To achieve reliable oscilloscope measurements, users should first determine the proper probe for their application. There are two main categories of oscilloscope probes — passive and active probes.
Passive probes do not need external probe power, and can be grouped into two major types — low-impedance resistor-divider probes and high-impedance-input probes.
Shipped with most mid- and low-range oscilloscopes, the high-impedance-input passive probe that features a 10:1 division ratio is today’s most commonly utilized probe.
Unlike active probes, passive probes are less expensive and more rugged. They provide wide dynamic range as well as high input resistance to match the input impedance of the oscilloscope. However, high-impedance-input probes require heavier capacitive loading and offer lower bandwidths than low-impedance (z0) resistor-divider passive probes or active probes.
Meanwhile, the key benefits offered by low-impedance resistor-divider probe include very low capacitive loading and remarkably high bandwidth, which helps achieve high-accuracy timing measurements. The probe also costs less than an active probe with a comparable bandwidth range. It can also be utilized in various applications, such as probing 50 Ω transmission lines, electronic circuit logic (ECL) circuits and microwave devices. However, one crucial trade-off of this probe is its relatively heavy resistive loading that can affect the signal’s measured amplitude.
Active probes are employed when utilizing an oscilloscope with over 500 MHz of bandwidth. It is the tool of choice when users need high-bandwidth performance. Costing more than passive probes, active probes offer limited input voltage. Their significantly lower capacitive loading enables them to deliver more accurate insight into fast signals.
Active probes require external probe power for its active components, such as amplifiers and transistors. Most contemporary active probes depend on intelligent probe interface that not only provide power, but also serve as communication links between the oscilloscope and the compatible probes. The probe interface usually identifies the kind of probe attached and sets up the proper attenuation ratio, input impedance, offset range and probe power needed.
To measure single-ended signals, users would usually select a single-ended active probe, while picking a differential active probe for determining differential signals. The ground plane of differential probes can effectively connect the device-under-test (DUT) to the probe tip ground than the ground connections of single-ended probes. This implies that differential probes are more ideal than single-ended probes when measuring single-ended signals.
Another clear advantage of active probes over passive probes is higher bandwidth. However, probe users often ignore the effect of the connection to the target, known as the “connection bandwidth”. Thus, even if an active probe offers remarkable bandwidth specification, the published specified performance is provided as ideal for probing conditions.
In addition, most people believe that probe impedance is constant. It is not. In fact, input impedance decreases over frequency.
At low frequency ranges and DC, the input impedance of the probe begins at the rated input resistance, and as the frequency rises, the probe’s input capacitance starts to shorten, while its impedance begins to drop.
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