ABC of Probing

Giacomo Tuveri

Overview

What is a probe?

Influence of the probing on DUT

Basic types of probe:

• Passive probes

• Active Probes

• Differential Probes

• Current Probes

Questions

What is a probe?

The probe is the most common connection method between your oscilloscope and your DUT (Device Under Test).

A cable connection such as a BNC cable is another possibility.

The probe is a physical connection between the DUT signal of interest and the oscilloscope.

In theory a probe could be a simple wire connected to a circuit, in practice it is a complex circuit along with the oscilloscope.

What happens....

...when you connect the probe to the DUT?

1. The probe cannot transfer the true shape of your waveform to the screen of your oscilloscope.

2. The probe will influence the real shape of the waveform.

2.1 You’ll observe a different waveform on your display.

2.2 Differences will depend on both probe and waveform characteristics

3. The probe can load the DUT.

3.1 The DUT may operate in a different way due to the probe load.

What happens.... in theory

CABLE Rin =

Cin = 0

Easy to connect

No affect on the

original signal

Absolute Signal Fidelity

What really happens.... DC and Low Freq

• The probe is an additional “load” driven by

the source

• Probes are designed with an high resistence

on the tip to reduce the energy drawn from the source

and reduce the load

• Mantaining high impedance across all frequencies

Is the only way to guarantee accurate, repeatable

and reliable measurements

What really happens.... AC

• High input resistance is important but only at DC or Low frequncy AC;

at higher frequencies the capacitance of a probe dominates the overall

impedance

• The resulting probe impedance can effect the signal displayed on the

oscilloscope and the signal transmitted in your device

Impedance

• Probe Impedance can be calculated as frequency increases

•This means passive probes are great at DC but have some limitations at

higher frequencies

Effects of Frequency on Probe Impedance

• At DC or low frequencies the High Input Resistance

dominates the overall impedance

• As frequency increases the capacitance dominates

the impedance and daramatically lowers the overall

impedance

At 1 Hz the impedance is 10 MΩ

At 100 Hz the impedance is 9.5 MΩ

At 100 kHz the impedance is 174 kΩ

At 1 MHz the impedance is 17.4 kΩ

At 10 MHz the impedance is 1.74 kΩ

At 100 MHz the impedance is 174 Ω

The result of high probe capacitance

shows up in the signal shape seen on the

oscilloscope

Effects of Frequency on Probe Impedance

100%

90%

10% 0%

Source Signal Effects of Input Resistance

100% 90%

10% 0%

Effects of Input Capacitance Source Signal

100% 90%

10% 0%

100% 90%

10% 0%

...what about Inductance?

• The final piece to the electrical characteristic of a probe is grounding.

• Any lead adds inductance to the probe tip or probe ground circuit.

• A lead can also act as an antenna and pick up electrical noise from the

environment. That noise is added to the signal.

• The longer the ground lead, the higher the probe inductance

Esource

RIn

CIn LGround Lead

Probe Tip Rsource

...what about Inductance?

Pulse Measured with 1:10 Passive probe

A: without Ground Lead

B: 50 cm Ground Lead

C: 10 cm Ground Lead

D: BNC Direct (true

signal shape)

...what about Inductance?

100% 90%

10% 0%

Effects of Inductance Source Signal

100% 90%

10% 0%

Signal integrity – Bandwidht Limitations

30% Frequency

10%

20%

3%

Am

pli

tud

e

Measured Signal

Real Signal

• Probe bandwidth should match oscilloscope bandwidth

A 100 MHz probe on a 600 MHz scope will degrade the

measurement bandwidth!

Oscilloscope and probe bandwidth specified as -3 dB point

At -3 dB, the measured

signal will have 30%

amplitude degradation!

FUNDAMENTAL

3° ARMONIC

5° ARMONIC

7° ARMONIC

WAVEFORM RESULT

Signal integrity – Bandwidth Limitations

5 Times Rule

BW of system > 5 x Fundamental Frequency

• Complex signals contain sine waves

at varying frequencies and amplitudes

added together.

• The components are called

Harmonics and classified by levels

Signal integrity – Insufficient rise time

Insufficient rise time affects the acquisition

Measured rise time depends on the signal and scope rise times

Choose the right Probe

Type of signal – voltage, current, single-ended,

differential…

Signal frequency content (bandwidth issue)

Signal rise time

Source impedance (R and C)

Signal amplitudes (maximum and minimum)

Test point geometries

Basic Types of Probes

• Passive Probe Supplied as standard with the scope

No active devices, only passive parts

Physically and electrically robust – rugged mechanical design with capability to

measure several hundred volts

Bandwidth limited ( more often to 500 MHz)

Higher voltage capabilities

• Active Probe Usually an optional probe powered by the oscilloscope or by an external device

Based on an active device such as a transistor or FET

Not as robust as passive probe but much wider bandwidths and much lower

capacitance

The ideal probe for high frequency measurements

Usually with limited voltage range

Basic Types of Probes

• Differential Probe Active probe based on a differential amplifier

Measures the difference between two signals when there is no ground reference

Come in two types: • High voltage for floating measurements in a power supply, lighting ballast, motor drives, 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 for AC signals.

