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Oscilloscope Selection Tip 11:
Probing
Part 11 of a 12-part series
Tip 11 Select an oscilloscope from a vendor that can
also provide the variety of specialty probes that you may require.
Your oscilloscope measurements
can only be as good as what
Agilent offers a broad range of your oscilloscope probe delivers
to the oscilloscope's BNC inputs.
oscilloscope probes to fit your
When you connect any kind of
specific voltage and current measurement system to your
circuit, the instrument (and
measurement applications. probe) becomes a part of your
device-under-test. This means
it can "load" or change the
behavior of your signals to some
degree. Good probes should not disturb the input signal and should ideally deliver an exact
duplicate of the signal that was present at the probe point before the probe was attached.
When you purchase a new oscilloscope, it typically comes standard with a set of high-
impedance passive probes -- one probe for each input channel of the oscilloscope.
These types of general-purpose passive probes are the most commonly used and enable
you to measure a broad range of signals relative to ground. But these probes do have
limitations. Figure 1 shows an electrical model of a typical 10:1 passive probe
connected to the high-impedance input (1-M input of an oscilloscope).
Figure 1: Typical model of a 10:1 passive probe
Inherent in all oscilloscope probe and oscilloscope inputs are parasitic capacitances. These
include the probe cable capacitance (Ccable), as well as the oscilloscope's input capacitance
(Cscope). "Inherent/parasitic" simply means that these elements of the electrical model are not
intentionally designed-in; but are just an unfortunate fact of life in the real world of electronics.
The amount of inherent/parasitic capacitance will vary from oscilloscope-to-oscilloscope and
probe-to-probe. Also included in this electrical model are designed-in capacitive elements
that are used to compensate for low-frequency pulse response.
The electrical model of any probe (passive or active) and oscilloscope can be simplified down
Figure 2: Simplified probe/oscilloscope
electrical model. to parallel combination of a single resistor and single capacitor. Figure 2 shows a typical
oscilloscope/probe loading model for a 10:1 passive probe. This is what gets connected
in parallel with your DUT when making oscilloscope measurements with a probe. For low
frequency or DC applications, loading is dominated by the 10 M resistance, which in most
cases should not be a problem. Although 13.5 pF may not sound like much capacitance, at
higher frequencies the amount of loading contributed by this capacitance can be significant.
For instance, at 500 MHz the reactance of 13.5 pF in this model is just 23.6 , which could
contribute to significant "loading" and signal distortion.
For higher frequency measurement applications, active probes should be used, such as
Agilent's InfiniiMode Series differential active probe shown in Figure 3. "Active" means
that the probe consists of an amplifier near the probe's tip. This can significantly reduce
the amount of capacitive loading and increase probing bandwidth. But the tradeoff with
high-frequency active probes is often reduced dynamic range.
Besides high-frequency active probes, there are many other specialty probing applications
that should be considered including high voltage measurements and current measurements.
Figure 3: Agilent's InfiniiMode Series
differential active probe
Broad range of probing solutions for
Agilent's InfiniiVision X-Series Oscilloscopes
If you are in the market today to purchase your
next oscilloscope, Agilent Technologies'
InfiniiVision X-Series oscilloscopes come in
various bandwidth models ranging from 70 MHz
up to 1.5 GHz. Agilent also offers a broad range
of probing solutions to fit your specific
measurement needs.
To learn more about Agilent's InfiniiVision X-Series oscilloscopes and mixed signal
oscilloscopes, go to www.agilent.com/find/InfiniiVision.
Product specifications and descriptions in this
document subject to change without notice.