Teledyne LeCroy DA1855A - Manual (Page 40)

DA1855A Differential Amplifier
34 922258-00 Rev A
two places right of the decimal (10mV) in the Precision Voltage Generator until the positive peak of
the waveform appears in the oscilloscope’s display. Continue incrementing and decrementing the
digits in the Precision Voltage Generator until the peak of the waveform is at the center line of the
oscilloscope’s display. The number in the Precision Voltage Generator display is the value of the
waveform’s positive peak voltage.
Press the ± button in the Precision Voltage Generator. Observe that the negative peak of the signal
is now at or near the oscilloscope display’s center line. By incrementing and decrementing the digits,
the negative peak can be positioned to the oscilloscope display’s center line. Now the number in the
Precision Voltage Generator’s display is the value of the waveform’s negative peak voltage.
Change the oscilloscope’s sensitivity from 50 mV/div to 10 mV/ div. Overall sensitivity, including the
DA1855A, is now 1 mV/div. Temporarily change the oscilloscope’s input coupling from DC to GND
(or OFF) and re-center the trace to center screen using the oscilloscope’s position control. Return its
input coupling to DC. Now press the X10 button on the DA1855A to invoke its Auto balance function.
(Note that pressing the gain button that is already selected causes the DA1855A to adjust its DC
balance, but does not change its gain.)
Change the Precision Voltage Generator’s reading to again place the negative peak of the waveform
at the oscilloscope’s centerline. Note that the Precision Voltage Generator’s display more accurately
represents the negative peak voltage of the waveform.
Return the oscilloscope’s sensitivity to 50 mV/div and again press the DA1855A's V
DIFF
button. The
V
DIFF
light will go out and the display on the oscilloscope will be centered about the center line.
Notice that the PVG retains its setting, but the output of the PVG is not applied to the amplifier.
Press the V
DIFF
button again and observe that the Precision Voltage Generator’s output is reapplied
internally to the DA1855A amplifier.
Following are a few observations on using the differential offset mode (V
DIFF
) of the DA1855A:
• Both the positive and negative inputs (AC, OFF and DC) are enabled and the DA1855A
remains a true differential amplifier.
• The value displayed by the Precision Voltage Generator indicates a waveform’s differential
voltage, with respect to the –INPUT, as it passes through the oscilloscope display’s center
line. It is very important that the oscilloscope’s trace be positioned to center screen if an
accurate measurement is to be made using this method.