Agilent Technologies 54621A, 54621D, 24A, 42A, 22A, 41A manual High-Frequency Probe Equivalent Circuit

MegaZoom Concepts and Oscilloscope Operation

Using Digital Channels to Probe Circuits

250 W		7.5 pF	150 W
1 pF	100 k W

High-Frequency Probe Equivalent Circuit

The impedance plots for the two models are shown in these figures. By comparing the two plots, you can see that both the series tip resistor and the cable’s characteristic impedance extend the input impedance significantly. The stray tip capacitance, which is generally small (1 pF), sets the final break point on the impedance chart.

	100 k
	10 k						High
							Frequency
							Model
	1 k W
Impedance	100				Typical
					Model
	10
	1	W
		10 kHz	100 kHz	1 MHz	10 MHz	100 MHz	1 GHz
				Frequency

Impedance versus Frequency for Both Probe Circuit Models

The logic probes are represented by the high-frequency circuit model shown above. They are designed to provide as much series tip resistance as possible. Stray tip capacitance to ground is minimized by the proper mechanical design of the probe tip assembly. This provides the maximum input impedance at high frequencies.

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