HP 54645A, User'S And Service Manual

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Using Digital Channels to Probe Circuits
You may encounter problems when using the HP 54645D 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 HP 54645D probes minimizes the
first problem, while the second is easily addressed by good probing practices.
Input Impedance
The logic probes used in the HP 54645D are passive probes, which offer high
input impedance and high bandwidths. They usually provide some
attenuation of the signal to the oscilloscope. This is 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 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
).
DC and Low-Frequency Probe Equivalent Circuit
Ω
Figure 4-13
Run/Stop/Single/Autostore/Erase Operation
Using Digital Channels to Probe Circuits
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