Teledyne Lecroy DA1855A User manual

Operator’s
Manual
DA1855A
Differential Amplifier

DA1855A Differential Amplifier
Operator’s Manual
February, 2018

© 2018 Teledyne LeCroy, Inc. All rights reserved.
Unauthorized duplication of Teledyne LeCroy documentation materials other than for internal sales
and distribution purposes is strictly prohibited. Customers are permitted to duplicate and distribute
Teledyne LeCroy documentation for internal training purposes.
Teledyne LeCroy is a registered trademark of Teledyne LeCroy, Inc. Windows is a registered
trademark of Microsoft Corporation. Other product or brand names are trademarks or requested
trademarks of their respective holders. Information in this publication supersedes all earlier
versions. Specifications are subject to change without notice.
929404-00 Rev A
February, 2018

Operator’s Manual
i
Table of Contents
Safety Instructions................................................................................................................... 1
Symbols..............................................................................................................................1
Precautions.........................................................................................................................1
Operating Environment.......................................................................................................2
Cooling................................................................................................................................2
Cleaning..............................................................................................................................2
Calibration...........................................................................................................................3
Power..................................................................................................................................3
Specifications........................................................................................................................... 4
Nominal Characteristics......................................................................................................4
Warranted Characteristics ..................................................................................................6
Typical Characteristics........................................................................................................7
Physical Characteristics......................................................................................................8
Overview................................................................................................................................... 9
Models.............................................................................................................................. 10
Standard Accessories....................................................................................................... 10
Optional Accessories........................................................................................................ 10
Operation................................................................................................................................ 11
General Information.......................................................................................................... 11
Dynamic Range................................................................................................................ 11
Front Panel....................................................................................................................... 12
Rear Panel........................................................................................................................ 17
Instrument Settings........................................................................................................... 21
General Operating Information............................................................................................. 24
Power Connection ............................................................................................................ 24
Comparator Mode............................................................................................................. 24
Differential Mode............................................................................................................... 24
Set Up Using the ProBus Interface................................................................................... 25
Set Up Without ProBus Interface...................................................................................... 30
Determining the Proper Offset Mode ................................................................................ 31
Avoiding Common Problems ............................................................................................ 31
Care and Maintenance........................................................................................................... 34
Cleaning............................................................................................................................ 34
Calibration Interval............................................................................................................ 34
Service Strategy ............................................................................................................... 34
Troubleshooting................................................................................................................ 34
Returns............................................................................................................................. 34

DA1855A Differential Amplifier
ii
Applications............................................................................................................................35
Voltage Measurements.....................................................................................................35
Measuring Current............................................................................................................35
Effects of Probes on Saturation Voltage Measurements...................................................36
Saturation Voltage Measurement......................................................................................38
Gate Drive Voltage............................................................................................................39
Upper and Lower Gate Drive ............................................................................................40
Avoiding Measurement Errors...........................................................................................40
Remote Control Commands..................................................................................................41
Command List...................................................................................................................41
Gain Control Mode............................................................................................................42
Attenuation........................................................................................................................42
Auto-Zero..........................................................................................................................43
Bandwidth Limit.................................................................................................................43
Coupling............................................................................................................................44
Gain ..................................................................................................................................45
Offset................................................................................................................................46
Input-Resistance...............................................................................................................47
Probe Attenuation.............................................................................................................48
Precision Voltage Generator.............................................................................................49
Volt / DIV...........................................................................................................................50
Performance Verification....................................................................................................... 51
Test Equipment Required.................................................................................................51
Preliminary Procedure ......................................................................................................53
Functional Check..............................................................................................................54
Verification Procedure.......................................................................................................55
DA1855A Performance Verification Test Record..............................................................72
Reference................................................................................................................................75
Differential Mode and Common Mode ..............................................................................75
Differential Mode Range and Common Mode Range .......................................................75
Common Mode Rejection Ratio........................................................................................75
Certifications.....................................................................................................................77
Warranty...........................................................................................................................80
Returning a Product..........................................................................................................81
Technical Support.............................................................................................................82

