Six Common Probe-Selection Mistakes

Probes allow you to observe the current state of the device under test (DUT). There are hundreds of probes available on the market, but selecting the appropriate probe for your specific measurement is critical to obtaining accurate observations. Many engineers believe that the passive probes supplied as standard with oscilloscopes are already good enough, but this is not the case. If your application uses the wrong oscilloscope probe, you may miss important events and obtain incorrect measurement results.

The probing method and the type of oscilloscope probe used have a significant impact on the accuracy of the measurement results you see on the screen. Before performing measurements, you should proactively learn about mistakes made by other engineers in the past and learn from them:

Mistake 1: Not understanding the key technical specifications of oscilloscope probes

Mistake 2: Choosing a probe with an inappropriate bandwidth

Mistake 3: Failing to consider probe loading effects

Mistake 4: Being unsure whether to choose an active probe or a passive probe

Mistake 5: Forgetting to consider connection bandwidth and loading effects

Mistake 6: Underestimating the attenuation ratio

Mistake 1: Not understanding key technical specifications

If you do not know which key technical specifications of oscilloscope probes you should pay attention to, it is impossible to determine your requirements. Technical documentation usually contains long lists of numbers. Which ones should you focus on?

The most familiar specification is bandwidth. Probe bandwidth ranges from DC to approximately 30 GHz. A common misconception about bandwidth is that greater bandwidth means more data can be observed. This is not always true. As bandwidth increases, many key specifications change, and these are also important factors to consider.

Figure 1. How increasing bandwidth affects other key specifications

Noise is distributed uniformly across frequencies. This means that the higher the probe bandwidth, the more frequencies are introduced and the more noise enters the signal. To prevent this, you should use only the bandwidth needed according to the calculation method described in the next section. Moreover, more specialized probes are required to measure higher frequencies. Of course, this requires greater development effort to create such specialized but costly products for sensitive components. Using a probe with more bandwidth than needed may introduce additional cost, effort, and noise, and these factors may greatly affect your measurement results.

Every type of probe has advantages and disadvantages; for the specific test you are performing, you need to select the more suitable probe. Fully understanding the key specifications and what they mean for you will make probe selection easier. We believe that, rather than reviewing the lengthy specification lists in technical documentation, it is more important to study the rest of the document.

Mistake 2: Choosing an inappropriate probe bandwidth

If the probe bandwidth used is incorrect, you may lose signal detail or introduce unnecessary noise into the system. To deepen understanding, let us first discuss what bandwidth means. Probe bandwidth is essentially the 3 dB point. The 3 dB point is the frequency at which the probe output decreases by 3 dB relative to its nominal response.

Figure 2. Display result of the 3 dB point

Assume a 1:1 probe is used to measure a low-frequency 1 Vpp sine wave. Because you are using a 1:1 probe, the probe output entering the oscilloscope will equal the actual signal input to the probe. However, if you continue to increase the frequency of this 1 Vpp signal, you will eventually reach a point at which the probe output is much smaller than the actual signal input to the probe. When you

see the output on the oscilloscope screen drop to 0.7 Vpp relative to the 1 Vpp input, this indicates that you have reached the 3 dB point, because the output has decreased by 3 dB relative to its nominal response.

                             Figure 3. Example of the 3 dB point appearing as frequency increases

Now that you understand the 3 dB point theory, you can use it to improve testing. The first step in selecting a suitable probe is to understand the signal bandwidth. To determine the signal bandwidth (BW), you can use the following simple formula: if we measure the 10% and 90% thresholds, the signal bandwidth equals the rise time divided by 0.35. If measuring the 20% and 80% thresholds, the signal bandwidth equals the rise time divided by 0.22.

After calculating the signal bandwidth, select the probe bandwidth according to the following two rules of thumb:

– The probe bandwidth should be 3 times higher than the fastest sine-wave frequency in analog applications

– The probe bandwidth should be 5 times faster than the highest digital clock rate in digital applications

Based on these quick calculation methods, you can roughly determine which probe bandwidth is suitable for your application. As rise time becomes faster, signal bandwidth increases accordingly, meaning you need a probe with higher bandwidth. But remember that excessively high bandwidth can also cause problems.

