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2017

If you are dealing with more than a couple tens of amperes of AC current and want to make flexible current measurements, consider a Rogowski current probe.

 

A Rogowski coil is an electrical transducer used for measuring AC currents such as high-speed transients, pulsed currents of a power device, or power line sinusoidal currents at 50 or 60 Hz. The Rogowski coil has a flexible clip-around sensor coil that can easily be wrapped around the current-carrying conductor for measurement and can measure up to a couple thousand amperes of very large currents without an increase in transducer size.

 

How does a Rogowski coil work?

The theory of operation behind the Rogowski coil is based on Faraday’s Law which states that the total electromotive force induced in a closed circuit is proportional to the time rate of change of the total magnetic flux linking the circuit.

Download the "6 Essentials for Getting the Most Out of Your Oscilloscope" eBook.

 

The Rogowski coil is similar to an AC current transformer in that a voltage is induced into a secondary coil that is proportional to the current flow through an isolated conductor. The key difference is that the Rogowski coil has an air core as opposed to the current transformer, which relies on a high-permeability steel core to magnetically couple with a secondary winding. The air core design has a lower insertion impedance, which enables a faster signal response and a very linear signal voltage.

 

An air-cored coil is placed around the current-carrying conductor in a toroidal fashion and the magnetic field produced by the AC current induces a voltage in the coil. The Rogowski coil produces a voltage that is proportional to the rate of change (derivative) of the current enclosed by the coil-loop. The coil voltage is then integrated in order for the probe to provide an output voltage that is proportional to the input current signal.

 

 

Advantages

Rogowski coil current probes offer many advantages over different types of current transducers or sensing techniques.

  • Large current measurement without core saturation -- Rogowski coils have the capability to measure large currents (a very wide range from a few mA to more than a few hundred kA) without saturating the core because the probe employs non-magnetic “air” core. The upper range of the measurable current is limited by either the maximum input voltage of a measuring instrument or by the voltage breakdown limits of the coil or the integrator circuit elements. Unlike other current transducers, which get bulkier and heavier as the measurable current range grows, the Rogowski coil remains the same small size coil independent of the amplitude of current being measured. This makes the Rogowski coil the most effective measurement tool for making several hundreds or even thousands of amperes of large AC current measurements.

 

 

  • Very flexible to use -- The lightweight clip-around sensor coil is flexible and easy to wrap around a current-carrying conductor. It can easily be inserted into hard-to-reach components in the circuit. Most Rogowski coils are thin enough to fit between the legs of a T0-220 or TO-247 power semiconductor package without needing an additional loop of wire to connect the current probe. This also gives an advantage in achieving high signal integrity measurement.
  • Wide bandwidth up to >30 MHz -- This enables the Rogowski coil to measure the very rapidly changing current signal – e.g., several thousand A/usec. High bandwidth characteristic allows for analyzing high-order harmonics in systems operating at high switching frequencies, or accurately monitoring switching waveforms with rapid rise- or fall-times.
  • Non-intrusive or lossless measurement -- The Rogowski coil draws extremely little current from the DUT because of low insertion impedance. The impedance injected into the DUT due to the probe is only a few pico-Henries, which enables a faster signal response and very linear signal voltage.
  • Low cost Compared to a hall effect sensor/transformer current probe, the Rogowski coil typically comes in at lower price point.

 

Limitations

  • AC only -- Rogowski cannot handle DC current. It is AC only.
  • Sensitivity - Rogowski coil has a lower sensitivity compared to a current transformer due to the absence of a high permeability magnetic core.

 

Applications

Rogowski coil current probes have a large number of applications in broad power industries and power measurement applications. The following are some examples of Rogowski coil applications:

  • Flexible current measurement of power devices such as MOSFET or IGBT device as small as TO-220 or TO-247 package or around the terminals of large power modules
  • To measure power losses in power semiconductors
  • To monitor currents in small inductors, capacitors, snubber circuits, etc.
  • To measure small AC current on a conductor with high DC current or in the presence of a high DC magnetic field.
  • To measure high frequency sinusoidal, pulsed, or transient currents from power line frequency to RF applications
  • To measure current in motor drives and, in particular, power quality measurements in VSD, UPS or SMPS circuits
  • To evaluate switching performance of power semiconductor switches (double pulse tester).
  • Power distribution line monitoring or utilities pole probe monitoring
  • Smart grid applications
  • Plasma current measurement

 

Conclusion

There are a number of different ways of measuring electric current where each method has advantages and limitations.

