Digital Channels 7
Keysight InfiniiVision HD3-Series Oscilloscopes User's Guide 115
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:
Increasing the ground inductance (L), increasing the current (di) or decreasing the
transition time (dt), will all result in increasing the voltage (V). When this voltage
exceeds the threshold voltage defined in the oscilloscope, a false data
measurement will occur.
Figure 22 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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