Operator’s Manual
1
Safety Instructions
This section contains instructions that must be observed to keep this oscilloscope accessory
operating in a correct and safe condition. Follow generally accepted safety procedures in addition to
the precautions specified in this section. The overall safety of any system incorporating this
accessory is the responsibility of the assembler of the system.
Symbols
These symbols appear on the instrument's front or rear panels and in its documentation to alert you
to important safety considerations.
CAUTION
of potential damage to instrument, or
WARNING
of potential bodily injury.
Attend to the accompanying information to protect against personal injury or damage. Do
not proceed until conditions are fully understood and met.
High voltage
. Risk of electric shock.
Measurement ground connection.
Safety (protective) ground connection.
Alternating Current.
Power On; connected to AC mains.
Power Off; disconnected from AC mains.
Precautions
Use proper power cord. Use only the power cord shipped with this instrument and certified for the
country of use.
Maintain ground. This product is grounded through the power cord grounding conductor. To avoid
electric shock, connect only to a grounded mating outlet.
Connect and disconnect properly. Do not connect/disconnect probes or test leads while they are
connected to a voltage source.
Observe all terminal ratings. Do not apply a voltage to any input that exceeds the maximum rating of
that input. Refer to the specifications for maximum input ratings.
Use only within operational environment listed. Do not use in wet or explosive atmospheres.
Use indoors only.

DA1855A Differential Amplifier
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Keep product surfaces clean and dry.
Do not block the cooling vents. Leave a minimum six-inch (15 cm) gap around the sides and back
between the instrument and the nearest object. The feet provide adequate bottom clearance. Keep
the underside clear of papers and other objects.
Do not remove the covers or inside parts. Refer all maintenance to qualified service personnel.
Do not operate with suspected failures. Do not use the product if any part is damaged. Obviously
incorrect measurement behaviors (such as failure to calibrate) might indicate impairment due to
hazardous live electrical quantities. Cease operation immediately and sequester the instrument
from inadvertent use.
Operating Environment
Temperature: 0 to 50 °C.
Humidity: Maximum relative humidity 80 % for temperatures up to 31 °C decreasing linearly to 50 %
relative humidity at 50 °C.
Altitude: Up to 2,000 m (6,562 ft).
Cooling
The instrument relies on forced air cooling with internal fans and vents. Take care to avoid
restricting the airflow to any part of the amplifier. Around the sides and rear, leave a minimum of 15
cm (6 inches) between the instrument and the nearest object. At the bottom, the amplifier feet (up
or down) provide adequate clearance.
CAUTION. Do not block vents. Always keep the area beneath the amplifier clear of paper and
other items.
The instrument also has internal fan control circuitry that regulates the fan speed based on the
ambient temperature. This is performed automatically after start-up.
Cleaning
Clean only the exterior of the amplfier using a damp, soft cloth. Do not use harsh chemicals or
abrasive elements. Under no circumstances submerge the instrument or allow moisture to
penetrate it. Avoid electric shock by unplugging the power cord from the AC outlet before cleaning.
CAUTION. Do not attempt to clean internal parts. Refer to qualified service personnel.

Operator’s Manual
3
Calibration
The amplifier is calibrated at the factory prior to being shipped. The recommended calibration
interval is one year. Calibration should be performed by qualified personnel only. Schedule an
annual factory calibration as part of your regular maintenance. Extended warranty, calibration, and
upgrade plans are available for purchase. Contact your Teledyne LeCroy sales representative or
[e-mail] to purchase a service plan.
Power
AC Power Source
100 to 240 VAC (±10%) at 50/60 Hz (± 10%).
Manual voltage selection is not required because the instrument automatically adapts to line
voltage.
Power Consumption
DA1855A: 28 Watts (39 VA)
DA1855A-PR2: 56 Watts (78 VA)
Power and Ground Connections
The amplifier is provided with a 10A/250V 18AWG rated grounded cord set containing a molded
three-terminal polarized plug and a standard IEC320 (Type C13) connector for making line voltage
and safety ground connections.
The AC inlet ground is connected directly to the frame of the instrument. For adequate protection
again electric shock, connect to a mating outlet with a safety ground contact.
WARNING. Interrupting the protective conductor inside or outside the device, or
disconnecting the safety ground terminal, creates a hazardous situation. Intentional
interruption is prohibited.
Power On/Off
The Power On/Off switch on the back of the amplifier controls the operational state of the device.
Press the side of the switch closest to either On or Off.

