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Equation 3
Ground Inductance Effects
Probe grounding techniques are an important factor in making accurate high
frequency measurements. The main limitation, probe resonance, is a function
of the input capacitance of the probe and the inductance of the ground return.
These two parameters in series form an LC resonant circuit that, when
connected to the circuit under test, becomes part of the circuit's response.
The probe resonance can cause overshoot and ringing on pulse edges that
contain energy in the same frequency band as the resonance. The true
response is masked, the false response gets transferred to the oscilloscope,
and the oscilloscope display shows an incorrect result. If overshoot and
ringing added by a probe during troubleshooting changes how the circuit
functions, it can produce an incorrect judgment about circuit operation.
To minimize the problem of ground ringing, use the shortest possible ground
with a probe that has the lowest possible input capacitance. Equation 3 can
be used to calculate the frequency where a certain probe and grounding
technique resonates.
1
------------------ -
f
=
r
2π LC
where
C is the probe input capacitance. (It is usually found in the probe data
sheet.)
L is the inductance of the ground return. (It can be approximated
using the constant of 25 nH per inch.)
Figure 11 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 also plots the inductive
reactance vs frequency for three different values of ground inductance. The
5-nH inductance represents a PC board socket, the 20-nH inductance a
spanner ground, and the 100-nH inductance a 4-inch ground wire. Where the
probe plots cross the inductance plots gives the resonant frequency of the
probe and ground combination. You can see from the graphs that in all three
cases the 6-pF probe resonates at approximately one-third the frequency of
the 54701A (0.6 pF). The lower resonance means that the effect of the
resonance is more likely to influence the representation of the signal.
Chapter 2: Probing Considerations
Resistive Loading Effects
27
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