Oscilloscope Probes and Accessories

Technical Overviews

Introduction

To get the most out of your oscilloscope, you need the right probes and accessories for your particular applications. Whether you need the high bandwidth and low loading of an active probe, an easy way to connect to surface mount ICs or a passive probe to measure high voltages, there are a wide selection of high-quality probes and accessories for your Keysight Technologies, Inc. oscilloscope.

How To Select A Probe

Selecting the correct probe for your oscilloscope measurement should not be difficult. This brochure provides suggestions on how to make the best decision. Following is a list of probe parameters you need to consider when you select a probe for a given measurement.

Attenuation

Choose the attenuation ratio of the probe (1:1, 10:1, 100:1, 1000:1) to match the test signal amplitude to the oscilloscope’s vertical sensitivity range. Higher attenuation probe allows the measurement range of a scope to be extended, and lower attenuation probe allows for lower noise measurement.

Bandwidth (BW)

The probe’s rated bandwidth should match the oscilloscope’s and be adequate for the test signal. However, at higher frequencies, grounded lead inductance and input capacitance often influence system performance more than probe bandwidth.

Input resistance (Rin)

Input impedance is used to describe the loading effects of a probe. At DC and low frequency ranges, the probe’s resistive component is the main factor that loads down the circuit under test. However, as the frequency goes up, the capacitance of the probe tip, in parallel with the DC resistance, starts to reduce the input impedance of the probe, resulting in greater loading and a more adverse effect to the target.

Input capacitance (Cin)

Excessive input capacitance (sometimes called tip capacitance) slows down the system’s pulse response. Usually the least input capacitance possible is best.

Maximum input voltage (Vmax)

To ensure user safety, help protect the oscilloscope input from destructive voltage, and avoid damage to the probe, select a probe that is rated for a higher voltage than the signal you intend to test.

Probe compensation range

Most passive probes have a specification that lists the oscilloscope input capacitance range over which they can be used. When choosing a passive probe, be sure that the oscilloscope’s input capacitance lies within the probe’s compensation range or you will not be able to adjust the probe to achieve a correctly compensated square wave signal.

Probe output termination

Most oscilloscopes have 1-MΩ and/or 50 Ω input resistance. For proper signal transfer and optimum signal integrity, it’s important that the probe’s R and C match the R and C of the oscilloscope it is to be used with. For example, 50 Ω terminated probes should be used with 50 Ω scope inputs. Similarly, 1 MΩ terminated probes should be used on scopes with a 1 MΩ input resistance.

Probe interface

Most Keysight oscilloscope probes offer either a BNC type of probe interface or the AutoProbe interface. The AutoProbe interface is an intelligent communication and power link between compatible probes and the Infiniium or InfiniiVision Series oscilloscopes. The AutoProbe identifies the type of probe attached and sets up the proper input impedance, attenuation ratio, probe power, and offset range as needed.

Probe tip form factor

Your probe must make a reliable connection to the test point, and you may want it to grab the test point. Generally, this requires a small and light probe and a tip or grabber that is compatible with the test point. SMT and fine-pitch geometries make this issue especially critical.

Single-ended, Differential or InfiniiMode Probe

Two most common voltage probe types are single-ended and differential probe.

Differential probes measure the voltage difference between any two input points in contrast to a single-ended probe, which measures input voltage relative to ground. Differential probes are especially popular for measuring high-frequency signals or signals where neither are referenced to ground. Differential probes use a differential amplifier to convert the difference between two signals into a voltage that can be sent to a typical single-ended scope input.

A differential probe can be used to make single-ended measurements by using its negative input as a ground reference contact and there are several advantages to using a differential probe to make single-ended measurement. New InfiniiMode probes offer multiple modes that can measure single-ended, differential, and common-mode characteristics of a differential signal with a signal connection.

Types of Probes

Passive probes

The most widely used type of oscilloscope probe is the “passive probe.” Passive probes are also the most rugged and economical. There are no active components such as transistors or amplifiers in the probe, and therefore passive probes do not need to be powered.

Single-ended active probes

Active probes contain a small, active amplifier built into the probe body near the probe tip. This arrangement makes it possible to keep the probe input capacitance very low, usually less than 2 pF. This low capacitance results in high input impedance on high frequencies. It has the best overall combination of resistive and capacitive loading. With such low loading, active probes can be used on high-impedance circuits that would be seriously loaded by passive probes. Active probes are the least intrusive of all the probes.

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