Digital Channels 6Keysight InfiniiVision 2000 X-Series Oscilloscopes User's Guide 103The impedance plots for the two models are shown in these figures. By comparingthe two plots, you can see that both the series tip resistor and the cable'scharacteristic impedance extend the input impedance significantly. The stray tipcapacitance, which is generally small (1 pF), sets the final break point on theimpedance chart.The logic probes are represented by the high-frequency circuit model shownabove. They are designed to provide as much series tip resistance as possible.Stray tip capacitance to ground is minimized by the proper mechanical design ofthe probe tip assembly. This provides the maximum input impedance at highfrequencies.Probe GroundingA probe ground is the low-impedance path for current to return to the source fromthe probe. Increased length in this path will, at high frequencies, create largecommon mode voltages at the probe input. The voltage generated behaves as ifthis path were an inductor according to the equation:Figure 11 Impedance versus Frequency for Both Probe Circuit Models100 k10 k1 k10010110 kHz 100 kHz 1 MHz 10 MHz 100 MHz 1 GHzHighFrequencyModelTypicalModelFrequencyImpedance
