Choose and use oscilloscope probes wisely

There are many kinds of oscilloscope probes on the market with differing functions and electrical/physical characteristics. Some suit low-frequency applications, while others function well at high frequencies or high voltages. In this article, we’ll go over some of the salient characteristics of common probe types and provide some tips on which serve best in given circumstances.

Some basic probe varieties:

Passive probe:

•          Standard oscilloscope probe supplied by all scope manufacturers

•          No active devices, only passive parts

•          Physically and electrically robust; handles hundreds of volts

•          Maximum bandwidth is 500 MHz but at the higher frequencies probe loading becomes an issue

Active Probe:

•          Typically an optional probe that is powered by the oscilloscope

•          Based on an active device such as a transistor or FET

•          Not as robust as a passive probe but has much wider bandwidths and much lower capacitance

•          The ideal probe for high frequency measurements

Differential Probe:

•          Measures the difference between two signals when there is no ground reference

•          Comes in two flavors: High voltage for floating measurements in a power supply, lighting ballast, motor drive, etc., and high bandwidth for differential serial data streams

Current Probe:

•          Active device that measures the current in a signal rather than the voltage

•          Three main types: Transformer based; Hall effect devices; or combination transformer/Hall effect

•          Most modern clamp-on current probes are combination transformer/Hall effect types

How Probes Affect the DUT

Each probe type has its own set of electrical characteristics. When we attach a probe to a circuit or device under test, it becomes a part of the circuit and its characteristics affect that circuit or device. Probes transfer some of the energy present in the circuit to the oscilloscope input. Thus, to the signal source, the probe constitutes another load. This load on the circuit can change the signal's shape and/or change the behavior of the DUT.

Broadly speaking, there are three possible outcomes when we connect a probe to a circuit. In the best case, the oscilloscope accurately reproduces the signal on screen. However, the probe may alter the signal in a way that misleads us about what is present at the probing point. A worst-case outcome is that the operation of the DUT changes radically, causing a well-designed device or circuit to malfunction (or vice-versa).

Probes are designed with high resistance at the point of contact in the hope of reducing the energy drawn from the circuit and, thus, to reduce the loading. High input resistance is important but it only makes a difference at DC or at low frequency AC (see Figure 1). At different frequencies, different characteristics of the probe gain:

•          At DC or low frequencies, the high input resistance dominates the overall impedance

•          As frequency increases, the capacitance dominates the impedance and dramatically lowers the overall impedance

•          The result of the high probe capacitance shows up in the signal shape seen on screen

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Figure 1. At different signal frequencies, different probe characteristics dominate the overall input impedance.

At 1 Hz, the impedance of a passive probe is 10 MΩ. At 1 MHz, that value decreases to 17.4 kΩ. At 100 MHz, impedance is just 174Ω. With such a sharp drop off in overall impedance, it's not surprising that the probe can have such a dramatic impact on what's seen on screen (Figure 2).

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Figure 2. The typical high-impedance passive probe has a 10:1 attenuation factor

There's one more major piece to the puzzle of a probe’s impact on the DUT: inductance. In 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. All measurements are fundamentally differential in that there has to be some kind of reference point to measure a voltage. In general, that reference point is earth ground.

Thus, it’s important to be aware that any lead added to the probe tip or the ground wire 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. So keep leads as short as possible to minimize these unwelcome inductance effects.

Passive Probes: One Size Fits All

Now for a closer look at the passive probe. A passive probe essentially constitutes an attenuator circuit due to the probe impedance and the oscilloscope's impedance (see Figure 3). If the coupling of the probe to the oscilloscope is set incorrectly, the result can be a signal that is over-attenuated. Fortunately, modern passive probes automatically set the correct coupling and attenuation factor.

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Figure 3. A characteristic impedance vs. frequency plot for a high impedance active probe

There also are high-impedance 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 600V, and rated bandwidths of up to about 350MHz. But be wary at bandwidths above 50MHz; Hi-Z probes present an appreciable amount of capacitive loading at high frequencies.

Thus, Hi-Z passive probes are best suited to general-purpose applications at 50MHz 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.

When it comes to impedance matching at the oscilloscope’s signal inputs, 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. 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. All oscilloscopes have a Cal (short for Calibration) output that provides a clean square wave for adjustment and compensation of passive probes. Adjusting the trimmer capacitor tunes the probe for that oscilloscope. Just turn the trimmer until you see a proper pulse shape on the display.

Active Probes: Higher Impedances

Passive probes are the basic, general-purpose probe type. However, active probes often suit more specialized applications. The main difference between the types is the passive probe contains no active components 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.

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 (see Figure 3). FET probes, then, are truly general-purpose probes at nearly all frequencies. Their low capacitive loading makes them usable on high-impedance circuits that would suffer severely from loading with passive probes.

Where should one use a passive probe vs. an active probe? Knowing which to use in a given measurement scenario avoids bad results or even damaging a probe:

•          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.

Differential Probes: When “Ground” is Relative

General-purpose single-ended probes (whether active or passive) can only accurately measure "ground-referenced" voltages. However, some measurements require probing test points with reference to each other, whether one of them is true earth ground or not. One example is VDS of a FET in a power supply; another is a serial-data link, when one is probing the positive and negative data lines of a differential signal.

Here is where differential probes come into play. Among the most common are high-bandwidth types, high-voltage types, and those with differential amplifiers offering a high common-mode rejection ratio.

High-bandwidth differential probes are best suited for applications such as probing differential serial-data lines. To function effectively, high-bandwidth probes must deliver high dynamic range at the higher bandwidths and a large offset capability. Another must for such probes is extremely low probe noise and impedance characteristics that minimize loading.

High-voltage differential probes (Figure 4) are built to handle common-mode voltages up to 1 kVRMS and 1.4 kV peak differential voltages (for example, Teledyne LeCroy’s model ADP305). Such probes suit troubleshooting of low-frequency power electronics in cases where ground is elevated, or the location of true earth ground is unknown. When looking at high-voltage differential probes (or any high-voltage probes, for that matter), be mindful of safety ratings.

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Figure 4. High-voltage differential probes handle common-mode voltages up to 1 kVRMS.

Teledyne LeCroy