ErinEast

What Current Probe Should I Choose?

Blog Post created by ErinEast Employee on Nov 15, 2017

To measure current with an oscilloscope, you have to use a current probe. The problem is, there are so many types of current probes available now, and it’s hard to know when to use what. When you think about current probing, there are three main categories that most applications fit into:

  1. High current

  2. General purpose

  3. Low current

Each of the current probes fits into a specific category. Breaking it up like this makes it a little bit easier to identify which probe will be the best for your test.

 

Table 1: Current probing categories

 

High Current

When I say high current, I mean HIGH current, as in hundreds or thousands of amperes. You will run into large currents like this when measuring inrush currents or switching transients of switching power supplies, motors, and more. How do you think it would go clamping on a standard current probe to something like this? For this special type of measurement, you need a special type of probe: a Rogowski coil. The technology used in a Rogowski coil allows for measuring extremely high currents, something that no other probing technology can do. This is due to the fact that it uses an air core, where traditionally a metalcore would be used. Normally, we would worry about the metal core getting saturated when measuring too high of a current, but with air we don’t have to worry about that. Make sure you check back in a couple weeks for more details on the N7040A/41A/42A Rogowski coil probes!

 

Image 2: N7040A/41A/42A Rogowski coil probes

 

There are a few other reasons why you would want to use the N704xA probes rather than the other one you see listed in this category in Table 1, besides the fact that the Rogowski coil can measure much higher current. One reason is the other probe listed is a clamp-on style. One of the big problems you can run into with this is that you aren’t able to actually fit the larger devices you want to measure in the clamp itself. You may have to add in some additional wires and do some extra work to get the measurement you need, often causing changes in the characteristics of the DUT. The N704xA has been getting a lot of attention from engineers because it’s extremely easy to use and connect directly to your device under test. The probe head is just a flexible loop that you can wrap around anything you want to test, as you can see in image 2.

 

General Purpose

Typically for more general current measurements, the most commonly used current probe is the clamp-on style, or the magnetic core current probe. This refers to measurements such as current consumption of high frequency digital circuit, ICs, or power supplies. You can also make measurements on motor drives, power inverters, to test lighting fixtures, and the list goes on.

 

Clamp-on style probes are convenient because they are really easy to use.

 

To use this style probe, all you have to do is clamp it around a wire and you’re ready to start measuring, there are no accessories or extra components to worry about. You’re able to quickly and easily make accurate measurements with low loading and a very flat response. Most Keysight clamp-on current probes employ a hybrid AC/DC measurement technology, integrating both the Hall effect sensory element for measuring DC/low-frequency contents and the current transformer for measuring AC into a single probe. With clamp-on probes, you can have the ability to measure AC and DC current, so you are able to account for the DC offset in your signal rather than blocking it out and not knowing what is going on there.

 

There are some advantages to the new N7026A over the other clamp-on current probes used for general purpose measurements. Keysight’s clamp-on current probe with 0.1V/A (or 10:1) conversion factor can only go down to 10 mA/div of vertical scale setting, which isn’t good enough for most lower level current measurements. The new N7026A now allows for more accurate low-level measurements, down to 1 mA/div sensitivity with much lower noise and higher accuracy. The higher sensitivity means you will find about 5x lower noise with this probe, which makes a huge difference when measuring in the mA range. This is evident in images 3 and 4 below. The N7026A also introduces a wider range, going from the mA’s up to 30 Arms or 40 Apk, along with the highest bandwidth of any clamp-on current probe available, at 150 MHz.

 

Image 3: 10mApp, 100kHz sine wave showing dramatically improved noise and sensitivity (Ch1 in yellow = N7026A 1V/A, Ch2 in green = N2893A 0.1V/A)

 

Image 4: CAN bus current shows improved noise means higher accuracy with low-level measurements (Ch1 in yellow = N7026A 1V/A, Ch2 in green = N2893A 0.1V/A)

 

Low Current

As power consumption becomes more important with battery powered devices, it becomes necessary to measure extremely low-level currents accurately.

 

To maximize battery life, engineers must minimize the power consumption.

 

Low current here refers to measuring down from the µA to the A range, on components like charging devices, memory chips, battery-powered devices, and more. When you need to measure at these low levels, your probe must be more sensitive than ever before with much lower noise and have high enough dynamic range to cover the input signal range.

 

To have sensitivity this high, Keysight offers breakthrough technology in the N2820A/21A high sensitivity current probe that allows you to connect a sense resistor directly to your current path and make high sensitivity measurements. This will allow you to measure the voltage drop across the resistor, which is proportional to the current flow through the resistor. The N2820A/21A current probes are optimized for measuring current flow within the DUT to characterize sub-circuits, allowing the user to see both large signals and details on fast and wide-dynamic current waveforms.

Many times, when engineers are working on these devices that require low-level measurements, they also must be able to analyze larger currents at the same time, such as analyzing a phone’s idle state. The solution to this is the 2-channel version of the N2820A. One channel will act as the low gain side that allows you to see the entire waveform, or the “zoomed out” view. The other channel is the high gain amplifier that provides the “zoomed in” view of the extremely low-level currents. Now, with the N2820A/21A current probes, engineers are able to see the details and the big picture on dynamic current waveforms like never before.

 

Image 5: 2-Channel N2820A shows “zoomed in” and “zoomed out” view

 

Conclusion

There are so many probes available! Now you are an expert and know exactly which current probes to use for your measurements. Using the probe that aligns with your corresponding category will enable you to make the most accurate measurements possible, therefore creating the best product possible.

 

To further help you select the correct probe for your applications, check out this probes and accessories selection guide

To learn more about the high current and general current measurements we discussed, download this application note

To learn more about the N2820A for extremely low-level current measurements, check out this application note 

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