Digital Channels 7
Keysight InfiniiVision 3000T X-Series Oscilloscopes User's Guide 145
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.
The 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.
Probe Grounding
A probe ground is the low-impedance path for current to return to the source from
the probe. Increased length in this path will, at high frequencies, create large
common mode voltages at the probe input. The voltage generated behaves as if
this path were an inductor according to the equation:
Figure 24 Impedance versus Frequency for Both Probe Circuit Models
100 k
10 k
1 k
100
10
1
10 kHz 100 kHz 1 MHz 10 MHz 100 MHz 1 GHz
High
Frequency
Model
Typical
Model
Frequency
Impedance
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