Operation
CONNECTING SIGNALS
A probe is usually the most convenient way to connect an input
signal to the instrument. Shielded to prevent pickup of elec
tromagnetic interference, the standard 10X probes supplied with
the instrument present a high impedance to a circuit under test.
While the 10 MQ and 13 pF of the probe-scope system present a
negligible load on most circuits, very fast circuits or very high
impedance circuits may be seriously affected.
Waveform Fidelity and Probe Grounds
A probe ground must be used for accurate measurements and
observations. Use the shortest ground connection possible if you
want good waveform fidelity.
The standard-accessory probe is a compensated 10X voltage
divider. To a circuit under test, it appears resistive for low-
frequency signals and capacitive for high-frequency compo
nents. The probe input capacitance can interact with the induc
tance of either a long signal lead or a long ground lead to form a
series-resonant circuit. This circuit can affect system bandwidth
and can ring if driven by a fast step. Always keep both the ground
lead and the probe signal-input connections as short as possible
to maintain the best waveform fidelity.
In some cases, a separate ground from the unit under test to the
ground receptacle on the oscilloscope front panel can reduce
interference from low-frequency hum and noise. For rough checks
of larger signals, such as 5 V logic, a ground lead separate from
the probe— or even the safety ground connection, which is
shared with the unit under test— may work for a signal ground.
Fast signal transitions will be highly distorted, and extraneous
noise will be induced without the probe ground connection.
2225 Operators
2-11
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