LeCroy DA1855A Instruction Manual

Operation
DA1855A-OM-E Rev A ISSUED: July 2002 3-1
3Operation
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 by pressing the Coupling button when the channel
connected to the DA1855A is selected. 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÷1Attenuation
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 attenua-
tion, while the Common Mode range is scaled by attenuation
only. Refer to table 3-1.
Table 3-1. Dynamic Range
* 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.
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

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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 ÷÷1mode 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 MΩattenuator. 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
X1and 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 Ωload
such as an oscilloscope with input impedance set to 50 Ω.
Automatic 50 Ωtermination is obtained when the DA1855A is
connected to a LeCroy oscilloscope through the ProBus
interface.
An instrument with only a 1 MΩinput impedance available should
have a 50 Ωcoaxial termination placed on its input connector.
The DA1855A is then connected to the oscilloscope through the
coaxial termination.

Operation
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Input Resistance
When the input ATTENUATOR is set to ÷÷1and no attenuating
probe is connected, the input resistance can be increased from
1MΩto 100MΩ. This is advantageous when measuring high
impedance circuits or when AC coupling is needed with a very
low frequency cut off. When the inputATTENUATOR 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 MΩwill
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 MΩinput resistance.
Auto Zero
Auto Zero is a feature invoked from the user interface in the
Coupling menu when connected via the ProBus interface or in
case the Differential Amplifier is not connected through a ProBus
interface by pushing either the X1 or X10 button, even if a
different gain is not selected. Auto Zero momentarily sets the
input coupling to OFFand 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 the user 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 MΩresistor, 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

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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 ATTENUATORset to
÷÷ 10, or set to ÷÷ 1with the INPUT RESISTANCE set to 1 M Ω , 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 M Ω , 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.
–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
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).

Operation
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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. Refer to figure
3-1 where the horizontal center line represents a power supply
voltage of 5.030 V, the next higher line 5.050 V and the line below
the center line 5.010 V. In this figure noise on a + 5.030 V signal
is easily displayed using 5.030 Volt offset and a vertical scale
factor of 20 mV/div.
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 theATTENUATION from ÷÷1to ÷÷10,
while the overall gain can still be set either to 1 or 0.1 by selecting
either X10 or X1 GAIN.
Figure 3-1. Voltage Measurement
5.030 V
5.010 V
5.050 V

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Note that 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 +INPUTsignal 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 LeCroy oscilloscope via the ProBus
interface, the oscilloscope OFFSET control increments or
decrements the PVG’s output voltage and the offset value will be
shown on the six PVG front panel indicators. The new offset
value will also be displayed on the oscilloscope’s screen for a few
seconds after a change has been made.
When connected to an oscilloscope not provided with a ProBus
interface, the PVG can be accessed by means of push buttons.
Above each digit is a push button which increments the
corresponding digit by one when pushed. When held, the digit
continues to increment, eventually incrementing the next higher
digit.
Similarly, below each digit is a push button which decrements the
corresponding digit.
The ±± button above the left-most digit changes the PVG output
polarity. The ZERO button below the left-most digit sets the
output to zero and invokes the PVG's Auto Zero function.

