LeCroy AP034 Instruction Manual

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REVISION D — JANUARY 2000

Warranty

LeCroy warrants this oscilloscope accessory for normal use and operation within
specifications for a period of three years from the date of shipment. Calibration after each 12
month interval is recommended to ensure performance to specification. Spare parts,
replacement parts and repairs are warranted for 90 days. The instrument’s firmware has
been thoroughly tested and thought to be functional, but is supplied without warranty of any
kind covering detailed performance.

In exercising its warranty, LeCroy will repair or, at its option, replace any assembly returned
within its warranty period to the Customer Service Department or an authorized service
center. However, this will be done only if the product is determined by LeCroy’s examination
to be defective because of workmanship or materials, and the defect is not caused by misuse,
neglect, accident, abnormal conditions of operation, or damage resulting from attempted
repair or modifications by a non-authorized service facility.

The customer will be responsible for the transportation and insurance charges for the return of
products to the service facility. LeCroy will return all products under warranty with
transportation prepaid.

This warranty replaces all other warranties, expressed or implied, including but not lim ited to
any implied warranty of merchantability, fitness, or adequacy for any particular purposes or
use. LeCroy shall not be liable for any special, incidental, or consequential damages, whether
in contract or otherwise.

Corporate Headquarters

700 Chestnut Ridge Road
Chestnut Ridge, NY 10977-6499
Tel: [phone], Fax: [phone]

Internet: www.lecroy.com
Copyright 2000, LeCroy Corporation. All rights reserved. Contents of this publication may not

be reproduced in any form without written permission of LeCroy Corporation.
LeCroy, Easywave, SMART Trigger, PowerMeasure, and ProBus are registered trademarks

of LeCroy Corporation.
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Table of Contents

Overview

Description .................................................................................................................................. 1-1

Applications ................................................................................................................................. 1-1

Conventions used in this Manual ................................................................................................ 1-2

Safety Information

Operator Safety ........................................................................................................................... 2-1

Operation

Connecting the Probe to the Test Instrument ............................................................................. 3-1

Connecting the Probe to the Test Circuit ......................................................................................3-1

Probe Input Loading .................................................................................................................... 3-4

Grounding the Probe ................................................................................................................... 3-4

Selecting the Proper Range ........................................................................................................ 3-6

Operation with LeCroy Oscilloscopes ......................................................................................... 3-7

AP034 Use with the ADPPS Power Supply...................................................................................3-8

Adding Offset ............................................................................................................................ 3-10

Autobalance .............................................................................................................................. 3-11

Designing Test Fixtures for the AP033 Probe ........................................................................... 3-12

Reference Information

Differential Mode and Common Mode ........................................................................................ 4-1

Differential Mode Range and Common Mode Range...................................................................4-1

Common Mode Rejection Ratio ................................................................................................... 4-2

Care and Maintenance

Cleaning ...................................................................................................................................... 5-1

Calibration Interval ...................................................................................................................... 5-1

Service Strategy .......................................................................................................................... 5-1

Troubleshooting .......................................................................................................................... 5-1

Trace Off Scale................................................................................................................5-1

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Incorrect Frequency Response........................................................................................5-2

DC Errors.........................................................................................................................5-3

Poor Common Mode Rejection.......................................................................................5-3

Returning a Defective Probe ....................................................................................................... 5-4

Replacement Parts ...................................................................................................................... 5-5

Matching Procedure for ÷10 Plug-On External Attenuator .......................................................... 5-8

Equipment Required .....................................................................................................................5-8

Procedure......................................................................................................................................5-9

Performance Verification

Test Equipment Required .............................................................................................................6-1

Preliminary Procedure...................................................................................................................6-3

Procedure......................................................................................................................................6-3

Check Gain Accuracy.......................................................................................................6-4

Check High Frequency CMRR.........................................................................................6-5

Check Low Frequency CMRR..........................................................................................6-8

Performance Verification Test Record........................................................................................6-11

Adjustment Procedure

Introduction .................................................................................................................................. 7-1

Test Equipment Required .............................................................................................................7-2

Preliminary Procedure...................................................................................................................7-4

Procedure......................................................................................................................................7-7

Adjust Coarse DC Balance (R226) ..................................................................................7-7

Adjust Fine DC Balance (R304).......................................................................................7-8

Adjust Low Frequency CMRR (R7)..................................................................................7-9

Adjust Offset (R330) ......................................................................................................7-11

Adjust Gain (R322C)......................................................................................................7-13

Adjust Final Attenuator Compensation (C17) ................................................................7-14

