Dc Errors; Poor Common Mode Rejection - LeCroy AP034 Instruction Manual

C. DC Errors
D. Poor Common Mode Rejection
AP034-OM-E Rev D
2. Connect the probe to another oscilloscope. If the probe
now measures properly, the problem may be in the
oscilloscope.
3. If the probe behaves as if it is ac-coupled at high frequency,
check for an open input connection.
4. Poor frequency or transient response and AC gain errors
may result when one of the two input connections is open.
5. Excessive "ring" and other transient problems can result
from excessive lead length. To test this, shorten the input
leads to less than 1 cm. If the transient response changes
significantly, the lead parasitics are the cause.
Incorrect DC gain requires recalibration or factory repair. This
can be determined by completing the gain checks in the
Performance Verification Procedure.
1. Extremely high source resistance will result in DC gain
errors. Check the probe accuracy with the oscilloscope
calibrator signal.
2. Verify that the probe is not being overdriven into clipping for
its current gain setting.
3. Excessive offset can result from large changes in ambient
temperature. Remove the input signal from the probe and
repeat the Autobalance cycle. With the Offset set to zero,
did the trace return to the center of the graticule?
Use the 1-kHz calibrator signal from the oscilloscope to check
common mode rejection. With both the + input and – input
connected to the calibrator signal, a flat line at zero volts should
be seen on the graticule.
1. Check the probe with the plug-on attenuator installed and
removed. If excessive common mode signal appears only
when the attenuator is present, the attenuator may need to
be rematched to the probe. Use the procedure listed in this
section to match the attenuator.
2. If the common mode signal appears when the probe is
connected to the test circuit, but not when it is attached to
ISSUED: January 2000
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