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PVG absolute mode: DA1855 PVG increment and decrement
buttons always function to increment or decrement the voltage
displayrespectively. When decrementing from a positive voltage,
the display always stops at zero. To obtain negative voltages, the
±± button must be pushed, and the increment button is used to
increase the magnitude of the negative voltage. This operation is
natural if simply setting a voltage, but unnatural if moving a
displayed oscilloscope waveform. This is known as the PVG
absolute mode, and the only mode available in the original
DA1855, (non “A” model). The DA1855A retains the option of
operating in this same manner as well as supporting PVG roll
through zero mode. Note:
When the DA1855A is controlled remotely through a
LeCroy oscilloscope, neither PVG absolute or PVG roll
through zero modes apply. When operated remotely, the
PVG value is controlled with the use of the OFFSET knob
on the oscilloscope, when in effect, operates in the roll
through mode.
PVG roll through zero mode: The DA1855A increment buttons
are oscilloscope waveform related by factory default. The
increment buttons move a displayed oscilloscope waveform
upward and the decrement buttons move the waveform
downward independent of the PVG polarity. Decrements from a
positive voltage will roll smoothly through zero. This is known as
roll through zero mode.
Toggle PVG modes: To change from roll through zero to
absolute mode of operation hold the PVG ZERO button and
press the ±button. Change back to the roll through zero mode by
repeating the same operation.
Differential Offset
VDIFF (differential offset voltage) is an instrument mode rather
than a type of input coupling. The VDIFF mode allows the PVG to
inject a calibrated offset signal into the DA1855A while still using
both inputs for full differential operation. This mode can be used
as a position control to move the trace on the oscilloscope screen
in preference to using the oscilloscope's position or offset control.
The oscilloscope's position and offset controls should always be
set to zero so that the DA1855A's dynamic range is properly
centered. (This is done automatically when using a LeCroy
oscilloscope with ProBus interface.) When the oscilloscope is set
to greater sensitivities (lower Volts/Div settings), the Differential

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offset provides much greater range than the conventional position
control. For example, at 50 mV/div, theVDIFF mode provides up to
±200 divisions of range.
Operation of the DA1855A using the VDIFF function is the same
as VCOMP except for the following:
•The –INPUT remains active, allowing full use of the
DA1855A as a differential amplifier.
•The maximum range of the PVG is ±10.000 Volt in X1
GAIN and ±1.0000 Volt in X10 GAIN. The effects of the ÷÷10
input ATTENUATOR and probe attenuation are the same
as when using VCOMP, i.e., any input attenuation multiplies
the effective offset.
The DA1855A's PVG display is changed to indicate the voltage
that, if applied between the+INPUTand –INPUT, would bring the
amplifier output to zero. When the DA1855A is used with
attenuating probes which feature readout, the PVG display is
scaled to include the effect of probe attenuation.
Effective Gain
Six indicators (LEDs) across the top of the DA1855A front panel
show the total gain from the instrument input to output. Logic
within the amplifier includes the gain, internal attenuation, and
probe attenuation factors (when readout encoded probes are
used) to determine the effective gain. When the X1 light is ON,
the overall amplifier voltage gain (amplification) is unity. Similarly,
X10 indicates an overall amplification of ten times. ÷÷10Indicates
the voltage amplification is 0.1, and so forth.
The DA1855A communicates the effective gain information to the
LeCroy oscilloscope when the ProBus interface is used. This
corrects the scale factor of the displayed waveforms, cursors and
measurements.
When LeCroy DXC series or other readout encoded probes are
used, the effective gain includes the probe’s attenuator factor.
BW Limit
FULL — The DA1855A amplifier's full bandwidth, over 100MHz,
is passed to the oscilloscope, spectrum analyzer or digitizer.
Frequency response and transient response are essentially
independent of the oscilloscope’s input impedance.
20 MHz — A 20MHz three pole (18dB/octave) filter allows the
DA1855A to reduce extraneous noise. This filter is a passive LC