Adjust Final Attenuator CMRR (C18A)...........................................................................7-15

Assemble Probe and Amplifier.......................................................................................7-16

Attenuator Matching and Final Check............................................................................7-17

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Specifications

Nominal Characteristics .............................................................................................................. 8-1

Warranted Electrical Characteristics ........................................................................................... 8-2

Typical Electrical Characteristics ................................................................................................ 8-3

General Characteristics ............................................................................................................... 8-5

Compliance and Certifications .................................................................................................... 8-6

Operator Safety.............................................................................................................................8-6

Standard Accessories ................................................................................................................. 8-7

Optional Accessories .................................................................................................................. 8-7

Oscilloscope Software Compatibility ........................................................................................... 8-7

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DESCRIPTION

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The AP034 is a wide-band, active differential pr obe. The probe
features low noise, low input capacitance, high common mode
rejection, and Field Effect Tr ansistor (FET ) buffer ed inputs in the
probe head. The user-selectable offset gives the probe the
flexibility to measure a large range of signal types. Plug-on
attenuators and AC coupling accessories further extend the
application range. Included interconnect accessories allow
connection to surface mount and through-hole com ponents with
minimal signal degradation. The input receptacles in the probe
head are compatible with standard 0.025 in. (0.635 m m) square
pins, which provide a convenient, low-cost method of creating
device characterization test fixtures.

The probe is powered directly from a LeCroy oscilloscope
through the ProBus interface. T he ProBus interface also allows
local control of the probe through the os cilloscope user interface
and remote control through the inter face buses, (G PIB, RS-232).
The optional ADPPS power supply allows the AP034 to be used
with other instruments such as spectrum analyzers, network
analyzers, and oscilloscopes without ProBus interface.

APPLICATIONS

The AP034 Active Differential Probe is ideal for acquiring high­speed differential signals su ch as those found in disk drive read
channels, differential LAN, video, etc. The probe can be used
with spectrum analyzers to acquire signals in some RF systems
(for example, balanced IF mixers in hand-held cellular
telephones). The high impedance characteristics of both inputs
allow you to use the probe as an FET probe to make single­ended measurements in digital systems without introducing a
ground loop, as a conventional FET probe would.

AP034-OM-E Rev D ISSUED: January 2000 ²

CONVENTIONS USED IN THIS MANUAL

The following conventions may appear in this manual:

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Note

A Note contains general information relating to
the use of the product.

Caution

A Caution contains information that should to be
followed to avoid possible damage to the
instrument or the device under test.

WARNING

A Warning alerts you to potential injury to
yourself. Failing to adhere to the statement in a
WARNING message could result in bodily injury.

The following symbol may appear on the product:
This symbol refers you to additional information contained in this

manual. The corres ponding inform ation in the m anual is denoted
with the same symbol.

CAT I Overvoltage Installation Category per EN 61010-1

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OPERATOR SAFETY

The AP034 Active Differential Probe is intended to be used only
with instruments that are connected to earth ground thr ough the
input BNC connector. When you are using the probe with an
ADPPS Power Supply Adapter, make sure the adapter is
connected to a BNC connector that is grounded by the test
instrument before connecting the probe inputs to the test circuit.

Do not use the probe in wet or explosive atmos pheres. Remove
any contamination from the probe housing before connec ting the
probe inputs to any circuit. Make sure that the surface of the
probe head is completely dry before connecting the inputs.

Use of the probe, and/or the instrument it is connected to, in a
manner other than that specified may impair the protection
mechanisms.

Do not use the probe if any part is damaged. All maintenance
should be referred to qualified service personnel.

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The input circuits in the AP034 Activ e Differ ential
Probe incorporate components that protect the
probe from damage resulting from electrostatic
discharge (ESD). Keep in mind that this is an
active probe, and it should be handled carefully
to avoid damage. When using the AP034 Active
Differential Probe, you are advised to take
precautions against potential instrument damage
due to ESD.

CONNECTING THE PROBE TO THE TEST INSTRUMENT

When you are using the AP034 Active Dif ferential Probe with a
LeCroy Oscilloscope equipped with ProBus, attach the probe
output connector to the oscilloscope input connector. The
oscilloscope will recognize the probe, set the oscilloscope input
termination to 50  DQG DFWLYDWH WKH SUREH FRQWURO IXQFWLRQV LQ
the user interface.

