Teledyne DA1855A User Manual
Operator’s
Manual
DA1855A
Differential Amplifier
DA1855A Differential Amplifier
Operator’s Manual
February 2013
© 2013 Teledyne LeCroy, Inc. All rights reserved.
Unauthorized duplication of Teledyne LeCroy documentation materials other than for internal sales
and distribution purposes is strictly prohibited. However, clients are encouraged to distribute and
duplicate Teledyne LeCroy documentation for their own internal educational 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.
Warranty
Teledyne LeCroy warrants this oscilloscope accessory for normal use and operation within
specification for a period of one year from the date of shipment. Spare parts, replacement parts and
repairs are warranted for 90 days.
In exercising its warranty, Teledyne LeCroy, at its option, will either repair or 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 Teledyne LeCroy’s
examination to be defective due to 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. Teledyne LeCroy will return all products under warranty with
transportation charges prepaid.
This warranty replaces all other warranties, expressed or implied, including but not limited to any
implied warranty of merchantability, fitness or adequacy for any particular purposes or use.
Teledyne LeCroy shall not be liable for any special, incidental, or consequential damages, whether in
contract or otherwise.
922258-00 Rev A
February 2013
Operator’s Manual
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
Model Description ............................................................................................................. 10
Rack Mount Versions ........................................................................................................ 10
Power Cable Option .......................................................................................................... 10
Standard Accessories ....................................................................................................... 10
Optional Accessories ........................................................................................................ 11
Operation ................................................................................................................................ 11
General Information .......................................................................................................... 11
Dynamic Range ................................................................................................................ 11
Front Panel ................................................................ ....................................................... 12
Rear Panel ........................................................................................................................ 17
Instrument Settings ........................................................................................................... 20
General Operating Information ............................................................................................. 23
Power Connection ............................................................................................................ 24
Comparator Mode ............................................................................................................. 24
Differential Mode ............................................................................................................... 24
Setting Up the Oscilloscope with ProBus Interface ........................................................... 25
Setting up the Oscilloscope without ProBus Interface....................................................... 30
Determining the Proper Offset Mode ................................................................................ 35
Avoiding Common Problems ............................................................................................ 35
Care and Maintenance ........................................................................................................... 38
Cleaning ............................................................................................................................ 38
Calibration Interval ................................................................................................ ............ 38
Service Strategy ............................................................................................................... 38
Troubleshooting ................................................................................................................ 38
Returns ............................................................................................................................. 38
Applications ........................................................................................................................... 39
922258-00 Rev A i
DA1855A Differential Amplifier
Introduction ....................................................................................................................... 39
Voltage Measurements ..................................................................................................... 39
Measuring Current ............................................................................................................ 39
Effects of Probes on Saturation Voltage Measurements ................................................... 40
Saturation Voltage Measurement ...................................................................................... 43
Gate Drive Voltage ............................................................................................................ 44
Upper and Lower Gate Drive ............................................................................................ 45
Avoiding Measurement Errors ........................................................................................... 45
Remote Control Commands .................................................................................................. 46
Command List ................................................................................................................... 46
Gain Control Mode ............................................................................................................ 47
Attenuation ........................................................................................................................ 47
Auto-Zero .......................................................................................................................... 48
Bandwidth Limit ................................................................................................................. 48
Coupling ............................................................................................................................ 49
Gain .................................................................................................................................. 50
Offset ................................................................................................................................ 51
Input-Resistance ............................................................................................................... 52
Probe Attenuation ............................................................................................................. 53
Precision Voltage Generator ............................................................................................. 54
Volt / DIV ........................................................................................................................... 55
Performance Verification ....................................................................................................... 56
Test Equipment Required ................................................................................................. 56
Preliminary Procedure ...................................................................................................... 58
Functional Check .............................................................................................................. 59
Verification Procedure ....................................................................................................... 60
Performance Verification Test Record .............................................................................. 77
Reference ................................................................................................................................ 80
Differential Mode and Common Mode .............................................................................. 80
Differential Mode Range and Common Mode Range ....................................................... 80
Common Mode Rejection Ratio ........................................................................................ 80
Certifications ..................................................................................................................... 82
Returning a Product .......................................................................................................... 85
Contact Teledyne LeCroy ................................................................................................. 86
ii 922258-00 Rev A
Operator’s Manual
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.
I
Power On; connected to AC mains.
Power Off; disconnected from AC mains.
