Fundamentals Oscilloscope Probing

8/10/2019 Fundamentals Oscilloscope Probing

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Fundamentals of Oscilloscope Probing

Presented by Roland E. Crop

Co ri ht Tektronix Inc.


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Agenda

4 Signal Integrity and Signal Fidelity Overview4 Evolution of Probing Issues

0 Resistive Era0 Early Capacitive Era0 Yesterday The RLC Era0

Today The Transmission Line Era4 Probe Loading Measurement Examples4 Summary Probing Recommendations



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There are two kinds of designers . . .those that have signal integrity problems

. . . and those that will.Sun Microsystems



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Textbook View of Digital Signals11 1

0 0 0

?

LogicSignal

+5 VoltSupply

Ground

LogicSignal

+5 VoltSupply

Ground

Real View of Digital Signals



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Signal Integrity vs. Signal Fidelity

4 Signal Integrity : Does my circuit operate as predicted bysimulation?

4 Signal Fidelity : Can I trust my measurement system as anaccurate representation of my signal?



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What Might You Be Missing?



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Equivalent Edge Bandwidth

3V

0V

150 psEdge Speed = 150 ps

90%

10%

125 ps

Risetime =125 ps

(10% to 90%)

Equivalent Edge Bandwidth:Rule of Thumb: BW ~ 0.35

tr

Note: IBIS Uses 20% - 80% Risetime



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Measurement System Bandwidth Also Affects Timing

Equal to Signal Edge BWTwice as fast as Signal Edge BWThree times as fast as Signal Edge BWFive times as fast as Signal Edge BW

When the Scope BW is:

41%12%5%2%

Risetime Slowing Error =

What you dont know can hurt you!

Actual Waveform when:

Scope BW = 5X Equivalent Edge BW(~2% Risetime Error)

41% Risetime Error when:Scope BW = Equivalent Edge BW



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Rule of Thumb for System Risetime

Sum of Squares Guideline for Displayed and Measured Results

t r (displayed) [tr (oscilloscope) 2+ t r (probe) 2+ t r (signal) 2 ]

1. The measurement system speed will affect the displayed result

2. For scope + probe, preferably use combined system risetime3. Do not try to solve for signal risetime

This formula assumes a one pole model for each rise time

(Scope, Probe and Signal)



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The Evolution of Probing Issues



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The Resistive Era

Clock rates ~ 1 MHzRisetimes ~ 100s of nanoseconds

4 Passive probes adequate4 Resistive loading is the main concern

1x Probe Model

Ground Lead

Probe

Distributed R for DC (0 Hz) signals.

Oscilloscope



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Typical Specifications

Type Bandwidth Rise Time Input C Input R

1X PassiveProbe 15 MHz 23 ns 100 pF 1 M



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1X Probe Model (Length of Cable)

PROBE

Probe TipPROBE CABLE

8 - 10 pF/ft *1.5 ns/ft

20 pF1 M

6 feet

Vsource

R source

SCOPEDUT

LGround Lead

| (

Disadvantages:4 Very High Reflections4 Very High Input C4 Very Low Bandwidth

Advantages:4 1X (No Attenuation)4 Inexpensive

* Typical 50 cable has about 30 pF/ft of capacitance



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The Early Capacitive Era

Clock rates ~ 10s of MHzRisetimes ~ 10s of nanoseconds

4 Capacitive loading begins to be an issue4 Passive probes


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10X PassiveProbe 100 MHz -500 MHz 3.5 ns -700 ps 13 pF -8 pF 10 M



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Passive VoltageDivider, Compensated

R1

R2 C2S

C2

C1 S C1

VIN

VOUT

| (

| (

| (

| (

Parasit ic capacitance, C1 s & C2 s areswamped by larger C1 & C2.



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Typical High Z10X Passive Probe Model

C18 - 12 pF

9 M R1

PROBE

Probe TipPROBE CABLE

8 - 10 pF/ft1.5 ns/ft

C220 pF

1 M R2

6 feet

SCOPE

LGround LeadVsource

Rsource

DUT

| (

| (

| (

500

C37 - 50 pF

R3

Disadvantages:4 Input C Too High4 Not Compatible with 50

Systems4 Must be Compensated

Advantages:4 High Input R4 Wide Dynamic Range4 Inexpensive4 Mechanically Rugged4 Low Input C vs 1X Probe



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Compensation Effects

50 kHz 50 kHz 50 kHz

1 s/div 1 s/div1 s/div

1 ms/div 1 ms/div 1 ms/div

COMPENSATEDUNDER

COMPENSATEDOVER

COMPENSATED



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Probe and Signal ImpedanceCan Reduce Rise Time on a Square Wave

Rise TimeIncreaseDue To

CapacitanceLoading

t r 2.2 (R source * Cin )~~ | (

Vsource

10 M C In

LGround Lead

Probe Tip

tRise Time 3 ns

R source 1 k

C in = 100 pF ~~220 nsec

for 1X Probe

22 nsecfor 10X Probe~~C in = 10 pF

100%90%

10%0%

100%90%

10%0%

Rise Time Waveform for the 1X Passive Probe

Rise Time Waveform forthe 10X Passive Probe


b d d d


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Probe Lead Inductance and CapacitanceCan Cause Ringing on a Square Wave