• Hall effect or combination transformer/Hall effect for AC&DC signals

• Rogowski effect for high currents (only AC signals)

Most modern clamp on current probes are combination transformer/Hall Effect

Passive Probe Basics

Probe

9 MΩ Scope

1 MΩ

Passive probes make an attenuator circuit with the

probe impedance and scope impedance

Attenuation ratio:

Passive Probe Basics.... Coupling importance

Probe

9 MΩ Scope

50 Ω

Signal is attenuated too much if coupling settings are

incorrect

Attenuation ratio:

Most modern passive probes have read-out option:

a sense pin that mates with a probe sense ring on

the oscilloscope. This automatically sets the correct

coupling and attenuation factor.

Passive Probe Basics.... No impedance matching??

Scope inputs also have resistance and capacitance

Probe

Passive Probe Basics.... No impedance matching??

Typical 10:1 Passive probes have an adjustment point that

allows the user to adjust the impedance of the passive

probe to match the impedance of the scope

Passive Probe Basics.... Compensation

Active Probe Basics

High Resistance

Low Capacitance

50 Ω

• 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 into the

scope 50 Ω input.

Active FET Probe

• Fixed or selectable attenuation - 1:1, 10:1, 100:1 (typical)

• Input capacitance - 0.5 – 3 pF (typical)

• Input resistance - 2 k – 10 MΩ (typical)

• Bandwidth - 500 MHz to 7.5 GHz (typical)

• Power source required

• Cable buffered, performance

independent of cable length

Active probes have limited input voltage range

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 or Passive?

Why Differential?

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• General purpose oscilloscopes can only measure “Ground Referenced”

voltages however not all measurements are ground referenced

• Consider power supply measurements where the test points are referenced to

each other and there is no ground

Upper VGS measurement required

between point “A” and “B”

Differential Voltage Measurements

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Methods for making differential measurements:

Float the scope

Channel “A” minus Channel “B”

Isolators

True differential

Floating the scope....

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...a shocking esperience

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Path to ground

UNSAFE!

Floating the

scope could be a

SHOCKING

experience!!!

“A” minus “B” method

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“A” – “B” limitations

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• 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 must be carefully calibrated to match probe attenuation - Standard probes and oscilloscope attenuators lack provision to precisely match

AC attenuation

Common Mode Rejection CMR

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• Common Mode Rejection is the ability of the differential amplifier to

eliminate the common mode voltage from the output

• Real world differential almplifiers do not remove all of the common mode

signal

• The measure of how effective the differential amplifier is in removing

common mode is CMRR – Common Mode Rejection Ratio

• Why do we care about CMRR

- Common mode feedthrough sums with the signal of interest and becomes

indistinguishable from the true signal

CMRR changes with frequency

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Isolators

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• An isolator allows the oscilloscope to make safe floating measurements - Consists of oscilloscope front end protected with insulation which drives a system based

on optical or transformer isolation

• Limitations od Isolators: - Unbalanced inputs

- Parasistic capacitance

- Low CMRR at high frequencies

True Differential Measurements

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+175 Volts

- 175 Volts

“A”

“B”

Output

Important Characteristics:

• Common Mode Range

• Common Mode Rejection Ratio

• True Balanced Inputs

Load “sees” high Impedance

Lead parasitic effects cancel out !

The differential solution

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True differential amplifiers – The Best Solution

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• Accurate differential measurements while oscilloscope is safely grounded

• Two high impedance 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 are

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

How can we measure currents?

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•Shunts - DC to High Frequency AC

- Embedded Sensors

•Transformers - AC Current Transformers (CT)

- AC Currents Probe

•Hall Devices - DC to Low Frequency AC

•Transformers + Hall Devices - DC to High Frequency AC

Shunts

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•Advantages:

- 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

- Inserts Impedance (resistence)

into Circuit Under Test

- Requires circuit to be broken

Current Transformer CT

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•Advantages:

- Precision transformers can be

very accurate

- Low Cost

•Disadvantages:

- Measures only AC

- DC component moves (lowers)

the dynamic range for

measuring AC components

- Requires circuit to be broken

- Inserts Impedance into Circuit

Under Test

Transformer / Hall Device Current Probes

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•Advantages:

- 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

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• 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

Best way to choose your probe

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First one you find available in a drawer!

Best way to choose your probe

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Distract a colleague and take theirs!

Questions?

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Thank you!!

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