User`s guide

Digital Channels 13
InfiniiVision Oscilloscopes User’s Guide 385
Digital channel signal fidelity: Probe impedance and grounding
When using the mixed-signal oscilloscope you may encounter problems that are related
to probing. These problems fall into two categories: probe loading and probe grounding.
Probe loading problems generally affect the device 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
).
DC
AND LOW-FREQUENCY PROBE EQUIVALENT CIRCUIT
8.5 pF 100 kW