13 Digital Channels
386 InfiniiVision Oscilloscopes User’s Guide
H
IGH-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.
I
MPEDANCE VERSUS FREQUENCY FOR BOTH PROBE CIRCUIT MODELS
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.
150 W1 pF
7.5 pF
100 kW
250 W
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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