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How to select a probe
Selecting the correct probe for your
oscilloscope measurement should not
be difficult. This brochure provides
suggestions on how to make the best
decision. Following is a list of probe
parameters you need to consider
when you select a probe for a given
measurement.
Attenuation
Choose the attenuation ratio of the
probe (1:1, 10:1, 100:1, 1000:1) to
match the test signal amplitude to
the oscilloscope’s vertical sensitivity
range.
Bandwidth (BW)
The probe’s rated bandwidth should
match the oscilloscope and be
adequate for the test signal. However,
at higher frequencies, grounded lead
inductance and input capacitance
often influence system performance
more than probe bandwidth does.
Input resistance (Rin)
Input impedance is used to describe
the loading effects of a probe. At DC
and low frequency ranges, the probe’s
resistive component is the main factor
that loads down the circuit under test.
However, as the frequency goes up,
the capacitance of the probe tip in
parallel with the DC resistance starts
to reduce the input impedance of the
probe, resulting in greater loading and
a more adverse effect to the target.
Input capacitance (Cin)
Excessive input capacitance (some-
times called tip capacitance) will slow
down the system’s pulse response.
Usually the least input capacitance
possible is best.
Maximum input voltage (Vmax)
To ensure user safety, help protect the
oscilloscope input from destructive
voltage, and avoid damage to the
probe, select a probe that is rated for
a higher voltage than the signal you
intend to test.
Probe compensation range
Most passive probes have a specifica-
tion that lists the oscilloscope input
capacitance range over which they
can be used. When choosing a passive
probe, be sure that the oscilloscope’s
input capacitance lies within the
probe’s compensation range or you
will not be able to adjust the probe
to achieve a correctly compensated
square wave signal.
Most oscilloscopes have 1-MΩ input
resistance. This input resistance is in
parallel with the input (shunt) capaci-
tance. Normally, high-frequency probes
with attenuation factors greater than
1:1 have adjustable compensation net-
works built into them. Adjusting this
compensation network provides the
best possible frequency linearity over
the oscilloscope’s designed frequency
range. Operating instructions provided
with the probe explain how to adjust
the compensation network to obtain
best signal fidelity.
Probe Interface
Most Agilent oscilloscope probes offer
either BNC type of probe interface
or the AutoProbe interface. The
AutoProbe interface is an intelligent
communication and power link
between compatible probe and the
Infiniium or InfiniiVision Series oscil-
loscopes. The AutoProbe identifies the
type of probe attached and sets up the
proper input impedance, attenuation
ratio, probe power and offset range as
needed.
Probe tip form factor
Your probe must make a reliable
connection to the test point, and you
may want it to grab the test point.
Generally, this requires a small and
light probe and a tip or grabber that is
compatible with the test point. SMT
and fine-pitch geometries make this
issue especially critical.
Under-compensated
The effects of passive probe compensation:
Over-compensated Properly compensated
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