Teledyne LeCroy HFP1500 - Manual (Page 26)

HFP1500 High Frequency Probe
20 922252-00 Rev A
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 0.7 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 (f
0
) of:
This frequency is well within the passband of the probe and will therefore show 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 0.4 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:
where C
total
is the combined probe and circuit capacitance and R
total
is combined circuit and probe
resistance.