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Caution

To use the AP034 Active Differ ential Probe with instrumentation
not equipped with a ProBus interface, it is necessar y to use the
ADPPS Power Supply. Attach the ADPPS connector to the probe
output connector. The output connector of the ADPPS is a
standard male BNC that can be directly connected to another
instrument. If necessary, the output of the ADPPS can be
interconnected with a 50 FRD[LDOFDEOH7RPLQLPL]HWKHHIIHFWV
of skin loss, this cable should be 1 m or less in length. The
AP034 Active Differential Probe is des igned to drive a 50 ORDG
The gain will be uncalibrated if the output is not correctly
terminated. If you are using the probe with an instrument with a
high input impedance, place a 50 %1&LQOLQHWHUPLQDWRURQWKH
instrument input before attaching the ADPPS.

CONNECTING THE PROBE TO THE TEST CIRCUIT

At the probe tip, two inputs and a ground connection are
available for connecting the probe to a circuit under test. For
accurate measurem ents, both the + and – inputs m ust always be

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connected to the test circuit. The ground connection is optional.
Positive voltages applied to the + input relative to the – input will
deflect the oscilloscope trace toward the top of the screen.

To maintain the high performance capability of the probe in
measurement applic ations, user care in connecting the probe to
the test circuit. Increasing the parasitic capacitance or inductance
in the input paths may introduce a “ring,” or slow the rise time of
fast signals. To m inimize these effects, use the shortest length
possible when connecting the probe to the circuit under test.
Input leads that form a large loop area (even shielded coaxial
cables) will pick up any radiated magnetic field that passes
through the loop, and may induce noise in the probe inputs.
Because this signal will appear as a diffe rential mode signal, the
probe’s common mode rejection will not remove it. You can
greatly reduce this effect by using short interconnection leads,
and twisting them together to minimize the loop area.

High common mode rejection requires precise matching of the
relative gain or attenuation in the + and – input signal paths.
Mismatches in additional parasitic capacitance, inductance,
delay, and a source impedance differ ence between the + and –
signal paths will lower the common mode rejection ratio.
Therefore, it is desirable to us e the sam e length and type of wire
and connectors for both input connections . W hen possible, try to
connect the inputs to points in the circuit with approxim ately the
same source impedance.

If AC coupling is desired, install the AC coupling accessory on
the probe tip before connecting it to the test circuit. The low-

frequency cutoff (–3 dB point) of the AC coupler is approxim ately
16 Hz.

If the voltage in the test circuit exceeds the probe’s capability,
add the external ÷10 or ÷20 attenuator* to the probe tip. If both
the external attenuator and AC coupler are used, install the
attenuator on the probe tip first, then install the AC coupler on the
attenuator input.

* Note

The external attenuators are precisely adjusted
during manufacturing to match the

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characteristics of the input of the probe with
which they were shipped. The input
characteristics of the external attenuator itself do
not exactly match those of the probe. Therefore,
using the ÷10 and ÷20 attenuators at the same
time is not recommended. The scale factor
encoding system will not operate correctly with
both attenuators installed simultaneously.

The input characteristics of the AP034 1 GHz
Active Differential Probe are significantly
different than those of the AP033 500 MHz
Active Differential Probe. The external
attenuators are not interchangeable. The
external attenuators currently supplied with the
probes are labeled with the appropriate model
number. The attenuators supplied with model
AP033 probes prior to the introduction of the
AP034 did not include the model number on the
label. When using an external attenuator with the
AP034 Active Differential Probe, make sure it is
labeled “AP034.”

Interchanging non-compatible attenuators will
not damage the probes; however, the transient
response of the measured signals will be
significantly in error.

In addition to being compatible with the included lead set, the
probe input connectors will mate with standard 0.025 in.
(0.635 mm) square pins in any rotational orientation. To avoid
damaging the input connectors, do not attempt to insert
connectors or wire larger than 0.036 in. (0.91 m m) in diameter.
Avoid rotating square pins after they are inserted into the input
connectors.

The included accessories simplify the task of connecting the
probe to the test circuit:

• Use the small (0.5 mm) mini clips with the flexible
lead set when connecting to fine-pitch surface mount
IC leads.

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PROBE INPUT LOADING

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• The larger (0.8 mm) mini clips can be used to
connect to through-hole leaded components.

• The offset round pins can be used for hand-held
probing applications. Reposition the pins by rotating
them to obtain the required spacing.