DA1855A Differential Amplifier
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Specifications
These specifications are valid when the following conditions have been met:
•The instrument is being operated from a power source, which meets the line voltage and
frequency specifications.
•The instrument has been operating for at least 20 minutes in an environment that is within
the operating environmental specifications.
•The instrument has been calibrated within the last 12 months. Calibration was performed in
a controlled environment of 25° C ± 5° C.
Nominal Characteristics
Nominal characteristics describe parameters and attributes which are guaranteed by design, but do
not have associated tolerances.
General
Input Configuration
True Differential, + and – Inputs Precision Voltage Generator can be
selected as – input source in VCOMP mode.
Offset Capability
Precision Voltage Generator can be used to provide true differential offset.
+Input Coupling Selections
AC, Off (Precharge), DC
–Input Coupling Selection
AC, Off (Precharge), DC, V
COMP
Input Connectors
BNC (+Input incorporates Probe Attenuation Coding sensing connector)
Maximum Input Voltage
Withstand up to ± 200 Vp continuous. Automatic input disconnect with
manual reset.
Output Configuration
Single ended, Ground referenced
Output Impedance
50 Ω
Intended Output Load
50 Ω
Output Connector
BNC
Amplifier Gain
X1 or X10
Input Attenuation
÷1 or ÷10
Bandwidth Limit Filters
100 kHz, 1 MHz, 20 MHz or
(Low Pass)
None (full bandwidth)
Bandwidth Limit Filter
Characteristics 3-pole Bessel, 18 dB/octave

Operator’s Manual
5
General, continued
Auto Zero
Amplifier initiates an automatic balance cycle, when either gain button is
depressed, to remove output offset drift
Effective Gain Indicator
Indicators show the effective system gain or attenuation, factoring Probe
Attenuation, Attenuator and gain settings. (Probe must have coding
connectors. ÷1, ÷10, ÷100 and ÷1000 probes are recognized)
Dynamic Ranges
Maximum Differential Mode Range
X10 Gain, ÷1 Attenuator
± 50 mV1
X1 Gain, ÷1 Attenuator
± 0.5 V1
X10 Gain, ÷10 Attenuator
± 0.5 V1
X1 Gain, ÷10 Attenuator
± 5 V1
Maximum Input Slew Rate
÷1 Attenuator
± 0.15 V/μsec
÷10 Attenuator
± 1.5 V/μsec
Maximum Common Mode Range
÷1 Attenuator
± 15.5 V1
÷10 Attenuator
± 155 V1
Differential Offset Range (VDIFF mode referred to input)
X10 Gain, ÷1 Attenuator
± 1 V1
X1 Gain, ÷1 Attenuator
± 10 V1
X10 Gain, ÷10 Attenuator
± 10 V1
X1 Gain, ÷10 Attenuator
± 100 V1
Comparison Offset Range (VCOMP mode, referred to input)
÷1 Attenuator
± 15.5 V1
÷10 Attenuator
± 155 V1
1. Voltages are referred to the amplifier input connector. Multiply by probe attenuation factor to obtain value referred to
probe input (e.g. ± 50 mV becomes ± 0.5 V at the probe tip when using a ÷10probe.)

DA1855A Differential Amplifier
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Precision Voltage Generator
Output Range
± 15.5 V
Output Impedance
≈ 10 Ω
Resolution
100 μV
Control
Individual increment and decrement digits carry over to the next decade
Reference Type
Oven stabilized buried zener diode
Output Routing
Can be applied to –Input and available at rear panel BNC connector
Power Requirements
Line Voltage Range
90 - 264 VAC
Line frequency Range
45 - 66 Hz
Warranted Characteristics
Warranted characteristics describe parameters which have guaranteed performance. Unless
otherwise noted, tests are provided on page 55, Performance Verification, for all warranted
specifications.
Gain Accuracy
± 1% + uncertainty of termination resistance
Bandwidth (-3 dB) x1 Gain
> 100 MHz
Rise Time
< 3.5 nsec (Calculated from bandwidth)
Common Mode Rejection
x1 or x10, ÷1 attenuation
70 Hz
≥50,000:1 (94 dB)
100 kHz
≥50,000:1 (94 dB)
10 MHz
≥316:1 (50 dB)
Precision Voltage Generator
Voltage Accuracy
+/- 1% of reading +/- 500uV accuracy (15° C to 45° C)

