Why, you may ask, would I want to spend money on an active probe for my oscilloscope when it came with free passive probes? As an oscilloscope probe designer I’m going to share with you a reason why you should consider upgrading to an active probe—and it’s not about bandwidth. I think a lot of folks who upgrade to an active probe do it because they need more bandwidth. Most passive probes top out at about 500 MHz so if you need more bandwidth than that you’ll need to buy an active probe. An active probe offers other benefits that should be considered even if you only feel you need a 100 MHz of so of bandwidth. I’m going to point out one that I think is most often overlooked.
Consider this, an active probe will provide significantly less probe loading than a passive probe. Probe loading is the effect that the probe has on your circuit when it comes in physical contact with it. Excessive probe loading will change the behavior of the signal being probed. With excessive probe loading, the signal that you see on the scope will be an accurate image of the signal being probed but it won’t be an accurate image of the real signal—the signal when the probe is removed. Here is how it works. When you look at the probe you are using, the label will say something like 10 MΩ:10 pf for a passive probe and 1 MΩ:1 pf for a general purpose active probe. What this is describing is a simplified circuit model for what the probe will look like to your circuit when the probe is connected to it. When in contact with your circuit the probe will appear as a resistor and capacitor, in parallel, connected to ground. It is easy to focus on the resistor value and overlook the contribution of the capacitance of the probe to probe loading. Considering only the resistor would lead one to conclude that a passive probe, with its 10 MΩ impedance will have much less loading than the active probe with its 1 MΩ impedance. Remember though that the impedance of a capacitor is inversely proportional to frequency. This means that the liability of the passive probe lies in its large input capacitance. Comparing the two probes, their input impedance (the impedance to ground when connected to your circuit) will be equal at 10 kHz. Therefore the active probe will produce less loading to any signal you are probing that has frequency content above 10 kHz. This is shown in figure above.
Now we will put this to the test. I’ve got a circuit that contains a 1.1 ns edge. Traditional guidance would suggest that I need about 300 MHz of bandwidth in my measurement system (scope and probe) to measure this signal. This is well within the capabilities of our free passive probe. I first probe the signal with my active probe and I measure the rise time. Just like I expected, 1.1ns. Now I remove my active probe and probe the signal with my passive probe. Oops, I’m measuring 1.5 ns. Is my measurement wrong? No, my measurement is correct. That is what the edge speed of my target signal has become due to the loading of my passive probe. The large capacitance to ground of the passive probe is creating a low impedance path to ground for the higher frequency content of my signal and my target cannot drive this load so the actual signal on my target is distorted.
What you can conclude is that a passive probe is good for making qualitative measurements and an active probe is good for making quantitative measurements. Qualitative measurements are things like: is the patient’s heart beating, is the 5V up, is the clock toggling..? Quantitative measurements are things like: what is the patient’s heart rate, how much ripple/noise is on the 5V supply, what is the rise time..? Do yourself a favor, next time you get a chance, splurge and buy an active probe.