Attaching any probe to a test circuit will add some loading. In
most applications, the high impedance of the AP034 Active
Differential Probe inputs im parts an insignificant load to the test
circuit. However at very high frequencies, the capacitive
reactance of the probe’s input capacitance may load the circuit
enough to affect meas urement accuracy. The equivalent model
of the probe input circuits is shown below:

GROUNDING THE PROBE

+

≈ 0.1 pF

1.5 pF

1 MΩ

Figure 1. AP034 Equivalent Input Model

The single lead along with one of the larger (0.8 mm) m ini clips
can be used to ground the probe to the test circuit. Insert the pin
end of the lead into the receptacle marked:

Note

Do not use the attenuator encoding receptacle
(unmarked socket near the – input) to ground the
probe. Connecting to the encoding receptacle
will not provide adequate grounding, and may
result in in correct scale factor indication.

-

1.5 pF
1 MΩ

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In most cases it is not necessary to ground the probe to the
circuit under test. However, if the test circuit is isolated from earth
ground, it is usually necessary to connect the probe ground to a
point in the circuit. Grounding test circuits that are referenc ed to
earth ground may improve the fidelity of high-frequency
components in the waveforms. The potential for improvement
with grounding will vary depending on the common mode source
impedance. However, connecting the probe ground to a circuit
that is referenced to earth ground c an create a ground loop that
may add noise to low-amplitude signals*. Rejection of high­frequency common mode signals is improved when the probe
head is ungrounded.

The best recomm endation for connecting or not connecting the
probe ground is to try both configurations and select the one that
performs the best.

Note

The AP034 Active Differential Probe transmits
the measured signal differentially through the
probe cable. This essentially eliminates signal
degradation from ground loop effects within the
probe. However, creating a ground loop may
introduce signal distortions in the test circuit
itself, or in any coaxial cable between the
ADPPS power supply and the test instrument.

AP034-OM-E Rev D ISSUED: January 2000 ²

SELECTING THE PROPER RANGE

The AP034 Active Differential Probe has a fixed gain of X1
(unity). Use of the ÷10 or ÷20 external attenuators reduces the
amplitude of the input signal when it is necess ary to extend the
dynamic operating range of the probe. Attenuating the input
signal increases both the differential mode range and comm on
mode range of the probe.

Refer to the block diagram below.

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ControlProbe

+

1

-

1

By using the plug-on attenuator, you can extend the attenuation
range to ÷20. Do not use the ÷10 and ÷20 plug-on attenuators
simultaneously. The maxim um ranges are given in the f ollowing
table:

Table 1. AP034 Dynamic ranges and i nput capacitance at different attenuator

Common Mode Range
Differential Mode Range *

X1

Σ

+

V

Offs

Σ

X1

Figure 2. AP034 Block Diagram

settings

÷1

Attenuation

(Probe Only)

±16 V ±42 V ±42 V

±400 mV ±4 V ±8 V

*Offset moves the center point of this range.

Attenuation

+

-

÷10

X1

50

÷20

Attenuation

² ISSUED: January 2000 AP034-OM-E Rev D

When us ing a differential probe or amplifier, be ca reful to avoid
exceeding the common mode range. Because the common
mode signal is rejected by the differential probe, and is not
displayed, changes in the amplitude of the common mode
component are not apparent to the user. Exceeding the c omm on
mode range may introduce distortion into the AP034 output.

Circuitry in the probe detects the presence of either attenuator,
and displays the effective gain of the probe on the probe front
panel.

OPERATION WITH LECROY OSCILLOSCOPES

When the AP034 probe is connected to a LeCroy oscilloscope
equipped with ProBus interface, the displayed scale fac tor will be
adjusted to account for the effective gain of the probe. The
channel O

FFSET knob will control the probe offset, rather than the

offset at the oscilloscope input. The probe contr ol menu can be
activated by pressing the COUPLING button while the channel to
which the probe is attached is selected.

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AP034-OM-E Rev D ISSUED: January 2000 ²

Figure 3. AP034 Probe Control Menu

When the AP034 Active Differential Probe is first connected to
the oscilloscope, the following message will appear:
“WARNING: Probe offset buttons are locked.” This alerts you
that offset control m ust be m ade through the os cilloscope, rather
than through the buttons located on the probe. It does not
indicate a failure in the probe or oscilloscope.

Correct display of scale factor with the ÷20
attenuator installed and correct operation of
probe offset require that software version 8.1 or
higher be loaded. Refer to the manual section
“Oscilloscope Software Compatibility” for
additional information.

