In that earlier post, we discussed some of the electrical characteristics of probes, such as the effects of frequency on impedance. There's one more piece to that puzzle: inductance.
In most typical measurement scenarios, you can't just connect the probe tip to the DUT, unless you're trying to make a floating measurement. The probe's ground lead must be attached to earth ground, or as close as you can get to it (that's the subject of another earlier post). The short version is that all measurements are differential in that there has to be some kind of reference point to measure a voltage. Most times, that reference point is earth ground.
Figure 1: The typical high-impedance
passive probe has a 10:1 attenuation factor
Now, let's look a little bit closer at one of the major categories of probes: the passive probe. A passive probe essentially constitutes an attenuator circuit due to the probe impedance and the oscilloscope's impedance (Figure 1). If the coupling of the probe to the oscilloscope is set incorrectly, the result can be a signal that is attenuated too much. Fortunately, modern passive probes are able to automatically set the correct coupling and attenuation factor.
Figure 2: Passive probes allow
adjustment to their impedance
to match a scope's input
they use only passive components, they're pretty robust mechanically and electrically. They'll also give you a wide dynamic range, with the low end of the amplitude range limited by the probe's attenuation factor and the oscilloscope's vertical sensitivity.
Low-impedance (Low-Z) passive probes generally provide a 10:1 attenuation factor into the oscilloscope's 50-Ω input termination. Where the high impedance probe uses capacitive compensation to provide flat frequency response with minimum capacitive loading, the low capacitance probe uses transmission line techniques to achieve extremely wide bandwidth with very low capacitance. Low-Z passive probes are best suited for wide-bandwidth or fast-transient measurements in circuits that can drive 50-Ω impedances. In such cases, low-Z probes offer excellent frequency response. And, unlike Hi-Z probes, Low-Z probes do not require compensation to match the oscilloscope's input impedance.
|Figure 3: All oscilloscopes have a "Cal|
Out" that provides a clean square wave
for passive probe adjustment and
In our next Back to Basics on probes, we'll turn to active probes and a bit about why you'd use one or the other in given applications. Stay tuned (pun intended!) for more.