Operator’s Manual
7
Typical Characteristics
Typical characteristics describe parameters, which do not have guaranteed performance. Tests for
typical characteristics are not provided in the Performance Verification Procedure.
Input Resistance
÷1 Attenuator
1 MΩ or 100 MΩ 1 MΩ only when used with attenuating probe
÷10 Attenuator
1 MΩ
Input Capacitance
20 pF
AC Input Coupling Capacitance
0.1 μF
÷10 Attenuator Accuracy
0.05%
Bandwidth, x10 Gain
100 MHz
Common Mode Rejection Ratio
Refer to Figure 22
Input Noise
Refer to Figure 23
(With DXC100A probe, 100 Ω between tip and ground)
Overdrive recovery
In X10 gain, settles within 1 mV referred to input within 100 nsec from
4 V input (8000% overdrive)
Output Zero1
≤ 2 mV referred to input
Input Leakage Current
<10 pA (0° C to 45° C)
Differential Offset Accuracy
X10 Gain, ÷1 Attenuator
0.1% + 50 μV2+PVG voltage accuracy
X1 Gain, ÷1 Attenuator
0.1% + 500 μV2+PVG voltage accuracy
X10 Gain, ÷10 Attenuator
0.15% + 500 μV2+PVG voltage accuracy
X1 Gain, ÷10 Attenuator
0.15% + 5 mV2+PVG voltage accuracy
Temperature Coefficient (power consumption)
DA1855A
≈ 28 W, ≈ 39 VA
DA1855A-PR2
≈ 56 W, ≈ 78 VA
1. Output Zero is the output voltage from zero with zero Volts applied between inputs. This specification is valid within 30
minutes from last Autozero cycle when the DA1855A is operating in an environment with stable ambient temperature.
2. Voltages are referred to the amplifier input connector. Multiply by probe attenuation factor to obtain value refer to probe
input. (e.g. 0.1% + 50 μV becomes 0.1% + 500 μV at the probe tip when using a ÷10 probe.)

DA1855A Differential Amplifier
8
Physical Characteristics
Height DA1855A 7.29 cm (2.87 inch)
DA1855A-PR2 8.75 cm (3.4 inch)
Width DA1855A 21.2 cm (8.36 inch)
DA1855A-PR2 43.9 cm (17.3 inch)
Depth DA1855A 23.2 cm (9.12 inch)
DA1855A-PR2 42.5 cm (16.7 inch)
Weight DA1855A 2.15 kg (4 lbs 12 oz.)
DA1855A-PR2 9.5 kg (21 lbs)
Shipping Weight DA1855A 3.12 kg (6 lbs 14 oz.)
DA1855A-PR2 11.3 kg (25 lbs)

