Operational "How to" Guides

Summary

What Every Engineer Should Know About Oscilloscope Probes: Passive vs Active

Description

Probe selection can seem awfully confusing and probably it happened you reached for a probe and made a measurement without even knowing which probe you were using. I’m guilty, too, for this “random selection.”

The fact is, probe selection can significantly affect your measurement, so let’s break this down and walk through the one simple thing every engineer should know about oscilloscope probes: what the difference is between passive and active. Use this guide as the first step to better, more accurate measurements.

Passive probes

Passive probes (Fig 1) are the most widely used type of oscilloscope probe. They are rugged and economical and they are what you’ll typically find shipping with your oscilloscope. They’re what you’re bound to find on every engineer’s test bench. These high impedance probes are what we consider “general purpose.”

Active Probes

Active probes (Fig 2) are a level above passive probes in terms of performance, complexity, and typically cost. You’ll generally purchase these probes separately from an oscilloscope, for a specific measurement application. Compared to passive probes, they need to be handled carefully due to the active circuitry inside the probe head.

The Big Differentiator: Power

Passive probes contain no active circuitry such as transistors or amplifiers and therefore require no power. A typical 10:1 passive probe and oscilloscope combined circuit is represented in Fig 3.

Reading from left to right, you’ll see a 9 Mega Ohm resistor that is one of two parts that make up the 10:1 divider of the probe. Continuing to the right, there’s an adjustable compensation capacitor which you can mechanically adjust to “match” the oscilloscope capacitance. This helps to make sure the probe and oscilloscope are equally compensated and thereby setting you up for an accurate measurement.

The oscilloscope makes up the second half of this combined circuit. The 1 Mega Ohm resistor completes the 10:1 divider ratio. In other words, the 9 Mega Ohm and 1 Mega Ohm resistors “divide down” the signal by a factor of 10. The input capacitance is the standard capacitance of your scope, typically printed on the front panel.

Active probes differ from passive probes because they have “active” circuitry, typically in the form of transistors instead of resistors as well as an amplifier. You can see the typical active probe configuration in Fig 4.

The physical look of an active probe is also much different from a passive probe. The head of the probe contains the active circuitry, where the filtering and conditioning are done. The pod interface that plugs into the scope tells the scope which probe has been plugged in, what we call “auto-sensing.” After sensing which probe is plugged in, there is auto-configuration that happens (i.e. settings are auto-adjusted based on the probe). This isn’t to say this isn’t possible with passive probes, it is just not as common.

Performance Difference: How to Select a Probe

Passive probes are great for making general purpose measurements. They have a wide dynamic range and bandwidth as high as 500 MHz when connected to the 1MOhm input of the oscilloscope. They work well if you’re working in the DC and low-frequency range. They can also be sufficient for making quick quantitative measurements, such as if a clock is running or if the source is on- simple “yes or no” questions where a high degree of accuracy isn’t required.

Active probes are often more expensive than passive probes, but they offer a superior level of performance that may be essential in certain circumstances. The real driving factor is when you need a high degree of signal integrity, you should use an active probe. But why? It really has everything to do with probe loading.

Signal Integrity: What Probe Loading Has to Do With It

When you touch a probe to your DUT, the probe loads the signal and this is called probe loading. To have high signal integrity, you need the least loading possible. The amount of loading on your signal is determined by the probe’s input impedance in relation to the source impedance.

The probe’s input impedance is a function of frequency. It stays pretty flat from DC up to a certain frequency but as frequency continues to go up, the probe’s input impedance goes down, as the capacitance of the probe starts to come into play. The more capacitance, the lower the impedance. As the frequency goes up above the crossing point of ~10kHz, this is where we can really see a difference in performance between an active and passive probe. The active probe has a low capacitance value, leading to higher impedance and less loading. You can see this effect plotted in Figure 5.

This effect trips up a lot of engineers. You might think that a passive probe is just fine because it worked well at low frequencies, but as soon as you try to use it to measure a higher frequency signal (past the crossover point of 70 MHz in the example in Figure 5), there’s a significant performance degradation where you’d be better off using an active probe.

Key Takeaway

Probe selection does affect measurement accuracy and signal integrity. Next time you go to make a measurement, consider your signal’s speed and the type of measurement you’re trying to make (quantitative vs qualitative) before deciding between a passive and active probe. Probe “random selection” will be a thing of the past.

Active probes differ from passive probes because they have “active” circuitry, typically in the form of transistors instead of resistors as well as an amplifier.

Active probes are often more expensive than passive probes, but they offer a superior level of performance that may be essential in certain circumstances.

To have high signal integrity, you need the least loading possible. The amount of loading on your signal is determined by the probe’s input impedance in relation to the source impedance.