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Reference
Active voltage probes, sometimes called “FET” probes, use active circuit
elements such as transistors. There are three classes of active probes:
High speed active
Differential active
Fixtured active
Active voltage measuring probes use active circuit elements in the probe
design to process signals from the circuit under test. All active probes require
a source of power for their operation. Power is obtained either from an exter-
nal power supply or from the oscilloscope itself.
NOTE
When you connect an active probe to the oscilloscope (such as the
P6205), the input impedance of the oscilloscope automatically
becomes 50
. If you then connect a passive probe (like the
P6139A)
you need to set the input impedance back to 1 M
Verti-
cal Control
on page 3-147 explains how to change the input imped-
ance.
High Speed Active Probes
Active probes offer low input capacitance (1 to 2 pF typical) while maintaining
the higher input resistance of passive probes (10 k to 10 M ). Like Z
O
probes, active probes are useful for making accurate timing and phase mea-
surements. However, they do not degrade the amplitude accuracy. Active
probes typically have a dynamic range of
V.
Differential Probes
Differential probes determine the voltage drop between two points in a circuit
under test. Differential probes let you simultaneously measure two points and
to display the difference between the two voltages.
Active differential probes are stand-alone products designed to be used with
50 inputs. The same characteristics that apply to active probes apply to
active differential probes.
Fixtured Active Probes
In some small-geometry or dense circuitry applications, such as surface
mounted devices (SMD), a hand-held probe is too big to be practical. You can
instead use fixtured (or probe card mounted) active probes (or buffered
Active Voltage
Probes