Operator’s Manual
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Overview
The DA1855A is a stand-alone high performance 100 MHz differential amplifier. It is intended to act
as signal conditioning preamplifier for oscilloscopes, spectrum analyzers and other instruments,
providing differential measurement capability to instruments having only a single-ended input.
When used with a DA1855A, high quality oscilloscopes can obtain common mode rejection and
overdrive recovery performance that was previously unobtainable in any product.
When used with a Teledyne LeCroy oscilloscope equipped with ProBus interface, the DA1855A can
be controlled through the oscilloscope user interface or remote commands.
When used with non ProBus oscilloscopes, the DA1855A settings can be controlled directly through
the front panel controls.
Amplifier gain may be set to 1 or 10. A built-in input attenuator may be separately set to attenuate
signals by a factor of 10, allowing gains of 10, 1, or 0.1 and common mode dynamic range of ± 15.5
V (÷1) or ± 155 V (÷10). Optional probes increase the maximum input signal and common mode
ranges in proportion to their attenuation ratio, but not exceeding their maximum input voltage
rating. Effective gain of the DA1855A, including probe attenuation, amplifier gain and attenuator
settings, is automatically displayed.
The DA1855A has a bandwidth of 100 MHz, but any one of the three 3-pole bandwidth limit filters
may be selected to reduce bandwidth to 20 MHz, 1 MHz or 100 kHz to limit noise above the
frequency of interest.
The DA1855A output is limited at ± 500 mV so that the oscilloscope is not overdriven by large
inputs. This allows an oscilloscope to directly measure the settling of D/A converters with 14 bit
(60ppm) precision.
The DA1855A features a built-in Precision Voltage Generator (PVG) that can be set to any voltage
between ± 15.5 Volt (± 10 Volt in Differential Offset mode) with 5-1/2 digit resolution. Each digit of
the voltage generator output can be individually incremented or decremented. Positive or negative
polarity can be selected. The PVG’s output can be selected as an input to the inverting (–) input of
the amplifier for operation as a differential comparator or applied internally as a true differential
offset voltage. The voltage is also available to be used externally through a rear panel connector.
To maintain the amplifier’s high Common Mode Rejection performance, special differential probes
such as the Teledyne LeCroy model DXC series are recommended and are available as optional
accessories.

DA1855A Differential Amplifier
10
Models
The DA1855A series is comprised of two models which differ in physical configuration. Both contain
the same 100 MHz differential amplifier which provides high common mode rejection, extremely
fast overdrive recovery, selectable ÷1 or ÷10 attenuation, selectable X1 or X10 gain, a 5-1/2 digit
Precision Voltage Generator (PVG), selectable upper bandwidth limiting filters, an effective gain
display, and ± 500 mV output swing limiting. The PVG is provides a calibrated reference for
Differential Offset or Comparison modes. The PVG reference voltage is also available through a BNC
connector on the rear panel.
DA1855A: Single channel unit packaged in a convenient desk top housing.
DA1855A-PR2: Two independent model DA1855A amplifiers packaged in one housing with
one line power input connection. The housing is intended for desk top use,
where it can be placed under an oscilloscope.
DA1855A-RM: Single channel rack mounted version.
DA1855A-PR2-RM: Dual channel rack mounted version.
Standard Accessories
Each DA1855A Differential Amplifier is packaged for shipment with the following items:
•Power cable, as specified by power cable option.
•ProBus interface cable with BNC signal cable. (-PR2 models include 2 ProBus cables)
•Certificate of Calibration traceable to NIST (United States National Institute of Standards
and Technology)
•Operator’s Manual, which includes a Performance verification Procedure.
Optional Accessories
•Service Manual, containing adjustments, repair and replacement part information P/N:
DA1855A-SM-E.
•DXC100A, ÷10 / ÷100 Passive Differential Probe.
•DXC200, ÷1 Passive Differential Probe.
•DXC5100, ÷100 2.5 K Passive Differential Probe Pair
•DA101, External ÷10 Attenuator.

Operator’s Manual
11
Operation
General Information
The DA1855A has been designed to be used with oscilloscopes equipped with a ProBus interface.
Connecting the Differential Amplifier to the oscilloscope through the ProBus interface will
automatically control all the required settings from the oscilloscope and will lock-out the DA1855A
front panel controls. All front panel controls are now accessible through the oscilloscope user
interface. The DA1855A user interface can be viewed from the Channel setup dialog for the channel
to which it is connected. The DA1855A front panel controls will operate manually when the
Differential Amplifier is connected to an oscilloscope not provided with a ProBus interface.
NOTE: Removing the ProBus interface cable with the differential amplifier still powered up, requires
the DA1855A to be turned OFF and ON to access the front panel controls.
Dynamic Range
The basic amplifier dynamic range in X1 Gain and ÷1 Attenuation is ± 0.500 V. Changing the gain
and or attenuation will affect both the Differential Mode and Common Mode ranges.
The Differential Mode range is scaled by both gain and attenuation, while the Common Mode range
is scaled by attenuation only.
Gain Atten* Differential Mode* Common Mode*
1 ÷1 ± 0.5 V ± 15.5 V
1 ÷10 ± 5.0 V ± 155 V
10 ÷1 ± 50 mV ± 15.5 V
10 ÷10 ± 0.5 V ± 155 V
* Attenuation, Common Mode and Differential Mode ranges are scaled with external probe
attenuation. A ÷10 probe will increase all these values by a factor of 10.

