HP 54645A User's And Service Manual page 123

Run/Stop/Single/Autostore/Erase Operation
Using Digital Channels to Probe Circuits
Figure 4-14
High-Frequency Probe Equivalent Circuit
The impedance plots for the two models are shown in these figures. By
comparing the two plots, you can see that both the series tip resistor and the
cable's characteristic impedance extend the input impedance significantly.
The stray tip capacitance, which is generally small (1 pF), sets the final break
point on the impedance chart.
Figure 4-15
Impedance versus Frequency for Both Probe Circuit Models
The HP 54645D logic probes are represented by the high-frequency circuit
model shown above. They are designed to provide as much series tip
resistance as possible. Stray tip capacitance to ground is minimized by the
proper mechanical design of the probe tip assembly. This provides the
maximum input impedance at high frequencies.
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