What to Consider When Selecting Probes for Your Oscilloscope

What to Consider When Selecting Probes for Your Oscilloscope

Published: 13th April 2022

What factors play into making the best oscilloscope probe decision? Depending on your oscilloscope and probe choice, you can detect signals in the frequency and time domains. In the time domain, synchronized analog waveforms, digital waveforms, and decoded serial protocols can be recorded. You can use an integrated spectrum analyzer signal path in the frequency domain. Because you are working to detect accurate measurements, the probe you choose for the work at hand is vital; here are some things to consider when picking probes.

Bandwidth

Your oscilloscope and probe have bandwidth specifications. Unfortunately, most datasheets do not cover this well. As a rule of thumb, your probe bandwidth should be greater than or equal to three to five times the fastest signal bandwidth on your oscilloscope. Add bandwidth specification to the top of your probe search list.

Probe Loading

Probes can have inductive and capacitance effects leading to degraded signals. Keep in mind that probes with higher attenuations have lower capacitive loading. For example, a passive probe typically has an input impedance of 10 MΩ and an input capacitance of > 10 pF. Active probes usually carry an input capacitance of < 1 pF at an input impedance of 1 MΩ and are especially suited for measurements on circuits with high-speed signals > 100 MHz.

Probe loading might not be a significant concern if you are using a higher-end oscilloscope; they use digital signal processing (DSP) that compensates for the difference. Your probe’s impedance value will depend on your design and how well your oscilloscope performs. For example, a 1:1 probe will reduce amplifier noise, and a 10:1 probe will reduce your capacitive loading.

Active or Passive Probes

Probes can have inductive and capacitance effects leading to degraded signals. Keep in mind that probes with higher attenuations have lower capacitive loading. For example, a passive probe typically has an input impedance of 10 MΩ and an input capacitance of > 10 pF. Active probes usually carry an input capacitance of < 1 pF at an input impedance of 1 MΩ and are especially suited for measurements on circuits with high-speed signals > 100 MHz.

Probe loading might not be a significant concern if you are using a higher-end oscilloscope; they use digital signal processing (DSP) that compensates for the difference. Your probe’s impedance value will depend on your design and how well your oscilloscope performs. For example, a 1:1 probe will reduce amplifier noise, and a 10:1 probe will reduce your capacitive loading.

Active or Passive Probes

As mentioned above, active probes are commonly used to measure high-speed circuits, but your oscilloscope comes equipped with one passive probe per channel. On the one hand, passive probes are generally less expensive, more durable, and offer a more comprehensive dynamic range and bandwidth than active probes. But on the other hand, the passive probe’s capacitance could be adding load to the system resulting in distorted output.

Passive probes should be used for qualitative measurements like checking for bugs, clock frequencies, and other general measurements. More costly and sensitive active probes should be used for measuring high-speed signals that have fast rise and fall times.

To discover the amount of probe loading to your device under test, connect one probe to a step signal you know, then connect another probe to the same location and look at the difference in edge speed.

Attenuation Ratio

Your probe’s attenuation ratio affects how its signals are delivered to the oscilloscope. So, your 10:1 probe will deliver one-tenth of its voltage to the input. Probes with high attenuation ratios allow you to extend the dynamic range of your oscilloscope and look at higher voltages. However, using high attenuation ratios can also lead to noise. You typically want your probe’s input resistance and capacitance to match the oscilloscope to get the cleanest reads possible. It is also important to keep in mind that probes have a dynamic range too; remember to choose probes with maximum voltages within the operating range you are looking to test in.

Deferential or Single-ended Measurements

When considering probes, think about your test design. What types of measurements do you need to take? If you are taking deferential measurements, you will need multiple probes. If you are taking measurements looking for asymmetry or common mode responses, you will want to do a single-ended test. When designing your system, identify what types of testing need to be done and think about which tests require floating or grounded measurements. Make sure your probes are designed to meet the requirements of your test design.

Power Rail Probes

To probe DC power rails, you need low voltage measurements and the ability to measure ripple, load transients, and chase fast spikes. Most passive probes you already have are not suited for this and will pick up too much noise for accurate measurements. It is best to opt for a power rail probe that is specially designed for power rail measurements.

Current Probes

Current probes are designed to measure current. Some offer extraordinarily high dynamic ranges for measuring active state and sleep currents. To get the best results from your testing efforts, use a current probe to measure currents.

EMC Near-Field Probes

These probes are configured for identifying problems with electromagnetic flows in circuits. They are commonly used to identify sources of current interference. These probes are usually more sensitive than a common current probe and need to be utilized if your oscilloscope does not have powerful FFT functionality.

Last Thoughts

When considering which probes to purchase, first have a good idea of your experimental design in mind. What do you want to get out of your measurements? After this, identify the probe specifications you need to accomplish what you have set out to do. Last, look for quality items that deliver value, so you are not wasting time and money returning failed products. TestEquity offers a wide range of probes to fit any design. Feel free to contact our experts with any questions you have about our probe performance; we are here to serve you.

TestEquity
Content Source: What to Consider When Selecting Probes for Your Oscilloscope | TestEquity

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TPR4000 Active Power Rail Probe

4 GHz Bandwidth, 1.25x Attenuation

The TPR4000 probe provides a low-noise measurement solution (oscilloscope and probe), which is critical to not confuse the noise of the oscilloscope and probe with the noise and ripple of the DC supply being measured.

Find out more about TPR4000 Active Power Rail Probe
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TPR1000 Active Power Rail Probe

1 GHz Bandwidth, 1.25x Attenuation

The TPR1000 probe provides a low-noise measurement solution (oscilloscope and probe), which is critical to not confuse the noise of the oscilloscope and probe with the noise and ripple of the DC supply being measured.

Find out more about TPR1000 Active Power Rail Probe
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Keysight 1147B AC/DC Current Probe, 50 MHz, 15A

Wide bandwidth: DC to 50MHz, High S/N Ratio

The 1147B is ideal for capturing transient current signals such as those found in motor controllers, switching power supplies, inverters and current amplifiers driving inductive loads.

Find out more about Keysight 1147B AC/DC Current Probe, 50 MHz, 15A
category.title

Keysight N7020A Power Rail Probe, 2 GHz

Power Rail Probe

The N7020A power rail probe is for users making power integrity measurements that need mV sensitivity when measuring noise, ripple and transients on their DC power rails. The probe is designed for measuring periodic and random disturbances (PARD), static and dynamic load response, programmable power rail response and similar power integrity measurements.

Find out more about Keysight N7020A Power Rail Probe, 2 GHz

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