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How To Pick The Right Oscilloscope Probe

The first step in making reliable measurements with the best oscilloscope is to select the right probe. There are several types of probes, but mainly there are two categories, passive and active. Active probes need an external power supply to power the active components, such as a built-in amplifier. They offer larger bandwidths than passive probes that do not require an external power supply. In both categories, there are different types, and each probe has its area in which it performs the best. reviewed the best oscilloscopes.

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Oscilloscope Probes: What You Need to Know

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When choosing the right tools to measure with the oscilloscope, many users do not pay enough attention to the appropriate probe.

At first, you only have to look at the oscilloscope itself and select it according to bandwidth requirement, sampling rate and some measuring channels. Only afterward do they think about how to get the measuring signals into the oscilloscope.

However, selecting the right probe for the application and its correct use is the first step in reliable oscilloscope measurements. A passive probe is a safe choice for everyday measurements and troubleshooting. In higher frequency ranges, however, an active probe provides much more accurate insights into fast measurement signals. Many active probes on the market have an impressive bandwidth specification on paper.

However, one should not forget in actual use that the performance of an active probe is essentially determined by the connection of the probe to the test object. A simple rule that should always be kept in mind for accurate measurements is: the shorter the connection to the test object, the better. Passive probes are the most common today. They can be divided into those with a high input impedance and those with a lower one. The most common are passive probes with high input impedance and an input divider of 10: 1. Such probes are now included with most entry-level to mid-range oscilloscopes ( Figure 1 ). The input resistance of the probe tip is typically 9 MΩ. If you connect such a probe with a 1-MΩ input of an oscilloscope, you get a voltage divider with a ratio of 10: 1. The total input resistance at the probe tip is then 10 MΩ. The voltage at the input of the oscilloscope is 1/10 of the voltage at the probe tip.

Passive probes are cheaper and more robust compared to active probes. They have a broad dynamic range (> 300 V for a typical 10: 1 probe) and high input resistance matching the input impedance of the oscilloscope input. However, they represent a more significant capacitive load than active probes or low-impedance passive probes, and they also have a lower bandwidth than this.

A low-impedance probe with resistance divider ( Figure 2 ) has an input resistance of either 450 Ω or 950 Ω. Together with the input resistance of the oscilloscope of 50 Ω, this results in a ratio of 10: 1 or 20: 1. The input resistance is followed by a 50 Ω measuring cable, which ends at a 50 Ω input of the oscilloscope and is terminated correctly here.

The main advantage of such a probe is the low capacitive load and huge bandwidth (a few GHz). This is helpful for accurate time measurements. Another advantage is the low price compared to an active probe with the same bandwidth. Such probes are used for example in ECL circuits (emitter-coupled circuit), in microwave applications or measurements on 50 Ω lines. The only drawback of this probe type is the relatively high resistive load which can affect the amplitude of the measured signal.

Active probes

If the oscilloscope has a bandwidth of 500 MHz or more, active probes are useful. Despite the high price, an active probe with high bandwidth requirements is the tool of choice. Active probes are more expensive than passive ones and have a small input voltage range, but due to their significantly lower capacitive load, they provide a closer look at fast signals. You always need an external power supply.

Many modern active Test probes come with "intelligent" interfaces that serve as a communication link between the oscilloscope and compatible probes and also provide power ( Figure 3 ). Typically, the probe interface recognizes the type of probe connected and appropriately adjusts the input impedance, divider factor, supply voltage, and offset range.

The higher bandwidth of an active probe is a clear advantage over its passive counterpart.

Frequently, however, users overlook the effect of the connection to the measurement object, ie the factor called "bandwidth of the connection". Although an impressive bandwidth may be specified in the datasheet of a given active probe, it will only work under ideal conditions. In actual use, where the user attaches any auxiliary parts to the probe tip, the real performance of the probe may be significantly worse than the specification indicates.

The performance of an active probe is in practice dominated by the connection to the test object. Parasitic components to the left of point V atn in Figure 4, in actual use in high-frequency applications, significantly determine the performance of a probe system.

For example, the Agilent N2796A single-probe probe with probe tip and 2 cm connection to ground provides a 2 GHz bandwidth.

If the user replaces the probe tip and the ground connection with a two-pole adapter with a length of 10 cm, the probe's bandwidth is reduced to 1 GHz.

With additional alligator clips, the bandwidth drops further to only 500 MHz.

So if you want the maximum performance of the probe, keep the connection to the input of the probe as short as possible ( Fig. 5 ).

What's that you were asking about oscilloscope probes?

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