Appendix E: Probe Selection
TDS 684A, TDS 744A, & TDS 784A User Manual
E–3
Active Voltage Probes
Active voltage probes, sometimes called “FET” probes, use active circuit
elements such as transistors. There are three classes of active probes:
H High speed active
H Differential active
H 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 external power
supply or from the oscilloscope itself.
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 external power
supply or from the oscilloscope itself.
NOTE. When you connect an active probe to the oscilloscope (such as the
P6245), the input impedance of the oscilloscope automatically becomes 50 W. If
you then connect a passive probe, you need to set the input impedance back to
1MW. The procedure To Change Vertical Scale and Position on page 3–10
explains how to change the input impedance. Also, please read Input Impedance
Considerations on page 3–5 for more information.
Active probes offer low input capacitance (1 to 2 pF typical) while maintaining
the higher input resistance of passive probes (10 kW to 10 MW). Like Z
O
probes,
active probes are useful for making accurate timing and phase measurements.
However, they do not degrade the amplitude accuracy. Active probes typically
have a dynamic range of ±8 to ±15 V.
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 W inputs. The same characteristics that apply to active probes apply to active
differential 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 amplifiers) to
High Speed Active Probes
Differential Probes
Fixtured Active Probes