DA1855A Differential Amplifier
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Front Panel
Input Connectors
Signals applied to the +INPUT and the –INPUT are connected either directly to the DA1855A
amplifier’s inputs or to the input attenuators. Maximum input voltage is ±200 Vp
A signal connected to the +INPUT will remain its polarity at the output connector. A signal
connected to the –INPUT will be inverted in polarity.
Attenuators
The input attenuators are passive networks which divide each signal by ten.
In ÷1 mode the front panel input connectors are directly connected to the DA1855A amplifier's
differential inputs.
In ÷10 mode each front panel input connector is connected to a passive 1 MΩ attenuator. The
attenuator output is connected to the DA1855A amplifier's corresponding differential input. The
signal at each input is attenuated by a factor of ten.
Gain
The DA1855A amplifier gain (amplification) is selectable between X1 and X10. The amplified signal
appears at the rear panel AMPLIFIER OUTPUT connector. Gain will affect the differential mode
output signal by amplifying the signal difference between the +INPUT and the –INPUT, but will not
affect the common mode signal, the signal common to the +INPUT and them –INPUT.
Output Termination
Proper gain is obtained when the DA1855A drives a 50 Ω load such as an oscilloscope with input
impedance set to 50 Ω. Automatic 50 Ω termination is obtained when the DA1855A is connected to
a Teledyne LeCroy oscilloscope through the ProBus interface.
An instrument with only a 1 MΩ input impedance available should have a 50 Ω coaxial termination
placed on its input connector. The DA1855A is then connected to the oscilloscope through the
coaxial termination.
Input Resistance
When the input ATTENUATOR is set to ÷1 and no attenuating probe is connected, the input
resistance can be increased from 1 MΩ to 100 MΩ. This is advantageous when measuring high
impedance circuits or when AC coupling is needed with a very low frequency cut off. When the input
ATTENUATOR is set to ÷10 or an attenuating probe with read out capability is attached, 1 MΩ (1M)
input resistance is automatically selected.
Unbalanced source impedances can have an adverse effect on common mode rejection. For
example, a differential source with impedances of 1000 and 2000 Ω, each loaded with 1 MΩ will
have a common mode rejection ratio (CMRR) of 1000 to 1. The common mode rejection ratio can be
improved to 100,000 to 1 by using 100 MΩ input resistance.

Operator’s Manual
13
Auto Zero
Auto Zero is a feature invoked from the Channel setup dialog when the amplifier is connected via
the ProBus interface. If the Differential Amplifier is not connected through a ProBus interface, push
either the X1 or X10 button, even if a different gain is not selected. Auto Zero momentarily sets the
input coupling to OFF and determines the offset necessary to set the output at 0 Volt. During this
process the front panel input signal to the amplifier is interrupted. When the Auto Zero cycle is
completed, the input coupling returns to its previous state. Auto Zero usually takes less than one
second to complete. This feature allows you to DC balance the DA1855A simply by pushing the
GAIN button which is already illuminated. When changing gains, the Auto Zero feature is
automatically invoked, adjusting the amplifier’s DC balance.
+ Input Coupling (AC – OFF – DC)
In OFF mode, the input connector is disconnected from the amplifier input, and the amplifier input is
connected to ground. The AC coupling capacitor is connected between the +INPUT and ground
through 1 MΩ resistor, independent of the INPUT RESISTANCE setting. In this mode, the AC
coupling capacitor is quickly charged to the average DC input voltage. OFF mode is also referred to
as precharge mode. Precharge is particularly useful prior to selecting AC coupling when the input
voltage has a DC component in excess of 19 V. The DA1855A input coupling is set to OFF and
connected to the circuit under test. When the +INPUT is changed from OFF to AC mode, the
coupling capacitor is already charged, and the trace properly centered on the oscilloscope screen.
Additionally, the risk of tripping the input overload detector and automatically disconnecting the
input is eliminated.
In the AC mode, the +INPUT is connected through an AC coupling capacitor to the amplifier input or
the input attenuator. The coupling capacitor retains its charge when the input is switched to DC,
making it possible to return to the same circuit without the precharge time. But this also makes it
possible to discharge the coupling capacitor into another circuit under test if its DC voltage differs
by more than approximately 19 V from the voltage on the coupling capacitor.
NOTE: The discharge current from the AC coupling capacitor is limited to about 70 mA. In some
situations this could damage sensitive circuits. To avoid the inrush current transient, it is therefore
recommended that the +INPUT coupling first be changed to the OFF (precharge) when measuring a
new circuit point. This will safely recharge the AC coupling capacitor in less than 0.3 seconds.
DC and low frequencies are attenuated by the AC coupling capacitor and the input resistance. With
the ATTENUATOR set to ÷10, or set to ÷1 with the INPUT RESISTANCE set to 1 MΩ, the low
frequency cut off (-3dB point) is approximately 1.6 Hz. When the input attenuator is set to ÷1, the
INPUT RESISTANCE may be set to 100 MΩ, and the –3 dB point is 0.016 Hz. This extremely low
frequency cut off is useful for observing low frequency noise riding on larger DC voltages.
In the DC mode, the +INPUT connector is connected to the amplifier either directly or through the
input attenuator, and the AC and DC attenuation are the same.