Safety Instructions
This section contains instructions that must be observed to keep this oscilloscope accessory
operating in a correct and safe condition. You are required to 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.
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.
922258-00 Rev A 1
DA1855A Differential Amplifier
Use indoors only.
Keep product surfaces clean and dry.
Do not block the cooling vents. Leave a minimum six-inch (15 cm) gap between the
instrument and the nearest object. 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.
2 922258-00 Rev A
Operator’s Manual
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
customersupport@teledynelecroy.com 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.
922258-00 Rev A 3
DA1855A Differential Amplifier
Input Configuration
True Differential, + and – Inputs Precision Voltage Generator
can be selected as – input source in V
COMP
mode.
Offset Capability
The 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
Specifications
These specifications are valid for instruments 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, which 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
4 922258-00 Rev A
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)
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 V 1
÷10 Attenuator
± 155 V1
Differential Offset Range (V
DIFF
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 (V
COMP
mode, referred to input)
÷1 Attenuator
± 15.5 V 1
÷10 Attenuator
± 155 V1
Dynamic Ranges
Operator’s Manual
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.)
922258-00 Rev A 5
DA1855A Differential Amplifier
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
Line Voltage Range
90 - 264 VAC
Line frequency Range
45 - 66 Hz
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
± 0.05% of reading + 500 Μv Accuracy
(15° C to 45° C)
Precision Voltage Generator
Power Requirements
Warranted Characteristics
Warranted characteristics describe parameters which have guaranteed performance. Unless
otherwise noted, tests are provided on page 60, Performance Verification, for all warranted
specifications.
6 922258-00 Rev A
Operator’s Manual
÷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)
X10 Gain, ÷1 Attenuator
0.1% + 50 μV
2
X1 Gain, ÷1 Attenuator
0.1% + 500 μV
2
X10 Gain, ÷10 Attenuator
0.15% + 500 μV
2
X1 Gain, ÷10 Attenuator
0.15% + 5 mV2
Precision Voltage Generator
<75 μV/°C
DA1855A
≈ 28 W, ≈ 39 VA
DA1855A-PR2
≈ 56 W, ≈ 78 VA
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
Differential Offset Accuracy
Temperature Coefficient (power consumption)
1
Output Zero is the output voltage from zero with zero Volt applied between inputs. This specification is valid within 30
minutes from last Autozero cycle, and when the differential amplifier 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.)
922258-00 Rev A 7
DA1855A Differential Amplifier
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)
Physical Characteristics
8 922258-00 Rev A
Operator’s Manual
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.
922258-00 Rev A 9
DA1855A Differential Amplifier
Model Description
The DA1855A series is comprised of 2 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, and 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.
Rack Mount Versions
Rack mounted versions available for models:
DA1855A-RM: Single channel rack mounted version.
DA1855A-PR2-RM: Dual channel rack mounted version.
Power Cable Option
(One must be specified at time of order)
Option – P01: 120 V North America / Japan
Option – P02: United Kingdom
Option – P03: Universal Europe
Option – P04: Australia / New Zealand
Option – P05: Switzerland
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.
10 922258-00 Rev A
Operator’s Manual
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
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.
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.
* 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.
922258-00 Rev A 11
DA1855A Differential Amplifier
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.
12 922258-00 Rev A
Operator’s Manual
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.
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
cut off (-3dB point) is approximately 1.6 Hz. When the input attenuator is set to ÷1, the INPUT
922258-00 Rev A 13
DA1855A Differential Amplifier
–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 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. Refer to Error! Reference source not found.
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 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 V
connector is not usable when V
mode, the amplifier is configured for single ended measurements. The –INPUT
COMP
is selected. The input signal applied to the + INPUT is referenced
COMP
to ground offset by the value set by the Precision Voltage Generator. Large calibrated offsets can be
obtained while making differential measurements by using V
mode.
DIFF
Precision Voltage Generator
The PVG generates the voltage which is used in the V
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
14 922258-00 Rev A
COMP
and V
modes and appears at the rear
DIFF
Operator’s Manual
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
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. PVG absolute
mode: DA1855 PVG increment and decrement buttons always function to increment or decrement
the voltage display respectively. 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 Teledyne 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.
922258-00 Rev A 15
DA1855A Differential Amplifier
Differential Offset
V
(differential offset voltage) is an instrument mode rather than a type of input coupling. The V
DIFF
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 Teledyne LeCroy
oscilloscope with ProBus interface.) When the oscilloscope is set to greater sensitivities (lower
Volts/Div settings), the Differential offset provides much greater range than the conventional
position control. For example, at 50 mV/div, the V
mode provides up to ± 200 divisions of range.
