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Choosing the Best Passive and Active Oscilloscope Probes for Your Tasks

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The passive voltage probe is the most commonly used type of scope probe today

Selecting the right probe for your application is the first step toward making reliable oscilloscope measurements. Although you can choose from a number of different types of oscilloscope probes, they fall into two major categories: passive and active. Passive probes do not require external probe power. Active probes do require external probe power for the active components in the probe, such as transistors and amplifiers, and provide higher bandwidth performance than passive probes. Each category offers many different types of probes, and each probe has an application for which it performs best.

Passive Probes

The most common type of scope probe today is the passive voltage probe. Passive probes can be divided into two main types: high-impedance-input probes and low-impedance resistor-divider probes. The high-impedance-input passive probe with a 10:1 division ratio is probably the most commonly used probe today. This probe is shipped with most low- to mid-range oscilloscopes today.The probe tip resistance is typically 9 MΩ, which gives a 10:1 division ratio (or attenuation ratio) with the scope’s input when the probe is connected to the 1 MΩ input of a scope. The net input resistance seen from the probe tip is 10 MΩ. The voltage level at the scope’s input is then 1/10th the voltage level at the probe tip, which can be expressed as follows: Vscope = Vprobe * (1 MΩ/ (9 MΩ + 1 MΩ))

Passive Probes

Compared to active probes, passive probes are more rugged and less expensive. They offer a wide dynamic range (>300 V for a typical 10:1 probe) and high input resistance to match a scope’s input impedance. However, high-impedance-input probes impose heavier capacitive loading and have lower bandwidths than active probes or low-impedance (z0) resistor-divider passive probes.

The low-impedance resistor-divider probe has either 450 Ω or 950 Ω input resistance to give 10:1 or 20:1 attenu­ation with the 50 Ω input of the scope. The input resistor is followed by a 50 Ω cable that is terminated in the 50 Ω input of the scope. Remember that the scope must have a 50 Ω input to use this type of probe. The key benefits of this probe include low capacitive loading and very high bandwidth—in the range of a couple of GHz—which helps to make high-accuracy timing measurements. In addition, this is a low-cost probe compared to an active probe in a similar bandwidth range. You would use this probe in applications such as probing electronic circuit logic (ECL) circuits, microwave devices or 50 Ω transmission lines. The one critical trade-off is that this probe has relatively heavy resistive loading, which can affect the measured amplitude of the signal.

Active Probes

If your scope has more than 500 MHz of bandwidth, you are probably using an active probe—or should be. Despite its high price, the active probe is the tool of choice when you need high-bandwidth performance. Active probes typically cost more than passive probes and feature limited input voltage but, because of their significantly lower capacitive loading, they give you more accurate insight into fast signals.

By definition, active probes require probe power. Many modern active probes rely on intelligent probe interfaces that provide power and serve as communication links between compatible probes and the scope. Typically the probe interface identifies the type of probe attached and sets up the proper input impedance, atten­uation ratio, probe power and offset range needed.

Typically you would choose a single-ended active probe for mea­suring single-ended signals (a voltage referenced to ground) and differential active probes for measuring differential signals (a plus voltage versus a minus voltage). The effective ground plane between the signal connections in differential probes is more ideal than most of the ground connections in single-ended probes. This ground plane effectively connects the probe tip ground to the device-under-test (DUT) ground with very low imped­ance. Therefore differential probes can make even better measurements on single-ended signals than single-ended probes can. The Keysight Technologies, Inc. InfiniiMax 1130A Series probing system allows either differential or single-ended probing with a single probe amplifier by using interchange­able probe heads that are optimized for hand browsing, plug-on socket connections or solder-in connections.


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