode Power Supply
Drain to Source Measurement
Common Mode Rejection
High Voltage Differential Probe – Both inputs
connected to the Source
Common Mode Rejection
High Performance Differential Amplifier – Both
inputs connected to the Source
CMRR Observations
 In this particular measurement there is so much
common mode feed through that the signal shape
can be seen when using either probe
 The difference is that the high performance
amplifier feed through is 5 to 6 times less than the
high voltage differential probe
 The 100,000:1 CMRR specification of the amplifier
helps maintain accurate measurements in a
variety of noisy scenarios where common mode
interface can corrupt measurements
CMRR changes with Frequency
ADP30x

High Performance
Differential Amplifiers
start at higher CMRR,
up to 100,000:1 and
can be maintained
across wide frequency
bands

HV Differential probes
have good CMRR at
DC and low frequency
but it cannot be
maintained through the
entire probe bandwidth
DA1855A
CMRR
100,000
10,000
1,000
10
100
1,000
10,000
Frequency (Hz)
100,000
1,000,000
10,000,000
How Do We Measure Current?
 Shunts
 DC to High Frequency AC
 Embedded Sensors
 Transformers
 AC Current Transformers (CT)
 AC Current Probes
 Hall Devices
 DC to Low Frequency AC
 Transformers + Hall Devices
 DC to High Frequency AC
Measuring Current - Shunts
 Advantages
ISOURCE = VSHUNT / RSHUNT
 Low inductance coaxial shunts
can be very accurate
 Wide Bandwidth - DC to High
Frequency
 Wide dynamic range - high crest
factor
 Disadvantages
 Requires differential voltage
measurement
+
TO SCOPE
-
 Inserts Impedance (resistance)
Into Circuit Under Test
 Requires circuit to be broken
Measuring Current – Current Transformer (CT)
ISOURCE = (VTERM / RTERM) / Turns Ratio
n = (TS / TP)
 Advantages
 Precision transformers can be very
accurate
 Low cost
RTERM
 Disadvantages
 Measures only AC
 DC component moves (lowers) the
dynamic operating range for
measuring AC components
 Requires circuit to be broken
 Inserts Impedance Into Circuit
Under Test
Measuring Current – Transformer/Hall Device
Current Probes
 Advantages
Hall Device
AC Winding
Measures both AC & DC
Easy to attach to circuit
Moderate cost
 Disadvantages
May require access wire to
be added to circuit
Inserts Impedance Into
Circuit Under Test
Oscilloscope Current Probes
 Most oscilloscope current probes use the combination
of transformer and Hall technologies
 These types of probes provide a more general
purpose solution because of the ability to measure
from DC to high frequency AC
 These types of probes also can be designed with a
split core so they can be clamped on to the circuit not
requiring a break in the wire
Typical Current Probe Specifications
 CP031 – 30 Arms, 50 Apeak, 100 MHz
 CP030 – 30 Arms, 50 Apeak, 50 MHz
 CP150 – 150 Arms, 500 Apeak, 10 MHz
 CP500 – 500 Arms, 700 Apeak, 2 MHz
 All current probes are active and require power
 Modern current probes do not require an external power supply
and can be powered through the oscilloscope
 All current waveforms displayed on the oscilloscope will be
correctly scaled in Amps calculated power traces in Watts
Examples courtesy LeCroy Corpor
Summary








What is an Oscilloscope Probe?
Why Worry About Probes?
Choosing the Right Probe
Active versus Passive
Probing Techniques and Probe Accessories
Why Differential?
How do we measure current?
Summary
Thank You
for Your Attention
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