Another way to consider bandwidth is based on harmonics. In general, the higher the probe bandwidth, the more harmonics are captured, and both will slightly improve signal accuracy. As shown in Figure 5 below, the original signal is the yellow trace, and the first-order harmonic is the green trace. You can see that they have the same period and duty cycle, but the rising edge of the first-order harmonic is noticeably slower, and the corners are very rounded. The blue trace shows the first- and third-order harmonics; their rising edge is faster, and the corners become sharper. But at the bottom of the image, we can see the first-, third-, and fifth-order harmonics. Their edges are smooth, the corners are sharp, and there is a lot of detail at the top and bottom of the signal. The higher the bandwidth, the more detail the waveform will reveal.

Figure 4. Higher bandwidth means more harmonics and richer signal detail

Having understood the above pattern, let us look at an example of measuring a 100 MHz clock with a 100 MHz probe. After completing this measurement, you will ultimately see a sine wave on the screen as shown in Figure 6. Therefore, you cannot obtain an accurate rise time or any real signal detail. This means that any measurement you make is inaccurate and meaningless.

Figure 5. Measuring a 100 MHz clock signal with a 100 MHz probe

However, if you use a 500 MHz probe to measure the same 100 MHz signal, you will have enough bandwidth to capture more harmonics, thereby obtaining a more accurate representation of the signal.

Figure 6. Measuring a 100 MHz clock signal with a 500 MHz probe

This shows how important it is to select the appropriate probe bandwidth for the signal you need to handle. But excessive bandwidth is also undesirable; too much is as bad as too little. When determining how much bandwidth is suitable for your application and selecting the right probe, these theoretical and quick calculation methods can help you avoid mistakes.

Mistake 3: Failure to consider probe loading effects

Once a probe is connected to the oscilloscope and contacts the device, it becomes part of the circuit. The problem is that the resistive, capacitive, and inductive loading effects that the probe presents to the device will affect the signal you see on the screen. This loading effect is an important factor you need to consider. Sometimes this effect is very small and may even go unnoticed, but if the loading effect is too large, it changes more than just what you see on the screen. It also affects the operating state of the device. Obviously, you want to minimize the loading effect. Unfortunately, because this is a parasitic loading effect, you will never be able to eliminate it completely, but the more you understand it, the more likely it is to help you reduce its effect on the device.

In the basic probe model in Figure 8, you can see the inductance, capacitance, and resistance of a passive probe. The resistor is a discrete component, which means it is designed at the end of the probe to isolate the probe from the circuit and minimize loading effects. The probe capacitance is the result of both the capacitive components in the design and parasitic capacitance.

Figure 7. Basic probe schematic

The graph in Figure 9 makes this easier to understand. Note the left side of the graph: in the DC frequency range, the input impedance is approximately 100 kΩ. To the signal, this looks like a 100 kΩ resistor to ground. However, on the right side of the graph in the MHz region, the impedance begins to fall because the impedance of a capacitor is inversely proportional to frequency. Therefore, as frequency increases, the capacitor begins to provide an easier path to ground than the resistor.

Figure 8. Capacitor impedance is inversely proportional to frequency

The loading effect of an active probe is far smaller than that of a passive probe, because a passive probe is made only of resistive and capacitive elements. The loading effect of an active probe is far smaller than that of a passive probe, because a passive probe is made only of resistive and capacitive elements.

Mistake 4: Being unsure whether to choose an active or passive probe

First, let us look at an example comparing a passive probe with an active probe, as shown in Figure 10. The comparison here is impedance versus frequency. Continuing with probe loading effects, most engineers focus on resistance when reading probe datasheets. They may compare the 10 MΩ resistance of a passive probe with the 1 MΩ resistance of an active probe and then think: 'The passive probe has higher resistance, which indicates lower loading effect.' However, what they should really consider is another important factor discussed above, namely capacitance. You can compare the 9.5 pF capacitance of the passive probe with the 1 pF capacitance of the active probe in Figure 10.