 

The Rogowski coil is similar to an AC current transformer in that a voltage is induced into a secondary coil that is proportional to the current flow through an isolated conductor. However, Rogowski coils have the capability to measure large currents (very wide range from a few mA to more than a few kA) without saturation because of its non-magnetic “air” core. The air core design also has a lower insertion impedance to enable a faster signal response and a very linear signal voltage and is very cost effective compared to its hall effect sensor/current transformer counterpart. This makes the Rogowski coil the most effective measurement tool to make several hundreds or thousands of amperes of large AC current measurement.

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 

Comparing different manufacturers’ oscilloscopes and their various specifications and features can be time-consuming and confusing. Streamline this process and select the oscilloscope that best fits your application with these top 10 considerations.

 

Streamline this process and select the oscilloscope that best fits your application with these top 10 considerations.

 

Bandwidth

The primary oscilloscope specification is bandwidth, and it is critical that you select an oscilloscope that has sufficient bandwidth to accurately capture the highest frequency content of your signals. Most oscilloscopes with bandwidth below 8 GHz have a Gaussian, or single-pole low-pass filter frequency response. The oscilloscope’s bandwidth is the frequency at which the input signal is attenuated by 3 dB. Because of this, you cannot expect to make accurate measurements near your oscilloscope’s specified bandwidth frequency.

 

Rule of Thumb:

  • Analog applications: Choose a bandwidth at least 3x higher than the highest sine wave frequencies you will measure
  • Digital applications: Choose a bandwidth at least 5x the highest clock rate in your system.

 

This will allow you to capture the fifth harmonic with minimum signal attenuation. This fifth harmonic of the signal is critical in determining the overall shape of your digital signals. This 5-to-1 rule-of-thumb does not take into account lower clock-rate signals that may have relatively fast edge speeds. These clock signals may contain significant frequency components beyond the fifth harmonic and require even higher bandwidths.

 

Keysight’s S-Series oscilloscope has bandwidth options of 1, 2, 2.5, 4, 6, and 8 GHz  

 

 

Figure 1 – Keysight’s S-Series Oscilloscope from 500 MHz to 8 GHz

 

Sample Rate

A digital oscilloscope can spend a lot of time calculating between the trigger event, the signal displayed, and the next trigger. This can result in only a few captures of your signal each second. Your oscilloscope will fail to capture intermittent errors or faults in your signal without a high enough update date.  This happens because the oscilloscope is busy calculating the last acquisition captured instead of acquiring. The higher the update rate, the higher your chances are of capturing that rare event.

 

Memory Depth

Closely related to an oscilloscope’s maximum sample rate is its maximum available acquisition memory depth. You should select an oscilloscope that has sufficient acquisition memory to capture your most complex signals with high resolution. Even though an oscilloscope’s banner specifications may list a high maximum sample rate, this does not mean that the oscilloscope always samples at this high rate. Oscilloscopes sample at their fastest rates when the time base is set on one of the faster time ranges. But when the time base is set to slower ranges to capture longer time spans across the oscilloscope’s display, the scope automatically reduces the sample rate based on the available acquisition memory. Maintaining the oscilloscope’s fastest sample rate at the slower time base ranges requires that the scope have additional acquisition memory. Determining the amount of acquisition memory you require is based on the time span of your signal and your desired maximum sample rate:

 

Acquisition Memory = Time Span x Required Sample Rate

 

Even though an oscilloscope’s banner specifications may list a high maximum sample rate, this does not mean that the oscilloscope always samples at this high rate. 

 

To better understand the relationship between bandwidth, sampling rate, and memory depth, let’s look at a real-world example. Consider trying to capture one frame data that lasts 1 ms and has serial data transmitted at 12 Mbps. So let’s assume that we have to capture a 12 MHz square wave for 1 ms.

  • Bandwidth — to measure the 12 MHz signal, we need an absolute minimum of 12 MHz, however, this will give a very distorted signal. So a scope with at least 50 MHz bandwidth should be selected.
  • Sampling rate — to reconstruct the 12 MHz signal, we need around 5 points per waveform, so a minimum sampling rate of 60 MS/s is required.
  • Memory depth — to capture data at 60 MS/s for 1 ms requires a minimum memory depth of 60,000 samples.

