Teledyne LeCroy DA1855A - Manual (Page 38)

DA1855A Differential Amplifier
32 922258-00 Rev A
Press the –INPUT’s V
COMP
button. This internally applies the Precision Voltage Generator’s output to
the DA1855A's –INPUT and the OFF light goes out (the –INPUT connector is disabled).
The positive and negative peaks of the waveform displayed on the oscilloscope are (respectively) 10
divisions above and below the display center line. Push the button above the digit that is two places
right of the decimal (10 mV) in the Precision Voltage Generator (PVG) until the positive peak of the
waveform appears in the oscilloscope’s display. Continue incrementing and decrementing Precision
Voltage Generator’s digits until the peak of the waveform is at the centerline of the oscilloscope’s
display. The number in the Precision Voltage Generator display is 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’s display centerline. By incrementing and decrementing the digits,
the negative peak can be positioned to the oscilloscope’s display centerline. At this point the
number in the Precision Voltage Generator’s display is the waveform’s negative peak voltage.
Change the oscilloscope’s sensitivity from 50mV/div to 10mV/div. Overall sensitivity, including the
DA1855A, is now 1mV/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 autozero 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 center screen. Note that the Precision Voltage Generator’s display represents
the negative peak voltage of the waveform with greater resolution.
Return the oscilloscope’s sensitivity to 50 mV/div and press the DA1855A's –INPUT OFF (or AC or
DC) button. The Precision Voltage Generator will retain its setting and the display on the
oscilloscope will be centered about the centerline. Press the V
COMP
button on the –INPUT again and
observe that the Precision Voltage Generator’s output is again connected to the –INPUT of the
DA1855A.
Following are a few observations on using the DA1855A comparison voltage mode (V
COMP
):
• The negative input and its AC, OFF and DC coupling are disabled. Instead of being a
differential amplifier, the DA1855A becomes a differential comparator. It compares the
voltage present at the +INPUT with the output of the Precision Voltage Generator and when
they are equal, the output of the DA1855A is zero volts.
• The value displayed by the Precision Voltage Generator indicates a waveform’s voltage, with
respect to ground, as it passes through the oscilloscope display’s centerline. 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.