Operation
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design and is intended to drive a 50 Ωload. Without the load, the
filter's frequency response and transient response are altered.
1 MHz — The 1MHz filter is of the same design as the 20 MHz
filter, and the same remarks apply.
100 kHz — The 100kHz filter is an active filter with a 50 Ωoutput
impedance. Transient and frequency response are independent
of the load impedance.
Overload
When a signal, which could damage the DA1855A, has been
applied to either input connector, the DA1855A protects itself by
disconnecting the signal. The input coupling mode changes to
OFF, and the OVERLOAD light is turned on.
To reset the amplifier to normal operation, remove the offending
input, press any of the input coupling modes (AC, OFF, or DC).
The Overload light will turn off indicating the amplifier is reset.
When the ATTENUATOR is set to ÷÷1, an input signal of
approximately ±19 Volt will activate the overload protection
circuit. Fast transients will draw up to about 70 mA of input
current for a brief period before the input coupling relay acts to
disconnect the input.
Caution:
Inputs in excess of 250 Volt may cause permanent dam-
age to the DA1855A.
The input is not disconnected when the ATTENUATOR is set to
÷÷10. The input attenuator can withstand up to 200 Volt continuous
input.
REAR PANEL
Power
Normal instrument operation is obtained with the power switch in
the 1(ON) position. The instrument can be used immediately,
however it requires a 30 minute warm up period to reach
specified performance. Prior to reaching operating temperature,
the amplifier offset will drift and the output from the Precision
Voltage Generator may not be within specification. In high-
humidity environments the time to stabilize may be much longer.
In high humidity environments or when warm-up time inhibits

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usage, the instrument may be left plugged in at all times and the
power switch left in the 1(ON) position.
Power up indication
Upon turn-on, the model number and firmware version are briefly
displayed in the PVG readout. For example, 1855.12 indicates
that the instrument is a model DA1855A and the firmware version
is 1.2.
Precision Voltage Generator Offset Voltage
The rear panel OFFSET VOLTAGE BNC (PVG) output
connector, is a monitor of the Precision Voltage Generator (PVG).
The voltage present on this connector is the same voltage as that
applied to the –INPUTwhen the –INPUTcoupling is set toVCOMP
or internally to the DA1855A when VDIFF is selected. The
OFFSET VOLTAGE output can be used to monitor the PVG with
a digital Voltmeter (DVM). A low pass filter between the PVG
output and the –INPUT removes radio frequency interference
(RFI) from the signal. This filter does not attenuate the PVG
signal.
The PVG output is not attenuated by the input attenuator or
probes, whereas the input signal is. Therefore the effective range
of VCOMP is increased by a factor of 10 when the ÷÷10
ATTENUATOR is selected or a÷10 attenuating probe is used to
attenuate the input signal. The PVG numerical display reflects the
attenuator setting and probe attenuation when the probe is
readout encoded. As an example, if there are no probes
attached, the ÷÷10 ATTENUATOR is selected and the display is
set to read –155.000, the PVG output will actually be –15.5 Volt.
The decimal in the display will be in the correct location to
indicate the voltage at the PVG output when no probes are
attached and ÷÷1 ATTENUATOR and X1 GAIN are selected.
The OFFSET VOLTAGE BNC (PVG) output also presents the
same voltage used internally for differential offset when VDIFF is
selected. Because the PVG is applied to the amplifier to create a
true differential offset, the relationship between VDIFF and the
voltage at the OFFSET VOLTAGE BNC (PVG) output (changes
with the amplifier gain selection according to the following table:
The maximum VDIFF is multiplied by any probe attenuation
factor.The DA1855A front panel displays the correct offset
referred to the instrument input.

Operation
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Table 3-2. VDIFF Range for Different Gain and Attenuator Settings
When using readout encoded probes which the DA1855A
senses, the PVG readout calculates the effective differential
offset at the probe tip. Of course, both probes must have the
same attenuation factor.
In the VCOMP mode, the maximum OFFSET VOLTAGEinput is
limited by the DA1855A common mode dynamic range. In the
VDIFF mode it is limited by the dynamic range of the internalVDIFF
amplifier.
Tables 3-3 and 3-4 will help the operator stay within the maximum
input voltage limits and understand the relationship between the
actual voltage applied and the effective voltage. Effective voltage
is always referred to the input of the DA1855A or the probe tip if a
probe is used. When using probes, the maximum effective
voltage range may be limited by the maximum voltage rating of
the probe.
Table 3-3. Effective Offset Range with ÷1÷1 Probe
Gain Attenuation Max. VDIFF
X1 ÷÷1± 10 V
X1 ÷÷10 ± 100 V
X10 ÷÷1± 1 V
X10 ÷÷10 ± 10 V
Front panel
Settings
Effective Offset
Range
Gain Attenuation VCOMP VDIFF
X1 ÷÷1± 15.5 V ± 10 V
X1 ÷÷10 ± 155 V ± 100 V
X10 ÷÷1± 15.5 V ± 1 V
x10 ÷÷10 ± 155 V ± 10 V

