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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)
The probe’s input resistance
must match the oscilloscope’s
input impedance to avoid
a characteristic impedance
mismatch. It also must be
appropriate to the test signal
to avoid excessive loading.
Input capacitance (Cin)
Excessive input capacitance
(sometimes 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
specification 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) capacitance.
Normally, high-frequency
probes with attenuation factors
greater than 1:1 have adjustable
compensation networks 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
5000/6000/7000 Series
oscilloscopes. 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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