DA1855A Differential Amplifier
14
- Input Coupling (AC–OFF – DC – VCOMP)
The –INPUT has the same coupling modes as the +INPUT plus one additional option, VCOMP
(comparison voltage).
The DA1855A contains a precision DC voltage source which is controlled by the oscilloscope
OFFSET control. (When the amplifier is used stand alone, without ProBus interface to a Teledyne
LeCroy oscilloscope, the voltage is controlled by the push buttons above and below the front panel
numerical display.) This voltage source is called the Precision Voltage Generator (PVG).
The DA1855A's amplifier subtracts the voltage applied to its inverting input from the voltage applied
to its non-inverting input. The DA1855A output is therefore zero whenever these two voltages are
equal. For this reason, the voltage applied to the inverting input is called a comparison voltage,
VCOMP. Stated another way, the value of the horizontal center line in the oscilloscope graticule is
the voltage read in the PVG display. Each graticule line above or below the center line will add or
subtract the Volts/div value from the PVG setting.
VCOMP can be used to make precise measurements of large signals by comparing the accurately
known VCOMP with the unknown signal. It can also be used to measure the actual voltage at any
point of a waveform.
Since the amplifier’s gain and input attenuator are individually selectable, the comparison range can
be changed from ± 15.500 V to ± 155.000 V by changing the ATTENUATION from ÷1 to ÷10, while
the overall gain can still be set either to 1 or 0.1 by selecting either X10 or X1 GAIN.
NOTE: While in VCOMP mode, the amplifier is configured for single ended measurements. The –INPUT
connector is not usable when VCOMP is selected. The input signal applied to the + INPUT is
referenced to ground offset by the value set by the Precision Voltage Generator. Large calibrated
offsets can be obtained while making differential measurements by using VDIFF mode.
Precision Voltage Generator
The PVG generates the voltage which is used in the VCOMP and VDIFF modes and appears at the rear
panel OFFSET VOLTAGE (PVG) output connector for use as a reference voltage.
The Precision Voltage Generator (PVG) output range is ± 15.500 Volt. The PVG is never attenuated
by the input attenuator. Attenuation of the +INPUT signal by the ÷10 input attenuator will cause the
PVG to null out an input voltage up to ± 155.00 Volt which is ten times larger than the actual PVG
voltage.
The increase in common mode voltage range also applies when using attenuating probes.
When the DA1855A is used with attenuating probes that feature readout, the PVG display is
changed to indicate the voltage at the +INPUT probe tip which will bring the amplifier output to zero.
When connected to a Teledyne LeCroy oscilloscope via the ProBus interface, the oscilloscope
OFFSET control increments or decrements the PVG’s output voltage and the offset value will be