342 6000 Series Oscilloscope User’s Guide
8 Reference
Figure 51 Impedance versus Frequency for Both Probe Circuit Models
The logic probes are represented by the high-frequency circuit
model shown above. They are designed to provide as much
series tip resistance as possible. Stray tip capacitance to ground
is minimized by the proper mechanical design of the probe tip
assembly. This provides the maximum input impedance at high
frequencies.
Probe Grounding
A probe ground is the low-impedance path for current to return
to the source from the probe. Increased length in this path will,
at high frequencies, create large common mode voltages at the
probe input. The voltage generated behaves as if this path were
an inductor according to the equation:
Increasing the ground inductance (L), increasing the current
(di) or decreasing the transition time (dt), will all result in
increasing the voltage (V). When this voltage exceeds the
threshold voltage defined in the oscilloscope, a false data
measurement will occur.
100 k
Ω
10 k
Ω
1 k
Ω
100
Ω
10
Ω
1
Ω
10 kHz 100 kHz 1 MHz 10 MHz 100 MHz 1 GHz
High
Frequency
Model
Typical
Model
Frequency
Impedance
VL
di
dt
-----
=