 

Triggering

Triggering allows you to synchronize the oscilloscope’s acquisition and display particular parts of your signal under test. Most digital oscilloscopes trigger on simple edge crossings, but you should select your oscilloscope based on the types of advanced triggering needed to help you isolate your most complex signals. Some oscilloscopes have the ability to trigger on pulses that meet a particular timing qualification. For example, trigger only when a pulse is less than 20 ns wide. This type of triggering (qualified pulse-width) can be very useful for triggering on unsuspected glitches. Pattern triggering is also very common and allows you to set up the oscilloscope to trigger on a logical/Boolean combination of highs (or 1s) and lows (or 0s) across two or more input channels. More advanced oscilloscopes even provide triggering that can synchronize on signals that have parametric violations. In other words, trigger only if the input signal violates a particular parametric condition such as reduced pulse height (runt trigger), edge speed violation (rise/fall time), or perhaps a clock to data timing violation (setup and hold time trigger).

 

Most digital oscilloscopes trigger on simple edge crossings, but you should select your oscilloscope based on the types of advanced triggering needed to help you isolate your most complex signals.

 

Display Quality

The quality of your oscilloscope’s display can make a big difference in your ability to effectively troubleshoot your designs. You should select an oscilloscope that provides multiple levels of trace intensity gradation in order to display subtle waveform details like noise distribution, jitter, and other signal anomalies. For the highest oscilloscope display quality in the industry, go to Keysight.com. An example is shown in Figure 2 below.

 

 

Figure 2 – High levels of display quality are required when viewing complex modulated signals such as video

 

Serial Bus Applications

To help you debug your designs faster, select an oscilloscope that can trigger on and decode serial buses. Serial buses such as I2C, SPI, RS232/UART, CAN, USB, etc., are pervasive in many of today’s digital and mixed-signal designs. Verifying proper bus communication along with analog signal quality measurements requires an oscilloscope. Many of today’s oscilloscopes have optional built-in serial bus protocol decode and triggering capabilities. If your designs include serial bus technology, then selecting an oscilloscope that can decode and trigger on these buses can be a significant time-saver to help you debug your systems faster.

 

If your designs include serial bus technology, then selecting an oscilloscope that can decode and trigger on these buses can be a significant time-saver to help you debug your systems faster. 

 

Connectivity and Documentation

Selecting an oscilloscope that meets your hardware connectivity, test automation, and electronic documentation requirements are key. Automated testing requires that the oscilloscope’s ports be programmable. So make sure any measurement that can be performed using the oscilloscope’s front panel and menu controls can also be programmed remotely via LAN or USB connectivity.

 

Keysight’s InfiniiVision X-Series and Infiniium Series oscilloscopes are all fully programmable via SCPI commands as well as National Instruments IVI drivers. Saved images (screen-shots) and data (waveforms) can also be easily imported into various word processors, spreadsheets, and applications such as MATLAB. Keysight’s N8900A InfiniiView offline analysis software lets you easily capture waveforms on your oscilloscope, save them to a file, and recall the waveforms into the application. With Keysight’s > 50 standard automated measurements with statistics and 16 independent math functions, you’ll be able to analyze a wide variety of tests.

 

Probing

Your oscilloscope measurements can only be as good as the data your probe delivers to the oscilloscope’s BNC inputs. Always keep in mind that when probing your circuit, your oscilloscope and probe become part of your device-under-test. This means it can change the behavior of your device-under-test signals due to capacitive or inductive loading. Select an appropriate probe for your measurement to minimize these loading effects. This will prevent disturbance of the input signal and deliver a true representation of your signal as it existed in your circuit before the probe was attached.

 

Select an appropriate probe for your measurement to minimize these loading effects.

