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 today, but choosing the right probe for your specific measurement is crucial for obtaining accurate results. Many engineers believe that a passive probe standard with an oscilloscope is good enough, but this is not the case. If your application uses the wrong oscilloscope probe, important events may be missed and incorrect measurement results may be obtained.

The method used for probing oscilloscope probes greatly affects the accuracy of the measurement results you see on the screen. Before conducting measurements, you should proactively learn about mistakes made by other engineers in the past and

learn from them

th 1 error: The key technical specifications of the oscilloscope probe are not understood

2 errors: The selected probe bandwidth is inappropriate

th 3 errors: Probe load effect not considered

4 errors: Unsure whether to choose active or passive probe

5 errors: Forgot to consider connection bandwidth and load effects

6 errors: Underestimating attenuation ratio

span style="font-family:微软雅黑"> 1 error: Key technical specifications are not understood

If you don't know which key technical specifications to pay attention to for your oscilloscope probe, it's impossible to determine your needs. Technical documentation usually contains a large list of numbers. Which ones do you need to focus on?

The most familiar technical metric is bandwidth. The probe's bandwidth ranges from DC all the way to about 30 GHz. A common misconception about bandwidth is that the larger the bandwidth, the more data can be seen. But that's not always the case. As bandwidth increases, many key technical indicators change, and they are important factors to consider

 

Figure 1. How continuously increasing bandwidth affects other key technical metrics

noise is evenly distributed across all 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 required bandwidth according to the calculation method introduced in the next section. Moreover, more specialized probes are needed to measure higher frequencies. Of course, this requires increased development efforts to create such specialized but costly products for sensitive components. Using probes with bandwidth exceeding required bandwidth may incur additional costs, workload, and noise, factors that can significantly impact your measurement results

Each type of probe has its pros and cons, and for your specific tests, you need to choose the probe that best suits you. Fully understanding key technical specifications and what they mean for you will make probe selection much easier. We believe that rather than just looking at lengthy lists of technical specifications in technical documents, it is more important to study the rest of the documentation

2 errors: The selected probe bandwidth is inappropriate

If the probe bandwidth used is incorrect, you may lose signal details or introduce unnecessary noise to the system. To deepen your understanding, let's first discuss the meaning of bandwidth. Probe bandwidth is essentially 3 dB points. A 3 dB point refers to the frequency at which the probe output decreases by 3 dB relative to the nominal response

 

Figure 2. Display results of 3 dB dots

Suppose you use 1:1 probe to measure a low-frequency sine wave of 1 VPP. Since you are using a 1:1 probe, the output of the probe entering the oscilloscope will be equal to 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 where the probe output is much smaller than the actual signal input to the probe. When you

When you see the output on the oscilloscope screen drop to 0.7 Vpp compared to it means you have reached the 3 dB point, because the output decreases by 3 dB relative to its nominal response

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

Now that you understand

After calculating the signal bandwidth, you can choose the probe bandwidth based on the following two experiences:

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

– The probe bandwidth should be 5 times faster than the maximum digital clock rate of a digital application

With these quick calculation methods, you can roughly determine which probe bandwidth is best suited for your application. As the rise time increases, the signal bandwidth increases, which means you need a probe with higher bandwidth. But remember, excessive bandwidth can also cause problems

Another way to consider bandwidth is based on harmonics. Generally, the higher the probe bandwidth, the more harmonics it captures, and both slightly improve signal accuracy. As shown in the figure below 5, the original signal is a yellow trace, and the first-order harmonic is a green trace. You can see they have the same period and duty cycle, but the rising edge of the first harmonic is noticeably slower, and the corners are very smooth. The blue trace shows the first and third order harmonics, with a faster rising edge and clearer corner points. But at the bottom of the image, we can see the first, third, and fifth order harmonics. Their edges are smooth, the corners 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 details

Now that we understand the above pattern, let's look at an example of measuring a 100 MHz clock using 100 MHz probe. After completing this measurement, you will eventually see the sine wave shown in Figure 6 on the screen. Therefore, you cannot get an accurate rise time or any real signal details. This means any measurement you make is inaccurate and meaningless

 

Figure 5. Use a 100 MHz probe to measure a 100 MHz clock signal

However if you use a

 

