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Figure 9
1 MΩ/6pF probe
54701A probe
Capacitive Loading Effects
The input capacitance of a probe causes the overall input impedance to
decrease as a function of frequency. For this reason, input capacitance
becomes one of the most important parameters that affect high frequency
measurements. Figure 9 plots the probe impedance vs frequency for two
probes: a 1-MΩ, 6-pF probe and the 54701A probe (100 kΩ, 0.6 pF). It shows
that because of the lower input capacitance, the 54701A probe actually has a
higher input impedance for frequencies above 240 kHz. At frequencies above
2.65 MHz, it has as much as 10 times the impedance of the 1-MΩ probe.
Probe Impedance vs Frequency
The input capacitance of a probe forms an RC time constant with the parallel
combination of source impedance and probe input resistance. This can cause
an increase in the circuit rise time and a time delay in a pulse edge.
Chapter 2: Probing Considerations
Resistive Loading Effects
25
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