DIFF
DIFF
Operation of the DA1855A using the V
function is the same as V
DIFF
except for the following:
COMP
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 +INPUT
and –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, ÷10 Indicates the voltage amplification is 0.1, etc.
The DA1855A communicates the effective gain information to the Teledyne LeCroy oscilloscope
when the ProBus interface is used. This corrects the scale factor of the displayed waveforms, cursors
and measurements.
When Teledyne 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 design and is intended to drive a
16 922258-00 Rev A
Operator’s Manual
50 Ω load. 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 Ω output 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 damage 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 power switch left in the 1 (ON) position.
Power Up Indicator
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.
922258-00 Rev A 17
DA1855A Differential Amplifier
Gain
Attenuation Max.
V
DIFF
X1
÷1
± 10 V
X1
÷10
± 100 V
X10
÷1
± 1 V
X10
÷10
± 10 V
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 –
INPUT when the –INPUT coupling is set to VCOMP 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 V
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 V
is selected. Because the PVG is applied to the amplifier to create a true
DIFF
differential offset, the relationship between V
output (changes with the amplifier gain selection according to the following table:
is increased by a factor of 10 when the ÷10 ATTENUATOR is
COMP
and the voltage at the OFFSET VOLTAGE BNC (PVG)
DIFF
The maximum V
is multiplied by any probe attenuation factor. The DA1855A front panel displays
DIFF
the correct offset referred to the instrument input.
Table 1, V
Range for Different Gain and Attenuator Settings
DIFF
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 V
dynamic range. In the V
18 922258-00 Rev A
mode, the maximum OFFSET VOLTAGE input is limited by the DA1855A common mode
COMP
mode it is limited by the dynamic range of the internal V
DIFF
amplifier.
DIFF
Operator’s Manual
Front Panel
Effective Offset
Settings
Range
Gain
Attenuation
V
COMP
V
DIFF
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
Front Panel
Effective Offset
Settings
Range with ÷100 Probe
Gain
Attenuation
V
COMP
V
DIFF
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
Table 2 and Table 3 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 2, Effective Offset Range with ÷11 Probe
NOTE: The effective voltage is always increased by the attenuator. It therefore follows that any
probe will increase the effective voltage of both V
For example, a probe with a 100X attenuation factor will increase the effective full scale range by
100.
COMP
and V
by its attenuation factor.
DIFF
Table 3, 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 OUTPUT BNC 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
Ω load, 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.
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DA1855A Differential Amplifier
Remote Operation
A REMOTE connector on the rear panel of the DA1855A allows total control of the instrument
through a Teledyne 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 oscilloscope. The DA1855A cannot be remotely controlled without a Teledyne
LeCroy oscilloscope. See page 46 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 6.6.0.5 or higher.
Probe Coding Input
This jack is to be used with Teledyne 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 +INPUT BNC 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 damaged 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.
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Operator’s Manual
Gain
X1
Attenuation
÷10
+ Input Coupling
Off
– Input Coupling
Off
Bandwidth Limit
Full
PVG Voltage
+00.000 V
V
COMP
Off
V
DIFF
Off
Input Resistance
1 MΩ
PVG Mode
Roll through zero
When used without ProBus interface, the instrument can be set to factory default settings by
pressing the VCOMP and VDIFF buttons simultaneously.
Table 4. 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 Teledyne LeCroy oscilloscope, the setup is automatic to prevent you 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 following rules:
922258-00 Rev A 21
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 V
or V
input. This assures that the oscilloscope is set to the center of the DA1855A's
COMP
DIFF
mode
dynamic range.
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.
DA1855A Differential Amplifier
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 the X10 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 X1 mode 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.
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, ÷1probes 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
MΩ input 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
22 922258-00 Rev A
Operator’s Manual
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 given
amplifier, each probe always be used on the same input (one probe always on the +INPUT and the
other always on the –INPUT).
DXC100A Differential Probe Pair
The DXC100A is a high performance matched passive differential probe pair designed for use with
Teledyne 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.
General Operating Information
This section will help you become familiar with the operation of the DA1855A and how it interfaces
with an oscilloscope. To carry out the following exercises, you will need an oscilloscope and a
general purpose function generator.
922258-00 Rev A 23
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