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20 November 2013

Back to Basics: Probes (Part II)

In a previous post, we provided some basic information about oscilloscope probes, including a brief survey of the different types and what can happen when the probe is connected to a DUT. In this installment, let's continue along those lines and take a closer look at passive probes.
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.

The typical high-impedance passive probe has a 10:1 attenuation factor
Figure 1: The typical high-impedance
passive probe has a 10:1 attenuation factor
With that in mind, it's important to pay attention to the inductance added by the leads to the circuit created by the probe's contact with the DUT. Any lead added to the probe tip or the ground lead adds inductance to the circuit. The inductance from leads can add overshoot and ringing to the signal seen on the oscilloscope's display. Moreover, leads can serve as antennas and pick up electrical noise from the environment. That noise may or may not be present in the circuit you're trying to measure. The moral of the story: keep leads as short as possible to minimize these unwelcome inductance effects on your measurements.

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. 

It's worth mentioning at this juncture that there are high-impedance passive probes and low-capacitance (or low-impedance) passive probes on the market. High-impedance (Hi-Z) passive probes are the most commonly used oscilloscope probes and offer attenuation factors of 10:1 (X10) and/or 100:1 (X100), a typical maximum input voltage of 600 V, and rated bandwidths of up to about 350 MHz. But be wary at bandwidths much above 50 MHz; Hi-Z probes present an appreciable amount of capacitive loading.

Passive probes allow adjustment to their impedance to match a scope's input
Figure 2: Passive probes allow
adjustment to their impedance
to match a scope's input
Thus, Hi-Z passive probes are best suited to general-purpose applications at 50 MHz or less. Because
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.

All oscilloscopes have a "Cal Out" that provides a clean square wave for passive probe adjustment and compenstion
Figure 3: All oscilloscopes have a "Cal
Out" that provides a clean square wave
for passive probe adjustment and
Speaking of which, it's important to remember that the oscilloscope signal input has an impedance too. Hi-Z passive probes always have an adjustment trimmer capacitor located at the connector end. The trimmer implements a simple RC compensation scheme that matches the time constant of the RC circuit in the probe to the time constant of the probe input resistance and shunt capacitance. Basically, the adjustment compensates for the capacitive load of the oscilloscope's input. It forms a high-pass path to compensate for the low-pass nature of the oscilloscope input. As a result, the probe and oscilloscope combination becomes an all-pass filter (Figure 2). All oscilloscopes have a Cal (short for Calibration) Out output which provides a clean square wave for adjustment and compensation of passive probes. Adjusting the trimmer capacitor allows the probe to be tuned properly for that oscilloscope. Just turn the trimmer until you see a nice pulse shape on the display and you're good to go (Figure 3).

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.

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