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Note
The effective voltage is always increased by the attenuator. It
therefore follows that any probe will increase the effective voltage
of both VCOMP and VDIFF by its attenuation factor. For example, a
probe with a 100X attenuation factor will increase the effective full
scale range by 100.
Table 3-4. Effective Offset Range with ÷÷100 Probe
Although the full scale range may be 10 kV or 15.5 kV, most
probes have a much lower maximum input voltage rating which
must not be exceeded.
Amplifier Output
The AMPLIFIER OUTPUTBNC is intended to be used with an
oscilloscope, spectrum analyzer or instrument having a 50 Ω
input resistance. The amplifier’s output impedance is 50 Ω.
Without the 50 Ωload, the amplifier gain will be uncalibrated and
will be approximately twice the amount indicated on the front
panel. Proper operation of the 1 MHz or 20 MHz bandwidth limit
filters requires an output load impedance of 50 Ω.
Remote Operation
A REMOTE connector on the rear panel of the DA1855A allows
total control of the instrument through a LeCroy oscilloscope
when connected to ProBus using the supplied cable. All of the
instrument functions can be controlled through the oscilloscope
user interface.
Remote control is also possible using commands sent through
the IEEE-488 bus or through RS-232 connected to the
Front Panel
Settings
Effective Offset Range
with ÷÷100 Probe
Gain Attenuation VCOMP VDIFF
X1 ÷÷1± 1.55 kV ± 1 kV
X1 ÷÷10 ± 15.5 kV ± 10 kV
X10 ÷÷1± 1.55 kV ± 100 V
x10 ÷÷10 ± 15.5 kV ± 1 kV

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oscilloscope. The DA1855A cannot be remotely controlled
without a LeCroy oscilloscope. Refer to Section 7 for a
description of the Remote Commands.
When the ProBus cable is installed, the buttons on the front panel
of the differential amplifier are disabled.
Note:
Remote operation requires software version V8.1.0 or higher.
Probe Coding Input
This jack is to be used with LeCroy DXC series probes to detect
the probe attenuation factor. Other manufacturer’s probes with
standard probe coding capability will be properly decoded
through the DA1855A's front panel +INPUTBNC connector.
INSTRUMENT SETTINGS
The DA1855A output is intended to connect directly to the input
of an oscilloscope, or other instrument, but it is important to
observe some rules so that the DA1855A delivers its specified
performance.
CAUTION
A properly terminated differential amplifier can deliver an
output voltage of ±0.5 Volt. The output is DC coupled and
will follow any DC component applied to the input. Some
instruments such as spectrum analyzers could be dam-
aged from overload or DC components.
Retained Settings
All front panel settings, including Precision Voltage Generator
(PVG) settings are retained when the instrument is turned off.
The DA1855A return to the same state they were in when power
was removed.
When used without ProBus interface, the instrument can be set to
factory default settings by pressing the VCOMP and VDIFF buttons
simultaneously. Table 3-4 lists the factory default settings.

