Do you want to accurately measure signals with minimal effects from your oscilloscope probe? The oscilloscope probe is a critical link in the quest for accurate signal measurements. The probe is more than just a connection between the circuit under test and the oscilloscope. It can affect both your measurement results and the circuit under test. It’s important to select the right probe, understand its loading effects, and factor in signal fidelity impacts. This blog will cover probe selection, circuit loading, and factors effecting measurement accuracy.
Step 1: Selecting the Probe Type –
Higher bandwidth is an obvious advantage of active probes over passive probes. Oscilloscope probe users often neglect the effect of the connection to the target. They tend to focus on the probe’s published specifications and do not factor in the effect of the probing accessories attached to the tip of the probe. The real-world, probe to system connection can drive the probe’s performance downward. Keeping this in mind, the system bandwidth of the probe and oscilloscope together should be three to five times greater than the frequency of the waveform being measured. This rule of thumb ensures adequate bandwidth for signals such as square waves with high-frequency content. In addition, when measuring rise times, you should ensure that the rise time of the oscilloscope and probe system should be three to five times faster than the pulse being measured. You may not always know the bandwidth of the target being measured but may know the rise time of the fastest signals to be measured. In these cases, there is a handy relationship for determining bandwidth: BANDWIDTH × RISE TIME = 0.35. Below is a chart to help with passive vs active probe selection.
High Dynamic Range
Bandwidth Limited to 500MHz
Heavy Capacitive Loading
Limited Input Dynamic Range
Figure 1: Passive vs Active Probes
Step 2: Factoring Circuit Loading Effects –
All probes have a somewhat similar impact on the circuit under test. A probe draws a portion of the circuit energy and supplies this energy to the oscilloscope. All probes present a capacitive, resistive, and inductive loading element to the circuit under test. The challenge is to ensure that these effects do not impact the circuit and change the signal from its original state.
Capacitive - Capacitive loading is the main culprit of measurement errors. For general-purpose measurements less than 700 MHz, passive high-impedance resistor divider probes are good choices. These rugged and inexpensive tools offer wide dynamic range greater than 300 V and high input resistance to match an oscilloscope’s input impedance. These probes often come with the scope when you purchase it. But, they begin to impose heavier capacitive loading as the frequency of the signal being measured goes up. The input capacitance of the probe and oscilloscope combine to create an impedance between the signal being measured and ground. As the frequency of the signal goes up, the impedance created by the capacitance drops. If the impedance drops too low it can affect your signal being measured, this is known as capacitive loading. For example, a capacitance of 10 pF presents only 100 Ohms of impedance to a 150 MHz signal so it is important to know the input capacitance of the passive probe and the scope you are using. As a rule, high-impedance passive probes are a great choice for general purpose debugging and troubleshooting on most analog or digital circuits.
Resistive - Resistive loading is not as troublesome because it is the least likely to induce nonlinear behavior in your circuit. Most common is resistive loading consisting of the circuit’s output resistance and the probe’s own resistance forming a voltage divider circuit. This divider circuit distorts the signal being measured because the probe is seen as a load to the circuit under test. Even a probe with a small output capacitance of 1 pF can substantially affect the measured circuit. A 1 pF probe looks like a 160-ohm load at 1 GHz, which is the highest frequency associated with a 0.5-ns rise or fall time.
Inductive - Many people think that probe input impedance is a constant number. Probes specifications may state that the probe has a kiloΩ, megaΩ or even a 10 megaΩ input impedance, but this is not constant over frequency because 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 MΩ for a 10:1 passive probe, but as the frequency goes up, the input capacitance of the probe starts to become a short, and the impedance of the probe starts to drop. Figure 2 below shows the limitations of the passive probe relative to an active probe. Note that the active probe in red tracks the non-loaded signal in blue very closely. But the passive probe in green has a slower slope or impulse response. This slower impulse response reflects the lower bandwidth and the ripple or lack of flatness because the displayed signal is missing the high frequency components needed for this measurement.
