4-38
MegaZoom Concepts and Oscilloscope Operation
Using Digital Channels to Probe Circuits
Using Digital Channels to Probe Circuits You may encounter problems when using the mixed-signal oscilloscope that are
related to probing. These problems fall into two ca tegories: 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 an d resistance. The resistance is the sum of t he tip resisto r value
and the in put resi stance o f the test instrum ent (see the foll owing fi gure). T he
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(Zo).
DC and Low-Frequency Probe Equivalent Circuit
8.5 pF 100 k