Figure 9. Comparison of active and passive probes

The graph at the bottom of Figure 10 helps you intuitively understand the difference in the impedance-versus-frequency relationship for two different capacitance values. The red trace for the passive probe and the blue trace for the active probe cross at a certain point. The frequency at which the two probes have the same impedance is 10 kHz. However, as frequency increases, their impedances look very different. At 70 MHz, the passive probe has only 150 Ω, while the active probe has 2.5 kΩ. The impedance of the active probe is much higher, which is exactly what you want. Higher impedance will reduce the capacitive loading effect. The much higher impedance of the active probe is mainly due to its lower capacitance. At such frequencies, capacitance plays a much greater role than resistance. Therefore, never forget to check the probe's capacitance specification.

Another important difference that loading effects cause between passive and active probes is accuracy. To analyze this situation, the simplest method is to use the two types of probes to measure the rise time of a signal. The green trace in Figure 11 shows the signal entering the oscilloscope directly through a special accessory rather than through a probe. It can be seen that the true rise time of the signal is 1.1 ns. When the passive probe is connected to the device, the red trace appears, and the rise time is measured as 1.5 ns. Compared with the true rise time, the deviation is 0.4 ns. After connecting the active probe (yellow trace), you will find that the measured rise time is almost indistinguishable from the actual value. You cannot even see the yellow trace. Because the active probe has a much smaller loading effect, its accuracy is also much higher.

Figure 10. Comparison of the effect of passive and active probe loading on rise time

Probe impedance changes with frequency, and the impedance of an active probe is greater than that of a passive probe. Active probes are more accurate but also more expensive. You may not always need high accuracy, so you want to evaluate the probe's loading effect within the specific frequency range you need to handle, thereby determining whether it is within the allowable range for the test. Remember to always pay attention to the two key attributes of probe loading effects: resistance and capacitance. The resistance value should be as high as possible, and the capacitance value should be smaller, so that a smaller loading effect can be obtained.

Mistake 5: Forgetting to consider connection bandwidth and loading effects

Connection bandwidth

Connection bandwidth is the bandwidth of the connection between the probe and the device under test. In many cases, an accessory is used at the probe tip to make it more convenient to establish a secure connection. However, such accessories become the weakest link in the system and limit bandwidth, even though you have already selected the oscilloscope and probe with the most suitable bandwidth. This must be taken into account when determining the required bandwidth and probe accessories.

Consider the 2 GHz active probe in Figure 12 and use it to understand why the loading effect is the weakest link in a measurement system. Many types of accessories can be paired with the probe. In the first example, a two-lead adapter and a clip grabber are connected to the device. This is a very short connection, so the bandwidth remains 500 MHz. If you remove the clip end and keep only the leads, the connection is shorter, so you can see better bandwidth. However, if all accessories are completely removed, it is possible to obtain the shortest connection and achieve the best bandwidth (2 GHz).

You now understand how the weakest link changes the measurement results you see on the screen. However, in practice, using such accessories is still necessary in many cases, or because the signal frequency is lower, a very wide bandwidth is not required, so accessories can be used. But for applications that require greater bandwidth to see faster rise times, do not forget to consider connection bandwidth. The connection should be as short as possible to achieve the greatest possible bandwidth.

Connection loading effect

The connection loading effect seen on the screen is parallel to connection bandwidth. You can observe the results of the same active probe in Figure 12 to understand the loading effect. When using the lead adapter and clip grabber, the loading effect on the signal is relatively low, but there is still some. If no accessories are connected at all, the loading effect will be minimized. You can see how it varies with accessories, similar to connection bandwidth. When no accessories are used, the load will be so small that you will likely not even notice it in the measurement. Knowing this rule is especially useful when you are trying to determine which accessories are most suitable for your application (if any).