Figure 2: Passive vs Active Probe loading effects
Step 3: Understanding Signal Fidelity Impacts
Ground lead – Ground lead issues are a constant concern because of the difficulty in determining a true ground reference point. This difficulty arises from the fact that ground leads have inductance and become circuits of their own as signal frequency increases. In addition to being the source of ringing and other waveform aberrations, the ground lead can also act as an antenna for noise. Suspicion is the first defense against ground-lead problems. Always be suspicious of any noise or aberrations observed on the signal. The noise or aberrations may be part of the signal, or they may be the result of the measurement process.
Ground length – All probe ground leads have inductance, and the longer the ground lead the greater the inductance. Ground lead inductance combined with probe tip capacitance forms a resonant circuit that causes ringing at certain frequencies. In order to see this ringing or any other aberrations, the oscilloscope system bandwidth and the probe bandwidth must be high enough to handle the high-frequency content of the aberrations.
Accessory Impact – The convenience offered by some accessories can be attractive, but may come at a cost. Any accessory that is placed between the probe and the target system has the potential to adversely affect the target because of the extra capacitance and inductance of the accessory. Extra capacitance will load the target system and the larger or longer the accessory, the more impact it will have on the measurement being made. Therefore, it may be prudent to reserve the use of larger or longer accessories for slower signal measurements.
Passive probes usually specify an input capacitance of 10-13pF. The inductance will be mainly determined by the ground connection and as we have stated, the shorter the ground connection, the smaller the inductance. Reducing this inductance will increase the resonance frequency to a range outside the bandwidth of the oscilloscope. Figure 3 below shows the impact caused by increasing a ground accessory from a one half inch ground blade to a 9 inch ground. The best response is the blue line with a sharp rise and a small overshoot but flat response from there on. The longer ground lead, in red, causes over a 3 nS delay resulting in a lower bandwidth solution. A good rule of thumb when looking at data like this is that .35 divided by the total rise time equals the bandwidth of the probing solution. In this case the 2 nS difference reduces the calculated bandwidth by 350 MHz relative to the blue signal bandwidth. Also note the additional ripple in the red signal. This is due to the resonant circuit created by the added capacitance and inductance due to the longer ground lead.
Figure 3: Loading Caused by a longer ground lead
Damping - As we have stated earlier, the performance of the probe and the resulting signal on the oscilloscope is highly affected by the probe connection. As signal speeds increase you may notice more overshoot, ringing, and other aberrations because probes and their accessories form a combined resonant circuit. If the resonance is within the bandwidth of the oscilloscope probe you are using, it will be difficult to determine whether the measured aberrations are due to your circuit or the probe resonating circuit. To reduce this effect, damping with the appropriate tip resistor can help. Figure 4 below shows the effects of damping on the measured signal reducing the aberrations on the output signal in blue. The resonance of the undamped circuit causes the oscillation seen in blue in the first screen shot using an active probe with a bandwidth greater than 1 GHz. However, this oscillation will not be seen using a 500 MHz passive probe because the frequency of the oscillation is above the bandwidth of the passive probe.
Figure 4: Undamped vs Damped waveforms
The purpose of these 3 steps was to walk you through the path to accurate probing measurements. The resultant waveform on the oscilloscope will now most accurately represent the signal from your device under test. It is important to remember that there will always be a tradeoff between measurement flexibility, usability, and resulting bandwidth. As we discussed, added capacitance and inductance create RC circuits that resonate within the measured bandwidth of your signal. These circuits will reduce your bandwidth and the ripple will impact your amplitude readings as well as the loading effect. Remember, your probing solution should not load the signal source. It should pass on the signal accurately to the oscilloscope and be immune to noise. In addition, an oscilloscope probe is not a wire. Sure it’s an electrical connection between a node on a board and the oscilloscope. But its resistance, capacitance and inductance can have serious consequences as signal speeds increase.
Additional probing resources:
See all of Keysight’s oscilloscope probes
Learn more about Keysight probes with the Oscilloscope Probe Resource Center
See oscilloscope probing solutions in action on the Keysight YouTube channel