LeCroy DA1855A Operation page 15

Gain Control Modes
DA1855A-OM-E Rev A
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 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.
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 and Manual. 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 200 V/
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.
ISSUED: July 2002
Operation
3-15