Figure 6. Use a 500 MHz probe to measure a 100 MHz clock signal

This shows how important it is to choose the appropriate probe bandwidth for the signal you want to process. However, too high a bandwidth is also not good; too much is worse. When determining how much bandwidth is suitable for your application and choosing the right probe, these theories and quick calculation methods can help you avoid mistakes

th 3 errors: Probe load effects not considered

Once the probe is connected to the oscilloscope and contacts the device, it becomes part of the circuit. The problem is that the resistance, capacitance, and inductance load effects the probe brings to the device will affect the signal you see on the screen. This load effect is an important factor you need to consider. Sometimes the effect is small or even unnoticed, but if the load effect is too great, it changes more than just what you see on the screen. It also affects the device's operating status. Obviously, you want to minimize load effects as much as possible. Unfortunately, since this is a parasitic load effect, you will never completely eliminate it, but the more you understand it, the more likely you are to help reduce its impact on your device

In the basic probe model of Figure 8, you can see the inductance, capacitance, and resistance of a passive probe. A resistor is a discrete component, meaning it is designed at the end of the probe to isolate it from the circuit and minimize load effects. The probe capacitance is the result of the design capacitive component and parasitic capacitance formed together

 

Figure 7. Basic principle diagram of the probe

The graph in The graph in Figure 9 makes this easier to understand. Note on the left side of the figure: within the DC frequency range, the input impedance is about 100 kΩ. To the signal, this looks like a 100 kΩ ground resistor. However, in the MHz region on the right side of the graph, impedance starts to decrease because the capacitor's impedance is inversely proportional to the frequency. Therefore, as frequency increases, capacitors become easier to ground than resistors

 

Figure 8. The impedance of a capacitor is inversely proportional to its frequency

The load effect of active probes is much less than that of passive probes, because passive probes are made only of resistor and capacitor elements. The load effect of active probes is much less than that of passive probes, because passive probes are made only of resistor and capacitor elements

4 errors: Unsure whether to choose active or passive probe

First, let's look at an example comparing passive and active probes, as shown in Figure 10. Here, the comparison is impedance and frequency. Continuing to consider probe load effects, most engineers focus on resistance when reading probe technical materials. They might compare the 10 MΩ resistance of a passive probe with the 1 MΩ resistance of an active probe and then think: "The higher resistance of a passive probe means the load effect is lower." However, what they should really consider is another important factor we discussed above: capacitance. You can compare the 9.5 pf capacitance of the passive probe in Figure 10 with the 1 pF capacitance of the active probe

 

Figure 9. Comparison of active and passive probes

image 10 The graphical at the bottom helps you intuitively understand the difference in impedance and frequency between two different capacitors. The red trace of a passive probe and the blue trace of an active probe intersect at a certain point. The frequency point of both probes with the same impedance is 10 kHz. However, as frequency increases, their impedance looks very different. At 70 MHz, passive probes have only 150 Ω, while active probes have 2.5 kΩ. Active probes have much higher impedance, which is exactly what you want. Higher impedance reduces capacitive load effects. The much higher impedance of active probes is mainly due to their lower capacitance. At this frequency, the effect of capacitance is much greater than that of resistance value. Therefore, never forget to check the technical specifications of the probe's capacitance

Another important difference between passive and active probes is accuracy. To analyze this situation, the simplest way is to use both probes to measure the signal rise time. The green trace in Figure 11 shows the signal entering the oscilloscope directly using a special attachment instead of the probe. It can be seen that the actual rise time of the signal is 1.1 ns. When the passive probe is connected to the device, a red trace appears, and the rise time is measured as 1.5 ns. Compared to the actual rise time, there is a deviation of 0.4 ns. After connecting the active probe (yellow trace), you will find that the rise time measurement is almost identical to the actual value. You won't even see the yellow trace. Because the load effect of active probes is much lower, the accuracy is also much higher

 

image10. Comparison of the Load Effects of Passive and Active Probe Sensors on Rise Time

The probe impedance varies with frequency, with active probes having higher impedance than passive probes. Active probes are more accurate but also more expensive. You may not always need high accuracy, so you want to evaluate the probe's load effects within a specific frequency range to determine if it is within the allowable testing range. Always pay attention to two key attributes of probe load effects: resistance and capacitance. The resistance value should be as high as possible and the capacitance value should be low to achieve a smaller load effect.

5 errors: Forgot to consider connection bandwidth and load effects

connection bandwidth

Connection bandwidth refers to the bandwidth between the probe and the device under test. In many cases, attachments are used at the tip of the probe to facilitate a more secure connection. However, these attachments become the weakest link in the system, limiting bandwidth, even if you have selected the oscilloscope and probe with the most suitable bandwidth. This must be considered when determining the required bandwidth and probe attachments

Consider the 2 GHz active probe in Figure 12 to understand why load effects are the weakest link in the measurement system. There are many accessories that can be paired with probes. In the first example, a dual-lead adapter and a fixture gripper are connected to the device. This is a very short connection, so the bandwidth is still 500 MHz. If you remove the clamp end and only keep the leads, the connection is shorter, so you can see better bandwidth. However, if all accessories are completely removed, it is possible to achieve the shortest connection and provide the best bandwidth (2 GHz).