AP034 USE WITH THE ADPPS POWER SUPPLY

The optional ADPPS Power Supply allows the AP034 Active
Differential Probe to be used with instruments that are not
equipped with the ProBus interface. When used with the ADPPS,
the AP034 must be terminated into 50  ,I WKH WHVW LQVWUXPHQW
input impedance is not 50  LQVHUW D   LQOLQH WHUPLQDWRU
between the ADPPS and the instrument input. If a coaxial
extension cable is used, the terminator should be located at the
instrument end of the cable. Note that the additional parasitic
losses of extension cables may reduce the usable bandwidth of
the system below the AP034 specification.

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Note:

To prevent signal distortion, it is necessary to keep the AP034
output less than 400 mV at all times.

With the ProBus interface (see note on following page), the
oscilloscope O
Without the ProBus, it is acceptable to use the oscilloscope to
position the waveform at high sensitivities , but it is a practic e that
can lead to erroneous measurements when the probe output
exceeds ±400 mV. Therefore, when the ADPPS is used, it is
preferable to use the AP034 offset function to perform all
waveform positioning.

The AP034 offset allows you to measure signals up to 2.0 V
(1.6 V of offset plus 400 mV of output signal). With accessory
plug-on attenuators, the effective offset is increased, and input
signals of 20 V (÷10) or 40 V (÷20) may be viewed. The peak

² ISSUED: January 2000 AP034-OM-E Rev D

FFSET control actually controls the AP034 offset.

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signal that can be viewed at any equivalent scale factor is the
Input Differential Mode Range plus the off set used. See Adding
Offset on the following page.

When using the AP034 with the ADPPS power supply on
oscilloscopes without ProBus interface, use the following table to
select the desired effec tive volts per division and determine the
offset available.

Equivalent

Scale

Oscilloscope
Scale Factor

Factor

2 mV/Div 2 mV/Div

5 mV/Div 5 mV/Div
10 mV/Div 10 mV/Div
20 mV/Div 20 mV/Div
50 mV/Div 50 mV/Div

100 mV/Div 100 mV/Div
200 mV/Div 20 mV/Div
500 mV/Div 50 mV/Div

1 V/Div 100 mV/Div
2 V/Div 100 mV/Div

Table 2. Recommended AP034 and Os cilloscope settings to obt ain desired

AP034

Attenuation

Common

Mode Range

÷1 ±16V ±8 mV
÷1 ±16 V ±20 mV
÷1 ±16 V ±40 mV
÷1 ±16 V ±80 mV
÷1 ±16 V ±200 mV
÷1 ±16 V ±400 mV

1

÷10
÷10
÷10
÷20

1

1

1

1
2

3

±42 V ±800 mV
±42 V ±2 V
±42 V ±4 V
±42 V ±8 V

Attenuation obtained using t he external ÷10 or ÷20 attenuator.
Input differential mode range displayed is limi ted by ±4 divisions of vertic al
scale on oscilloscope.
Limited by probe output and ±4 divisions of vertical scale on oscilloscope.
Oscilloscope OFFSET and POSITI ON must be set to zero.

equivalent Volts/Divi sion

Input

Differential

Mode Range

2

2

2

2

2

3

2

2

3

3

Maximum

Offset

Maximum

Observable

Signal with

Offset

±1.6 V ±1.608 V
±1.6 V ±1.620 V
±1.6 V ±1.640 V
±1.6 V ±1.680 V
±1.6 V ±1.800 V
±1.6 V ±2.0 V

±16 V ±16.8 V
±16 V ±18 V
±16 V ±20 V
±32 V ±40 V

Input Noise

(nV/√Hz),

Typical

35
35
35
35
35

35
350
350
350
700

AP034-OM-E Rev D ISSUED: January 2000 ²

ADDING OFFSET

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The AP034 Active Differential Probe has true differential offset
capability. This allows you to remove a DC bias voltage from the
differential input signal while maintaining DC coupling. By using
probe offset rather than the P
the full dynamic range of the probe remains centered around the
offset level.

OSITION control on the oscilloscope,

+700 mV

+500 mV

+300 mV

0 mV

Input Differential

Waveform

+700 mV

+500 mV
+400 mV
+300 mV

0 mV

-400 mV

Waveform “clipped”

+700 mV

±400 mV window
re-centered around
the probe offset.

with no offset

+500 mV

+300 mV

±400 mV
centered
at 0 V

Figure 4. Effect of Dif f erential Offset

0 mV

Offset used to re-center
the differential dynami c
range around +500 mV

When the AP034 Active Differential Probe is used with a LeCroy
oscilloscope equipped with ProBus interface, the probe offset
can be controlled with the channel O

FFSET knob.* The buttons on

the probe housing will be disabled.
When used with the ADPPS Power Supply on instruments that

lack ProBus support, of fset can be c ontrolled with the buttons on
the probe’s front panel. The offset can be returned to zero at any
time by briefly pressing both the Î and Ï O
same time. Pres sing both the Î and Ï O

FFSET buttons at the

FFSET buttons for more

than 2 seconds will initiate an Autobalance cycle.