There is a simple technique to determine how much connection loading effect your device will encounter. Its name is: two-probe system. Its rationale is to use two probes at the same location in the design to estimate the loading effect of one probe. Basically, the difference between measurements using one probe and two probes is roughly the same as the difference between measurements without a probe and with one probe.

1. Connect the probe and accessories to the device under test

2. Measure the rise time of the signal and save the trace shown on the screen for later comparison

3. Connect another probe to the same point using the same setup/accessories.

Figure 11. Different connection and bandwidth effects

You can roughly see how large the loading effect is on the oscilloscope screen (Figure 13). This is the difference between the single-probe trace and the two-probe trace. With this information, you can know how much loading effect different probes and accessories will introduce into the system. To repeat, the shorter the accessory or connection between the probe tip and the device under test, the smaller the connection loading effect you will see on the signal.

Figure 12. Two-probe method demonstration

Mistake 6: Underestimating the attenuation ratio

Another important specification of a probe is the attenuation ratio. Most applications require different attenuation ratios. The attenuation ratio you need is determined by the magnitude of the signal to be measured. The attenuation ratio determines how the signal is fed into the oscilloscope and, ultimately, how it is displayed on the screen. Let us step back and think about why different attenuation ratios are used. An oscilloscope is a very sensitive instrument and can only operate within a certain voltage range to prevent damage. If you are dealing with a larger voltage, you need to reduce the voltage through voltage division to within the safe voltage range of the oscilloscope. The voltage-division ratio is the attenuation ratio, and different signals and applications require different attenuation ratios.

If you are dealing with higher voltages, you need to use a higher attenuation ratio, for example 10:1. This means that when the probe is connected to the device, the signal seen on the oscilloscope screen will be only one tenth of the actual output signal of the device. If you are dealing with higher voltages, it should be reduced to one tenth of the original to fit the ADC range and be processed correctly. If you are dealing with small voltages, you need to use a smaller attenuation ratio, such as a 1:1 probe. This makes the most of the ADC and lets you view small signals more accurately. If a device such as a 1:1 probe is used to measure a large voltage, the signal will be clipped, and the entire signal cannot be viewed on the screen. The attenuation ratio can range from 1:1 for low-voltage applications all the way up to 1000:1 for high-voltage applications. Different attenuation ratios have their own advantages and disadvantages, but again, this is determined by the magnitude of the signal. Using a higher attenuation ratio introduces more noise from the oscilloscope amplifier, while using a lower attenuation ratio means less noise, but at the same time it also produces a greater loading effect on the signal, and the useful frequency range is significantly reduced.

Figure 13. The same signal is input to two channels, with the vertical axis set to 5 V/div; you can see the difference between the 10:1 probe (yellow trace) and the 1:1 probe (green trace).

Understanding probe and oscilloscope specifications helps you better understand the attenuation ratio. Take the 10:1 probe schematic in Figure 15 as an example. This type of probe is usually the standard probe supplied with an oscilloscope. It connects a 9 MΩ probe tip resistor in series with the oscilloscope's 1 MΩ input impedance. The resistance at the probe tip then becomes 10 MΩ, meaning that the signal voltage entering the oscilloscope is 1/10 of the voltage at the probe tip connection.

Figure 14. Schematic of probe and oscilloscope connection

With this knowledge, you can easily select a probe with a suitable attenuation ratio for your application. It depends entirely on the signal voltage you are dealing with.

Understanding all these key probe specifications will help you avoid the common mistakes other engineers make when selecting oscilloscope probes.

Remember the biggest mistake not to make: never underestimate the capability of the probe! Otherwise, there will be a significant difference between the signal you are measuring and the signal displayed on the screen. Ensure that the probe you select meets the requirements of the important technical specifications in the test.

Reprinted from Keysight Technologies: 6 Common Mistakes When Choosing Oscilloscope Probes