Ring Frequency Usinga 10 pF Input Capacitance10X High Z Passive Probe

and 3 Lead Loop Diameter

For a 10X Passive Probe with C in = 10 pFand a 3 Lead Loop Diameter

Ring Amplitude ~~

Typical RingFrequency from3 Lead LoopDiameter

= ~~

50% Amplitude Error

100 - 160 MHz

| (

Vsource

10 M C In

Probe TipR source 50

Lsource LGround Lead

0.1-0.2 H

10 pF

1

2 LC



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Calculating Resonant Frequency

Resonant Frequency =1

2 LCIn practice, the resonant frequency should be >5 times the signals BWequivalent based on r ise and /or fall t imes. This gives a guideline to themaximum inductance (or maximum allowable probe-connection loop.)

Rules of Thumb:L 20 nH/inch for typical lead lengths around 0 - 3 (probe tip and ground lead)C Probe rated C plus 1 to 2 pF/inch of added lead length (probe tip)


P b I I d


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Probe Input Impedancevs. Frequency (RC Model Only)

Signal Frequency (Hz)

InputImpedance ( )

100M

10M

1M

100k

10k

1k

100

10

1

100 1k 10k 100k 1M 10M 100M 1G 10G

10X Z0

0.15 pF/500

Active1.0 pF/1 M

1X Passive

100 pF/1 M

10X Passive10 pF/10 M

10X Passiveprobe loadinggoes to 159

at 100 MHz



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Yesterday The RLC Era

Clock rates ~ 100s of MHzRisetimes ~ 1 to 2 nanoseconds

4 Capacitive loading ~ 1 pF probe required0 Active probes only

4 Inductive loading0 1 - 2 ground leads OK with low C

probe4 Otherwise, model as RLC circuit

0If ground lead > 20 If tip lead length added



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10X PassiveProbe

100 MHz -500 MHz

3.5 ns -700 ps

13 pF -8 pF 10 M

Z0 PassiveProbe

3 GHz -9 GHz

120 ps -40 ps

1 pF -0.15 pF 500


50 Di ider Probe (Z0)


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50 Divider Probe (Z0)Model (10X)

450

0.5 pF

50 Vsource

R source

SCOPEDUT PROBE

LGround Lead

Probe Tip PROBE CABLE50

6 feet

|(

Disadvantages:4 Low Input R4 Must be Terminated into

50

Advantages:4 Low Input C4 Wide Bandwidth4 Compatible with 50 Systems

and 1 M with TerminationResistor

4 No Compensation Necessary



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Typical Specifications for Voltage Probes



10X PassiveProbe

100 MHz -500 MHz

3.5 ns -700 ps

13 pF -8 pF 10 M

Z0 PassiveProbe

3 GHz -9 GHz

120 ps -40 ps

1 pF -0.15 pF 500

Active Probe 500 MHz -6 GHz700 ps -100 ps

2 pF -0.4 pF

10 M -100 k



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Active Probe Model

Advantages:4 Low Input Capacitance4 Wide Bandwidth4

High Input R4 Compatible with 50 Systemsor 1 M with TerminationResis tor R t

4 No Compensation Necessary

Disadvantages:4 Higher Cost4 Limited Dynamic Range4 Mechanically Less Rugged4 Requires Power

1 M /50

SCOPEPROBE

PROBE CABLE50 Vsource

R source

DUT

LGround Lead R t50

6 feet|( |(

| ( BUFFER AMP

ProbeTip


Active Probe Resonance: How Does Added


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Active Probe Resonance: How Does AddedInductance Affect the Measurement?

Trise of 1 ns ~ 350 MHz BW Equivalent

| (

Vsource

1 M C In

Probe TipR source 50

LGround Lead1.5 pF

Trise 1 ns

0.05 - 0.1 H(combined typical)

For a 10X Active Probe with C in = 1.5 pFand a 6 Ground Lead

Ring Frequency Usinga 1.5 pF Input Capacitance

10X High Z Active Probeand 6 Ground Lead.

=1

2 LC350 MHz to 500 MHzRing Frequency =



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Algebraic Difference Probing


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Algebraic Difference ProbingProvides Limited Differential Measurements

Advantages:4 Already Have Standard

Passive Probes

4 Wide Dynamic Range4 Mechanically Rugged4 Probes Test Points That Are

Large Distances Apart

Disadvantages:4 Very Low CMRR4 Few Matching Adjustments

in the Probe or the Amplifier 4 Must be LF Compensated

Scope Ch 2 Amplifier


AnalogCH1 +

Inv Ch2

PROBES SCOPE

Typical CMRR100 : 1 @ DC

20 : 1 @ 1 MHz

| ( | (

| ( | (



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Passive Differential Probing

