File name 5991-2325EN Oscilloscope Selection Tip 11_ Probing - Application Note c20130724 .pdf
Oscilloscope Selection Tip 11:
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