Active Probes
An active probe, like the HP 54701A, has a buffer amplifier at the tip. See
figure 1-14. This buffer amplifier drives a 50-
Ω
cable terminated in 50
Ω
at
the oscilloscope input. Active probes offer the best overall combination of
resistive loading, capacitive loading, and bandwidth, even though an active
probe does not have the highest resistance, highest bandwidth, or lowest
capacitance available. The disadvantages of active probes, besides their
higher cost, are the larger size of the tip and a somewhat limited input
dynamic range. Previous active probe designs were more susceptible to
damage, particularly to ESD, and required careful handling. The HP 54701A
is designed to withstand 200 V peak ac and 12 kV of ESD, so it functions
reliably in adverse conditions.
The HP 54701A has sufficient bandwidth (2.5 GHz), sufficiently small
capacitive loading (0.6 pF), and sufficiently high resistance (100 k
Ω
) to be
useful for both ECL and CMOS circuits, and for most analog circuits.
For the high-speed CMOS state machine in the example, the HP 54701A
active probe offers the best combination of measurement accuracy with
minimal circuit loading.
Active probe
Figure 1-14
How the Oscilloscope Works
System Bandwidth
1–30
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