 

Ease of Use

Your oscilloscope should be user-friendly and intuitive. This can be just as important as specified performance characteristics.  Oscilloscopes have evolved over the years with many additional features and capabilities but ease of use should not be compromised. Although most oscilloscope vendors will claim that their oscilloscopes are the easiest to use, usability is not a specified parameter that you can compare against in a product’s data sheet. Ease-of-use is subjective, and you must evaluate it for yourself. However, there are a few things you can look for when evaluating ease of use:

  • Built-in help menus, which reduce the need to reference manuals
  • Large color displays, which allow views of waveforms and measurement data at the same time
  • Voice control, which allows for hands-free control

Always request a demo when picking out your oscilloscope with a reputable company providing field engineers to help demo the scope or go to trade shows that provide hands-on demos.

 

Learn more about these and other oscilloscope selection topics:

Recently I was visiting the SIPI lab (Signal Integrity and Power Integrity) of a large corporation showing them how to make some solid power integrity measurements that they could trust (check out this application note if you’d like some tips). We had just finished making some ripple and noise measurements on one of the key supplies inside their system (using our power integrity analyzer) when the PI Lab manager shared a thought with us. He said “You know, I am a bit of a perfectionist. There is always more work we could do to clean up the supplies but I have limited resources, so I wish there was a button I could push that would tell me if it is worth it or not. What can I gain if I clean up the DC supplies.” I think many folks working on power integrity (PI) have this same thought. “Before I model, simulate, redesign, re-layout, fabricate, load and test a new revision, I’d like to know if there is a lot or little to be gained by cleaning up the DC supplies.

 

The issue that the above group was struggling with was power supply induced jitter (PSIJ). PSIJ is frequently the biggest source of clock/data jitter. The delay through a device varies as a function of the voltage applied to that device. Therefore, a system with very little DC supply noise will have very little PSIJ and conversely for a system with a lot of supply noise. The difficult part is getting an idea how much PSIJ your supplies are causing, because it varies from supply to supply, from device to device and target to target. To illustrate PSIJ, consider the example in Figure 1 where we are probing the 1.1V supply to an FPGA and one of the data lines from the FPGA. Initially, the supply has about ±5% Vpp ripple, noise and transients on the supply. We built an eye diagram of the transitions on the data line and could see that the eye width was around 70ps. Next, we did the heavy lifting and cleaned up the supply so that it is rock solid with <1% Vpp ripple, noise and transients. Again, we built up an eye diagram on the FPGA data line and found the eye width increase to about 115ps or about a 55% increase. The only thing that had changed was the amount of noise on the power rail.


 

Figure 1: The effects of power supply noise on the data lines of an FPGA

 

Don’t be fooled into thinking that this problem is reserved for those working on very high-speed designs. I have seen power supply induced data corruption on a little IoT device that is only clocked at a few MHz’s If you’d like to see this example, check out this video.

 

Understanding the impact of power supply noise on data lines, sensors, clocks, displays, cameras, et cetera can be difficult. Some users who have been doing PI for a long time may have a ‘gut feel’ that they trust but even these folks would find comfort in some ‘hard data’ to back up their intuition. Traditionally, the way to find answers to this question has come from doing extensive modeling and simulation—power-aware, signal integrity simulations. This approach is usually reserved for the few who work for institutions that can afford the simulation tools and the dedicated staffing to operate these tools.

 

Worry not, there is an answer for the rest of us. It is even as simple as pushing a button like our friend the SIPI lab manager wished for (okay, truth be told, I think it is about 3 or 4 mouse clicks not one but close enough).  The new Keysight N8846A Power Integrity Analysis application. To give you an idea of how the PI Analysis application can be helpful let’s return to our FPGA example. The setup is the same as our previous example except for this time we use the N8846A PI Analysis application to estimate what the eye width would be without the negative effects of power rail noise (we didn’t do anything to clean up the supply this time). Figure 2 below shows the results from the N8846A. You’ll notice the applications estimate for eye width with a clean supply matches what it really was when we cleaned up the supply.

 

Figure 2: The results from the N8846A Power Integrity Analysis Application. Note that the prediction from the application matches the results from Figure 1 where the supply was physically changed.

 

The N8846A PI analysis application lets users define a dc supply as either a victim of or an aggressor to, one other periodic transitioning signal and predicts the amount of adverse interaction involved. In this way, users can see what their dc supply and/or toggling signals would look like if they were immune to the negative effects of each other. With this insight, users can make informed decisions about what, if any, next steps they would take to clean up the dc supplies. This is the “button” that our friend at the SIPI lab was asking for.