² ISSUED: January 2000 AP034-OM-E Rev D

*Note

Probe offset is controlled with the channel

O

FFSET knob in oscilloscope software versions

8.1.0 or higher. The current offset value is

AUTOBALANCE

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displayed above the graticule for a few seconds
after a change is made by turning the knob (refer
to Figure 3). In prior software versions, the
channel O
oscilloscope input rather than the probe offset.
When the AP034 Active Differential Probe is
used with LeCroy oscilloscopes with older
software versions, it is important that the channel
offset (controlled by the channel O
remain set to 0.0000 V. Moving the displayed
waveform with the channel offset rather than
probe offset will not re-center the dynamic
operating range of the probe. Offsetting the trace
with the oscilloscope channel offset introduces
the possibility of erroneous waveforms that result
from overloading the probe.

The AP034 Active Differential Probe incorporates an
Autobalance function to remove the DC offset from the output.
Autobalance must be invoked by you. When power is first
applied, the probe will return to the internal values resulting from
the last Autobalance cycle. For Autobalance to function properly,
all signals must be removed from the input.

FFSET knob adjusted the offset of the

FFSET knob)

After several minutes of warmup, or when the probe is exposed
to a large shift in ambient tem perature, s ome DC off set dr ift m ay
occur from thermal ef fects in the am plifier cir cuitry. To initiate an
Autobalance cycle, remove the probe from the test circuit and
select the AUTOBALANCE menu selection in the oscilloscope
“COUPLING” menu for the channel the probe is c onnected to. If
the probe is being used with the ADPPS power supply, remove
the input signal, then push and hold both O

FFSET buttons for two

seconds. The AP034 must be terminated into 50 IRUVXFFHVVIXO
Autobalance.

Upon successful completion of the Autobalanc e c ycle, all three of
the EFFECTIVE GAIN indicators will be briefly illuminated. If an
input signal is present during auto balance and the routine fails ,
the EFFECTIVE GAIN indicators will not illuminate. The probe
will then revert to the offset values resulting from the last

AP034-OM-E Rev D ISSUED: January 2000 ²

successful completion of the Autobalance cycle. In many
situations, this will be adequate to make routine measurements.

DESIGNING TEST FIXTURES FOR THE AP034 PROBE

Often it is desirable to connect the probe directly to user­fabricated test fixtures, such as those used for semiconductor
characterization. To facilitate use with custom test fixtures, the
input receptacles used in the AP034 probe are compatible with
commercially available 0.025 in. (0.635 mm) square pins. The
receptacles do not require a specif ic rotational orientation for the
square pin. The dimensions listed below can be used as a layout
guide for a test fixture circuit board. T he recomm ended insertion
depth of the pins is 0.100 in. (2.5 mm) to 0.200 in. (5.0 mm).

Top View

Looking toward circuit board

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Side View

0.100”-

0.200”

0.110”

0.105”

0.170”

0.100”

Figure 5. Layout dimensions f or test fixtures

0.025” REF.

# # #

² ISSUED: January 2000 AP034-OM-E Rev D

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DIFFERENTIAL MODE AND COMMON MODE

Differential probes amplify the voltage difference that appears
between the + and – inputs. This voltage is referred to as the
Differential Mode or Normal Mode voltage. The voltage
component that is refer enced to earth ground, and is identic al on
both inputs, is rejected by the amplifier. This voltage is referred to
as the Common Mode voltage, because it is common to both
inputs. The common mode voltage can be expressed as:

V

VCM =

DIFFERENTIAL MODE RANGE AND COMMON MODE RANGE

The Differential Mode Range is the max imum signal that can be
applied between the + and – inputs without overloading the probe
amplifier, resulting in “clipping” or distortion of the waveform
measured by the oscilloscope.

+Input

+ V

2

-Input

The Common Mode Range is the max imum voltage with respect
to earth ground that can be applied to either input. Exceeding the
common mode range can result in unpredictable results.
Because the Common Mode s ignal is normally rejected, and is
not displayed on the oscilloscope, you need to be careful to avoid
accidentally exceeding the common mode range.

AP034-OM-E Rev D ISSUED: January 2000 ²