Back to Basics: Probes (Part III)

Figure 1: Active oscilloscope probes
sport high resistance and low
capacitance at their tips, but
terminate into a scope's 50Ω input.

In the first two installments of this series on probe basics, we examined some broad probe categories (Part I) and some of the issues that come with probe inductance (Part II). In the present installment, we'll delve a bit deeper into the topic of active probes. We'll also discuss when it's best to use passive probes and when to use active types.

So what's the main difference between a passive probe and an active probe? Basically, the passive probe contains no active components that must be powered, while the active probe contains an amplifier near the probe tip, most commonly based on a transistor or FET. Such probes typically provide higher overall impedance than passive types, presenting high resistance to DC voltages and low-frequency signals and low capacitance to high-frequency signals. Active probes have high resistance at the probe tip but terminate into the 50Ω input of the oscilloscope (Figure 1).

Figure 2: An impedance vs. frequency plot for a high-
impedance active probe

When considering active versus passive probes, probe impedance is an important factor. Passive probes provide the highest impedance below frequencies of 20 kHz. Their high input capacitance causes circuit loading at high frequencies or with low-frequency signals containing high-frequency content.

Meanwhile, active FET probes provide high impedance from DC to 20 kHz, maintaining that impedance out to about 1.5 GHz thanks to their low capacitance (Figure 2). FET probes, then, are truly general-purpose probes that can be used at nearly all frequencies. Their low capacitive loading makes them usable on high-impedance circuits that would suffer severely from loading with passive probes.

So based on all of the foregoing, where does it make sense to use a passive probe vs. an active probe and vice versa? Both types have their strengths and weaknesses, but knowing which to use where can go a long way toward avoiding misleading measurement results:

• Passive probes are an excellent choice for low-frequency measurements, especially if high voltages may be encountered
• Active FET probes are better suited for measurements requiring high bandwidths
• Active FET probes are a great general-purpose choice for all frequencies out to the multi-GHz range, but watch out for higher voltages, which could damage the probe amplifier.

In the next installment of this series, we'll look at differential probes.