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HFP1500 High Frequency Probe
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
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 nsec 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
is the combined probe and circuit capacitance and R
total
resistance.
= 0.7 pF and the source resistance is 250 Ω, the measured rise time will be
In a setup where C
t
0.385 nsec, which will correspond to a bandwidth of 909 MHz, assuming no inductive loads.
(
t
=2.2 x 0.7 X 10
rise
5–2
1
------------------------------------------------------------------ -
f
=
0
9 –
×
2π 50 10
0.35
--------- -
t
=
=
rise
BW
t
=
2.2
rise
-12
x 250 Ω = 0.385 ns
=
851 MHz
–
12
×
×
0.7 10
0.35
-------------------- -
=
0.4 ns
851MHz
×
×
C
R
total
total
total
)
) of:
0
is combined circuit and probe
HFP1500-OM-E Rev E
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