Teledyne LeCroy DA1855A - Manual (Page 41)

Operator’s Manual
922258-00 Rev A 35
• By using the DA1855A in the differential offset mode and the oscilloscope in a high
sensitivity setting, high resolution voltage measurements can be made. The –INPUT is the
reference for these measurements.
• The Precision Voltage Generator can be used as a position control which allows the
DA1855A to operate in its most linear region.
Determining the Proper Offset Mode
The operation of the Comparison (V
COMP
) and Differential Offset modes (V
DIFF
) are quite similar. The
Comparison mode is easier to understand and has a wider range, 15.5 Volt vs. 10.0 Volt. The
Differential Offset mode provides offset operation while allowing the DA1855A to function as a true
differential amplifier.
For most applications, the Differential Offset (V
DIFF
) mode has advantages over the Comparison
(V
COMP
) mode. When using the Comparison mode, the Precision Voltage Generator’s output is
subtracted from the +INPUT. Except for the PVG’s offset, operation is the same as a standard single-
ended oscilloscope...only one DA1855A input is available. In the Differential Offset mode, the
DA1855A functions as a differential amplifier…both
+INPUT and –INPUT function. This allows the operator to choose a measurement reference point
other than ground. Even in ground referenced measurements, signal degradation can be reduced by
using the –INPUT probe to select a ground reference point with the least noise. This method is
especially useful in eliminating hum and noise from ground loops.
There is one instance in which the Differential Offset (V
DIFF
) mode might result in more noise.
Magnetic pick-up is proportional to the area between the probes. If twisting the probe leads
together is not sufficient to reduce magnetic pick-up, the Comparison Offset (V
COMP
) mode may be
preferable.
The Differential Offset (VDIFF) mode is usually the mode of choice if the wider range or higher
accuracy of the Comparison (VCOMP) mode is not needed.
Avoiding Common Problems
There are a few situations the operator of either the DA1855Ashould is aware of to avoid some
potential measurement traps.
Exceeding the Common Mode Range
The DA1855A Differential Amplifiers have the largest common mode range available for this type of
amplifier and are very good at measuring small differences between two large signals. However,
care still must be taken not to allow a large common mode signal to exceed the available common
mode range.
Unlike the differential mode signal, which is viewed on the oscilloscope, the common mode signal is
normally rejected. Thus, you may not notice exceeding the common mode signal range.