Teledyne DA1855A, Operator'S Manual

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Operator’s Manual
922258-00 Rev A 37
properly set). Thus, a sine wave large enough to overdrive the DA1855A will appear as a square
wave on the oscilloscope.
The DA1855A is designed to cleanly limit the output signal to± 500 mV. The DA1855A goes into limit
when its output reaches ± 500 mV and is designed to recover very quickly once its input signal level
decreases sufficiently to allow the amplifier to return to its linear range. The DA1855A will recover
from overdrive to its full accuracy much more quickly than most oscilloscopes. Keeping the
oscilloscope’s position at center screen and using oscilloscope sensitivities between 100 mV/div and
2 mV/div (or the oscilloscope’s most sensitive setting) will insure good signal integrity. When the
displayed signal contains mostly low frequency components, the operator can use the oscilloscopes
100 mV/div setting to allow large signals to be completely shown on screen.
Failure to Terminate the Amplifier into 50
Ω
(D
OES NOT APPLY WHEN USING A
T
ELEDYNE
L
E
C
ROY OSCILLOSCOPE WITH
P
RO
B
US INTERFACE
)
“All the signals displayed on my oscilloscope seem to be twice as large as they should be.” This
comment results from not having the output of the DA1855A properly terminated into 50
Ω
. When
interfaced using ProBus to a Teledyne LeCroy oscilloscope, you do not need to be concerned about
the termination resistance, as the oscilloscope sets the termination mode automatically.
The DA1855A output impedance is 50
Ω
. The cable connecting the DA1855A to the oscilloscope or
spectrum analyzer should be 50
Ω
and be terminated with a 50
Ω
load. If the termination at the end
of the connecting coaxial cable is missing, the amplifier will not be properly terminated.
In addition to the error in scale factor, operating the DA1855Awith the output unterminated may
result in poor high frequency, linearity and transient response. Proper operation of the 1 MHz and
20 MHz bandwidth limit filters also require 50
Ω
termination.
Poor Overdrive Recovery
The DA1855A output is limited at ± 500 mVp-p to prevent the amplifier from being overdriven by
large inputs. Poor recovery may still occur when the oscilloscope vertical scale is set to too high a
sensitivity, causing the oscilloscope to be overdriven rather than the DA1855A amplifier. Care must
be taken to set the vertical sensitivity such that the oscilloscope is not being overdriven.