328 6000 Series Oscilloscope User’s Guide
8 Reference
Digital channel signal fidelity: Probe impedance and grounding
You may encounter problems when using the mixed-signal
oscilloscope that are related to probing. These problems fall
into two categories: probe loading and probe grounding. Probe
loading problems generally affect the circuit under test, while
probe grounding problems affect the accuracy of the data to the
measurement instrument. The design of the probes minimizes
the first problem, while the second is easily addressed by good
probing practices.
Input Impedance
The logic probes are passive probes, which offer high input
impedance and high bandwidths. They usually provide some
attenuation of the signal to the oscilloscope, typically 20 dB.
Passive probe input impedance is generally specified in terms of
a parallel capacitance and resistance. The resistance is the sum
of the tip resistor value and the input resistance of the test
instrument (see the following figure). The capacitance is the
series combination of the tip compensating capacitor and the
cable, plus instrument capacitance in parallel with the stray tip
capacitance to ground. While this results in an input impedance
specification that is an accurate model for DC and low
frequencies, the high-frequency model of the probe input is
more useful (see the following figure). This high-frequency
model takes into account pure tip capacitance to ground as well
as series tip resistance, and the cable’s characteristic
impedance (Z
o
).
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