Teledyne LeCroy ZS2500 - Manual (Page 18)

ZS Series High-Impedance, Active Probes
The resonant frequency of a series LC circuit can be raised by decreasing the
inductance, capacitance or both. Since the input capacitance is already very low
and cannot be reduced, you can only try to reduce the inductance. This can be
accomplished by using the shortest possible input lead as well as the shortest
possible ground lead.
For example, to obtain the shortest possible ground lead when measuring IC
related signals, attach a small piece of copper clad material to the top of the IC
package and connect this to the package grounding wires.
Using the shortest ground lead and input lead available makes probing signals on
the package easier and makes for the shortest lead length for the best signal
fidelity. To illustrate how dramatic this effect is, we will work a simple example.
Assuming an input capacitance of 0.9 pF and a total lead length (input and
ground) of 2 inches (inductance of ≈ 25 nH/inch) such a setup may cause ringing
with a resonant frequency (f0) of:
This frequency is well within the passband of the probe and therefore shows up
as part of the measured signal at faster time/div settings. To determine how fast
a waveform to be measured can be without causing ringing on a probe like this,
divide the BW (ringing frequency) of the probe into 0.35:
Any input signal with a rise time faster than 0.47 ns can cause ringing.
Capacitive Loading
Capacitive loading is usually the most troublesome of the three loading effects.
It can affect the rise time, bandwidth and delay time measurements. At higher
frequencies the capacitive loading can affect the amplitude as well as the
waveshape of the measured signal by introducing an exponential response to
the waveform.
For a simple RC network the time constant of this exponential response is:
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