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Table 3-5. DA1855A Factory Default Settings
Sensitivity, Position and Offset
Oscilloscopes are designed to maintain their accuracy for that
portion of a signal that is displayed on-screen. When the signal is
large enough to drive the display off-screen, the oscilloscope’s
amplifier must limit the signal in a non-linear mode. Oscilloscopes
are designed so that no matter how the sensitivity, position and
offset controls are set, the operator cannot view this distorted
portion of the signal.
When used with a LeCroy oscilloscope, the setup is automatic to
prevent the user from entering a mode which could result in
displaying a distorted signal resulting from overload.
When used with instruments lacking ProBus interface, the
instrument’s gain and position controls should be properly set to
avoid displaying the non-linear portion of the DA1855A's output
signal when it is in overdrive. This can be accomplished by
observing the two following rules:
1. Turn the oscilloscope input coupling to “OFF” or “GND”,
set the oscilloscope position control to center screen,
and do not change it! If the oscilloscope has an OFFSET
control, it too should be set to zero. Return the oscilloscope’s
input coupling to “DC”. Subsequently adjust the trace position
on the oscilloscope screen using the DA1855A PVG and
VDIFF mode or VCOMP input. This assures that the oscillo-
scope is set to the center of the DA1855A's dynamic range.
Gain X1
Attenuation ÷÷10
+ Input Coupling Off
– Input Coupling Off
Bandwidth Limit Full
PVG Voltage +00.000 V
VCOMP Off
VDIFF Off
Input Resistance 1 MΩΩ
PVG Mode Roll through zero

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2. Set the oscilloscope deflection factor to no greater than
100mV/div. The most useful range for the oscilloscope
deflection factors will be between 1mV/div and 100mV/div.
Using a scale factor of 200 mV/Div will allow the nonlinear
portion of the DA1855A's output to be viewed on screen.
More sensitive settings (e.g. 100µV/div) available on some
oscilloscopes can be used, but their usefulness may be limited by
noise, particularly with the DA1855A FULL bandwidth limit
selection and without averaging. With the oscilloscope set to
100µV/div and the DA1855A in the X10 GAIN mode, the overall
scale factor will be 10µV/div.
In theX10 GAIN mode, the DA1855A has lower noise than many
oscilloscopes, so it is preferable to use the /DA1855A X10 GAIN
mode and a lower oscilloscope scale factor. For example, to
obtain the best noise performance at 1mV/div, set the DA1855A
to X10 mode and the oscilloscope to 10mV/div rather than the
use X1mode and 1mV/div. This also maximizes the bandwidth,
as some oscilloscopes give up some bandwidth at their most
sensitive settings. Some oscilloscopes give up bits of resolution
to obtain 1mV or 2 mV/div sensitivity. The loss of resolution can
be avoided by using this technique. Any oscilloscope bandwidth
limit setting may be used so long as the unlimited signal does not
exceed full screen before invoking bandwidth limit.
Gain Control Modes
When the DA1855A is connected to a LeCroy oscilloscope
equipped with ProBus interface, the displayed scale factor and
measurement values will be adjusted to account for the effective
gain of the differential amplifier.
With software version 8.1.0 and higher, there are two modes of
gain control,Auto andManual. The oscilloscope defaults to Auto
mode when the amplifier is first attached. In Auto mode, the
VOLTS/DIV knob controls the oscilloscope scale factors Gain
and Attenuation to give the full available dynamic range 200µV/
div to 1 V/div without external probe attenuation, or 20 mV/div to
100 V/div with ÷100 probe installed.
Some of the transitions in scale factor will result in a change of
the attenuation of the external probe. Changing the amplifier’s
gain or attenuation will alter the common mode range and noise
performance of the differential amplifier. For Volt/Div settings
which can be produced with more than one combination of
amplifier gain and attenuation settings, Auto mode selects the
combination which results in greater common mode range.

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In some situations, the user may wish to select amplifier settings
optimized for the lowest noise performance with lower common
mode range. Or, the user may require the amplifier not to change
noise or common mode range when theVOLTS/DIV setting is
changed. These requirements can be met by selecting the
Manual mode in the menu box of user interface window.
Figure 3-2. DA1855A Control Menu in Manual Mode.
When set to Manual mode, the dynamic range of the VOLTS/DIV
knob is limited to the scale factors which can be obtained without
changing the DA1855A gain or attenuation. Thus in Manual gain
control mode, only the scale factor of the oscilloscope will be
changed. In this mode, dedicated menu boxes are displayed for
amplifier Attenuation and Gain. (Refer to figure 3-2).
The available attenuation values which appear in the Value box
will be /10 or /1 independent of the attenuation of any external
probe.
With software version 8.1.0 or higher, the channelOFFSET knob
will control the amplifier’s offset rather than the amplifier’s PVG
buttons.

