118 Keysight InfiniiVision 3000 X-Series Oscilloscopes User's Guide
6 Digital Channels
Passive probe input impedance is generally specified in terms of a parallel
capacitance and resistance. The resistance is the sum of the tip resistor value and
the input resistance of the test instrument (see the following figure). The
capacitance is the series combination of the tip compensating capacitor and the
cable, plus instrument capacitance in parallel with the stray tip capacitance to
ground. While this results in an input impedance specification that is an accurate
model for DC and low frequencies, the high-frequency model of the probe input is
more useful (see the following figure). This high-frequency model takes into
account pure tip capacitance to ground as well as series tip resistance, and the
cable's characteristic impedance (Z
o
).
Figure 21 DC and Low-Frequency Probe Equivalent Circuit
Figure 22 High-Frequency Probe Equivalent Circuit
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