Agilent Technologies 42A, 54621D, 24A Using Digital Channels to Probe Circuits, Input Impedance

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 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 (Zo).

8.5 pF 100 kW

DC and Low-Frequency Probe Equivalent Circuit

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