Teledyne LeCroy DA1855A - Manual (Page 22)

DA1855A Differential Amplifier
16 922258-00 Rev A
Differential Offset
V
DIFF
(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
DIFF
mode provides up to ± 200 divisions of range.
Operation of the DA1855A using the V
DIFF
function is the same as V
COMP
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 +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