Now you understand how the weakest link can affect the measurement results you see on the screen. However, in many cases, using such accessories is still necessary, or because the signal frequency is low and you don't need a wide bandwidth, accessories can be used. But for applications that require larger bandwidth to see faster rise times, don't forget to consider connection bandwidth. Connections should be as short as possible to achieve maximum bandwidth

Connect load effects

The connection load effect shown on the screen is parallel to the connection bandwidth. You can observe the results of the same active probe in Figure 12 to understand the load effect. When using lead adapters and clamp grabbers, the load effect on the signal is relatively low, but there is still some effect. If no accessories are connected at all, the load effect is minimized. You can see how it varies with the accessory, similar to the connection bandwidth. Without any accessories, the load will be so small that you might not even notice it during measurement. Knowing this pattern is especially useful when trying to determine which accessories are best suited for your application (if any).

There is a simple trick to determine how much connection load effect your device will experience. Its name is: Dual-probe system. Its theoretical basis is to use two probes at the same position in the design to estimate the load effect of one probe. Basically, the difference between measuring with one probe and two probes is roughly the same as measuring with no probe versus measuring 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 traces displayed on the screen for later comparison

3. Connect another probe to the same point using the same device/attachment

image 11. Different connection and bandwidth effects

You can roughly see how much load effect is on the oscilloscope screen (Figure 13). This is the difference between "single probe" and "dual probe" traces. With this information, you can see how much load effect different probes and accessories will introduce to the system. Again, the shorter the attachment or connection between the probe tip and the device under test, the less connection load effect you see in the signal

 

Figure 12. Demonstration of the dual probe method

6 errors: Underestimating attenuation ratio

Another important technical metric for probes is the attenuation ratio. Most applications require different attenuation ratios. The attenuation ratio you need depends on the signal size to be measured. The attenuation ratio will determine how the signal is fed into the oscilloscope and ultimately displayed on the screen. Let's take a step back and consider why different attenuation ratios are used. Oscilloscopes are very sensitive instruments and can only operate within a certain voltage range to prevent damage. If you are dealing with higher voltages, voltage division is needed to reduce the voltage to the oscline's safe voltage range. The voltage divider is the attenuation ratio, which requires different attenuation ratios for different signals and applications

If you are handling 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 from the device. If you are handling higher voltages, you should reduce it to one-tenth of the original range to fit the ADC's range and handle it correctly. If you are dealing with low voltages, you need to use a smaller attenuation ratio, such as a 1:1 probe. This maximizes the full use of the ADC, allowing you to view small signals more accurately. If devices like 1:1 probes are used to measure large voltages, the signal will be clipped and cannot be viewed on the screen. The attenuation ratio can increase from 1:1 at low voltage to 1000:1 at high voltage. Different attenuation ratios have their pros and cons, but again, this depends on the signal size. Using a higher attenuation ratio introduces more noise from the oscilloscope amplifier, while using a lower attenuation ratio means less noise but also imposes a greater load on the signal and significantly reduces the useful frequency range

 

Figure 13. Input the same signal into two channels, set the vertical axis to 5V/grid. You can see the difference between the 10:1 probe (yellow line) and the 1:1 probe (green line

).

Understanding the technical specifications of probes and oscilloscopes helps you better grasp attenuation ratios. Take the 10:1 probe schematic diagram in Figure 15 as an example. This type of probe is usually the standard probe for oscilloscopes. It connects a 9 MΩ probe end resistor in series with a 1 MΩ input impedance of the oscilloscope. The resistance at the probe tip then becomes 10 MΩ, meaning the signal voltage entering the oscilloscope is 1/10 of the voltage at the probe end connection

 

Figure 14. Schematic diagram of the connection between probe and oscilloscope

With this knowledge, you can easily select a probe with the appropriate attenuation ratio for your application. It all depends on the signal voltage you want to handle

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

Remember the biggest mistake you should never make: never underestimate the probe's capabilities! Otherwise, the signal you measure will differ greatly from what is displayed on the screen. Make sure the probe you choose meets the requirements of important technical specifications during testing

 

reprinted from SideTech "6 Common Mistakes When Choosing Oscilloscope Probes."