Selecting the right probe for your application is the first step toward making reliable oscilloscope measurements. There are a number of different types of oscilloscope probes to choose from. Probes fall into two major categories: passive and active.
A simple distinction between the two types is that active probes require an external supply to power the active components in the probe such as transistors and amplifiers. Active probes also offer higher-bandwidth performance than passive probes, which do not require external power. There are many different types of probes under each category, and each has an area where it performs best.
The most common type of scope probe today is the passive voltage probe (Fig. 1). Two major categories of passive probes are high-impedance-input passive probes and low-impedance, resistor-divider passive probes. The high-impedance-input passive probe with a 10:1 division ratio is probably the most commonly used probe, and most of today’s low- to mid-range oscilloscopes ship with them (Fig. 2).
The probe tip resistance is typically 9 M?, which provides a 10:1 division ratio (or attenuation ratio) with the scope’s input when it’s connected to a 1-M? input of a scope. The net input resistance seen from the probe tip is 10 M?. The voltage level at the scope’s input is then one-tenth of the level of the voltage at the probe tip, which can be expressed as:
Vscope = Vprobe * (1 M?/(9 M? + 1 M?))
Compared to active probes, passive probes are more rugged and inexpensive. 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, they 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-? or 950-? input resistance to give 10:1 or 20:1 attenuation with the 50-? input of the scope. That input resistor is followed by a 50-? cable, which is terminated in the 50-? input of the scope. It is important to remember that the scope must have a 50-? 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 gigahertz, which helps to make high-accuracy timing measurements (Fig. 3). 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 emitter-coupled logic (ECL) circuits, microwave applications, or looking at 50-? transmission lines. The one critical tradeoff is that this probe has relatively heavy resistive loading, which can affect the measured amplitude of the signal.
If you have a scope with more than 500 MHz of bandwidth, you’re 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 their input voltage is typically limited. 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 as needed.
Higher bandwidth is a clear advantage of active probes over passive probes. Probe users often overlook the effect of the connection to the target, known as connection bandwidth. Even though a particular active probe may have an impressive bandwidth specification, the published specified performance may be under ideal probing conditions.
In a real-world probing situation, which would include using probing accessories to attach to the probe tips, the active probe’s performance may be much worse than the published specified performance. The real-world performance of an active probing system is dominated primarily by the “connection” system. Consider an example in which parasitic components to the left of the point labeled VAtn are the driving factors in determining the performance of a real-world active probing system in high-frequency applications (Fig. 4).
As an example, Agilent’s N2796A 2-GHz single-ended active probe provides 2 GHz of bandwidth with a probe tip and a 2-cm long offset ground. With this best-case setup, you get 2 GHz of probe bandwidth. If you take the tip and ground off and replace them with a 10-cm dual-lead adapter, the probe bandwidth drops to 1 GHz. With additional clips attached to the dual-lead adapter, the probe bandwidth drops further to 500 MHz. The key here is that shorter input leads are better if you are looking for probe performance.
Probe Loading Effect
Many people think that probe input impedance is a constant number. You might hear that the probe has a 1-k?, a 1-M?, or even a 10-M? input impedance, but that isn’t constant over frequency. Input impedance decreases over frequency.
At dc and low frequency ranges, the probe’s input impedance starts out at the rated input resistance, say 10 M? for a 10:1 passive probe. But as the frequency rises, the input capacitance of the probe starts to become a short, and the impedance of the probe starts to drop. The higher the input capacitance, the faster the impedance slope drops.
Compare a 500-MHz passive probe and a 2-GHz active probe. At about 10 kHz of a crossover point and beyond, the input impedance of the active probe is higher than that of the passive probe (Fig. 5). Higher input impedance means less loading on the target signal, and less loading means less effect or less disruption of the signal.
If we go out to a bandwidth of 70 MHz in the chart, the input impedance of the passive probe goes down to about 150 ?, while the input impedance of the active probe is about 2.5 k?. There is a significant difference between them. If, for example, you had a system that had something like 50-? or 100-? source impedance, that passive probe is going to have a significantly higher effect on the signal due to probe loading.
In that frequency range, connecting that passive probe is like hanging a 150-? resistor on your circuit. If you can tolerate that, this passive probe is going to be fine. If you cannot tolerate that, then this probe would be an issue, and you will be better off choosing a probe with higher impedance at a high frequency range like an active probe.
When considering the right measurement tools for scope applications, probing is often an afterthought. Many engineers select the scope first based on the bandwidth, sample rate, and channel count needs, worrying about how to get the signal into the scope later. Selecting the right probe for your application and learning how to use it are the first steps toward reliable scope measurements.
A passive probe is a safe choice for general-purpose probing and troubleshooting, while for high-frequency applications, an active probe gives you much more accurate insights into measuring fast signals. Although many active probes in the market have an impressive bandwidth specification, remember that the real-world performance of an active probe is dominated primarily by how you connect the probe to the target. A simple rule of thumb is that a shorter input lead is better if you’re looking for high-fidelity measurement.