Agilent Technologies 6000 Series manual Probe Grounding

8 Reference

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

Figure 51 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.

Probe Grounding

A probe ground is the low-impedance path for current to return to the source from the probe. Increased length in this path will, at high frequencies, create large common mode voltages at the probe input. The voltage generated behaves as if this path were

an inductor according to the equation: di

V = L----

dt

Increasing the ground inductance (L), increasing the current (di) or decreasing the transition time (dt), will all result in increasing the voltage (V). When this voltage exceeds the threshold voltage defined in the oscilloscope, a false data measurement will occur.

342 6000 Series Oscilloscope User’s Guide