Operation
DA1855A-OM-E Rev A ISSUED: July 2002 3-17
Probes and Differential Amplifiers
When using a differential amplifier it is very important to
understand the role probes play in the overall measurement
system performance. Probes not only make attachment to the
circuit under test more convenient, ÷10 and ÷100 attenuating
probes also extend the common mode range of the differential
amplifier. For example, the DA1855A amplifiers have a common
mode range of ±15.5 volts when their internal attenuators are set
to ÷1 and 155 volts when set to ÷10. The addition of a probe with
an attenuation factor of ten will extend the common mode range
to 1550 volts or the rating of the probe, whichever is less.
There is a trade-off, however. The Common Mode Rejection
Ratio (CMRR) capability of even highly matched differential probe
pairs is seldom as good that of the amplifier. In order to preserve
as much of the amplifier’s performance as possible at the probe
tips, it is important to use probes that are designed for differential
performance. Attempting to use normal ÷10 or ÷100 attenuating
oscilloscope probes, even high quality probes, will result in very
poor CMRR performance. Nominally matching ÷1 probes
however, will provide excellent common mode rejection and are
recommended.
For applications which do not require additional attenuation, ÷1
probes present relative high capacitive loading to the circuit
under test, limiting their usefulness to low frequency
measurements.
When making differential measurements, accurate probe
compensation is much more important than in single-ended
measurements. Most probes depend on the accuracy of the
oscilloscope’s 1 MΩinput resistor to determine the accuracy of
the probe’s attenuation factor. Two probes with a 1% accuracy
specification can yield a CMRR as low as 50 to 1 at DC while the
amplifier CMRR may be higher than 100,000 to 1. At high
frequencies, the CMRR will be worse.
A differential probe pair must allow for matching at DC as well as
over their useful frequency range. Changing the compensation of
a differentially matched probe set without following the proper
compensation procedure can result in a significant decrease in
the CMRR capability of any differential probe pair.
It is a good practice to compensate a probe pair for a given
amplifier and then leave the probe pair and amplifier together as
a system. Similarly, it is important that, once compensated for a
given amplifier, each probe always be used on the same input

DA1855A Differential Amplifier
3-18 ISSUED: July 2002 DA1855A-OM-E Rev A
(one probe always on the +INPUT and the other always on the –
INPUT).
The DXC100A Differential Probe Pair
The DXC100A is a high performance matched passive differential
probe pair designed for use with LeCroy DA1855A series
differential amplifiers. The probe pair consists of two well-
matched individual probes that share a common compensation
box to allow the attenuation factor on both probes to be
simultaneously switched between ÷10 and ÷100. When used with
the DA1855A Differential Amplifier, the probe’s attenuation factor
is automatically incorporated into the effective gain display and
the decimal properly located in the Precision Voltage Generator
(PVG) display.
Probe Grounding
The DXC100A Probe Pair is supplied with accessories that allow
for three methods of connecting probe grounds.
In most cases, when the common mode portion of the signal
consists mainly of low frequencies (1 MHz and below), the probe
ground leads should not be connected to the ground of the circuit
under test. They should be connected to each other. This
minimizes the effects of ground loop currents. The signal
corruption caused by not having the probes connected to the
ground of the circuit under test will be common to both inputs and
will be rejected by the differential amplifier.
However, when working in an environment with high RF ambient
noise, it is best to connect the probe ground leads to a good RF
ground near the point where the signal is being measured.
The best way to determine which probe grounding technique
should be used is to try both methods and use the one that gives
the least corruption of the differential signal.
When adjusting the compensation and probe CMRR, the use of
probe tip to BNC adapters is required. They provide the best
performance of the three grounding method.
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