Advantages:4 Larger CMRR Than Standard

Passive Probes

4 Wide Dynamic Range4 Mechanically Rugged4 Probes Test Points That Are

Large Distances Apart

Disadvantages:4 Requires Differential Input4 Not Compatible with 50

Systems4 Must be Compensated

Typical CMRR10,000 : 1 @ DC

100 : 1 @ 20 MHz

SCOPE

Differential Amplifier

+

| (

PROBE

| (

| ( | (



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Active Differential Probing

Advantages:4 Lower Input Capacitance4 Higher CMRR vs Frequency

Than Passive DifferentialPair 4 Compatible With 50 and

1 M Single-ended Systems

Disadvantages:4 Higher Cost4 Limited Dynamic Range4 Mechanically Less Rugged

and Larger Size4 Requires Power

Typical CMRR10,000 : 1 @ DC

2000 : 1 @ 20 MHz

VOUT

PROBE SCOPE


+

_


d h


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Today The Transmission Line Era

Risetimes ~ 100s of picoseconds

4 All signals at these speeds propagatewithin transmission line systems

4 RLC lumped elements alone no longeradequate

4Model circuit as transmission line withlumped and distributed elements

4 Differential probing techniques need to beconsidered


Signal Fidelity & Impedance


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Signal Fidelity & ImpedanceConsiderations4 Transient Response

0 Risetime / Fallt ime0 Overshoot / Undershoot

4 Signal Fidelity4 Loading

4 TDR Analysis Capability4 Impedance Characterization

0 Connectors, backplanes, etc.


P bi Ch ll


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Probing Challenges

4 High-speed Signals4 Differential Clock and Data

4 Capacitance Effects4 Inductance Effects4 Grounding Effects4 Probe Loading4 Connectors4 Fine Pitch Parts4 Real Estate4 Density


P b L di M t E l


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Probe Loading Measurement Examples

4 MeasurementSetup:

0 Source = Quickstart8, ~200ps TR signal

0 SMA directconnection to

CSA8000 andTDS74040 Probe input to

TDS7404


T t S t


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Test Setup

CSA8000 TDS7404

Quickstart 8

Signal Board

Setup #1: Signal SMA-cabled to Sampling Scope

SMA Cables


Example Probe Loading


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Example Probe Loading(Setup #1, Using CSA8000 Sampling Scope)

Reference Signal vs. P7240 Loading

Red = Reference

YellowYellow = probe loading effect(observed at SMA output)


Test Setup


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Test Setup

Setup #2: Signal SMA-cabled toTDS7404 + Probe Connection

CSA8000 TDS7404

TekConnectProbe

Quickstart 8

Signal Board

SMA Cable




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p g(Setup #2 Using TDS7404)

Reference Signal vs. P7240 Probe Output (pink trace)

White = Reference(unloaded signal)

Pink = P7240 output(probe through response)

Green = probe loading(SMA output)


Challenges of Connecting to the DUT


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Challenges of Connecting to the DUT


Probe Tip Adapters


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Probe Tip Adapters

How Do Tip Adapters Affect Probe Performance?Tip Accessories Will Degrade Signal Fidelity.


Effect of P7240 with 3 Inch Ground Lead + SMK4


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(Setup #2 Using TDS7404)

Pink = P7240 output(probe through response)


White = Reference(unloaded signal)


Effect of P7240


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with Y-Lead Adapter and SMG50


Effect of P7240 with Y-Lead Adapter and SMG50s


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White = Reference

Pink = P7240 output(OFF THE CHART)





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P7330 Active Differential Probe

P7330 - ShowingVariable Spacing Adapter and Square Pin Adapter


Example Probe Loading( l )


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(Setup #1, Using CSA8000 Sampling Scope)

Reference Signal vs. P7330 Loading (No Adapters)

Red = Reference

YellowYellow = probe loading


Reference Signal vs. P7330 Output(S #2 U i TDS7404)


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White = Reference

Blue = P7330 output



Effect of P7330 Square Pin Adapter


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q p


Reference Signal vs. P7330 + Square PinAd t + Pi (S #2 U i TDS7404)


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Adapter + Pins (Setup #2 Using TDS7404)

White = Reference

Blue = P7330 output



Effect of P7260 6 GHz Active Probe withPogo Ground


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Pogo Ground


Reference Signal vs. P7260/TDS6604Probe/Oscilloscope Bandwidth = 6 GHz


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Probe/Oscilloscope Bandwidth = 6 GHz

Same asSetup #2

except withP7260 probeand TDS6604oscilloscope

White = Reference(loading effect not visible)

Pink = P7260 output

YellowYellow = probe loading(SMA output)


Summary Probing Recommendations


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4 Choose probes with sufficient BW / Risetime:0 >3x BW of signal under test if possible0 Use Equivalent Edge BW of signal

4 Match probe to scope: consider probe + scope as a system4 Choose the right probe type:

0 Active vs. Passive vs. Z00

Single-ended vs. Differential4 Use the shortest circuit connections possible0 Limit additional L, C due to added tip accessories or leads0 Or allow for performance degradation due to adapters

4 Be aware of probe dynamic range limits4 Always de-skew for multi-channel timing measurements


Resources


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Fundamentals Oscilloscope Probing