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5 Posts authored by: KennyJ Employee

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.

KennyJ

"Hey, I Need a Current Probe"

Posted by KennyJ Employee Jan 18, 2017

When I hear someone say this and I ask “why,” the answer is nearly always “I need to measure how much power this thing is using”.  Based on this experience I have come to consider current probe use as synonymous with power measurements. Usually someone is trying to figure out how much power they are using or determine if the supply they have designed or are using is working properly and efficiently. It seems that sooner or later nearly everyone needs to make a current measurement so I thought I would share some of my experience with current probes and making current measurements (I’ve designed some current probes and have some current probe patents). My goal is to help you understand your options, what they are useful for, and what their limitations or drawbacks are.

 

Of course, one of the most common methods for measuring current is to use a digital multimeter. For this article we’re going to skip the DMM because you probably want to see a waveform of how the current changes over time since you are using an oscilloscope.

 

Let’s start with a review of the types of current probes. There are two classes of measurement capabilities—AC only and AC/DC—and primarily two different measurement methods—magnetic field sensing and Ohm’s law. The most general purpose measurement is the AC/DC measurement so we’ll focus on that. When it comes to magnetic field sensing there is a wide range of options—Hall Effect sensors, transformers, Rogowski coils, giant magnetoresistance (GMR), tunnel magnetoresistance (TMR), and many more cool and exotic-sounding approaches, but the most common is to use a Hall Effect sensor teamed with an AC transformer.  For the Ohm’s law approach you throw down a resistor and measure the voltage drop across it. Some people balk at the idea of inserting a resistor in series because of the voltage drop it induces. This is called the burden voltage. These same people that balk at using a current sense resistor tend to not think twice about using a DMM. What these individuals are overlooking is that a DMM inserts a sense resistor in series, usually about 1-5 Ω’s, and measures the voltage drop across the resistor. There is also the contact resistance of the connection points and the resistance of the test leads. In my experience, the DMM series resistance is about 100X greater than the resistance typically used by the Ohm’s law approach.

 

I like to think about the two types of probes this way. If you are going to measure large current (5—100’s A) or fast current (like a switch mode power supply) then you want to use a magnetic field probe. If you need to measure small current or current in sub circuits or current in a physically small product then you want to use the Ohm’s law approach.

 

The most ubiquitous current probe and the one you have probably used or seen is the split-core AC/DC current probe (one example is shown here). These probes use a split ferrite core—a core that has two separable pieces. Usually one part of the core slides back-and-forth or opens like scissors to allow you to clamp it around the wire in your circuit carrying the current you want to measure. The core has a series of windings around it that are connected to the internals of the current probe for measuring the current. Most probes have a negative feedback loop inside that is used to generate an opposing magnetic flux in the core to keep the core from saturating. Sandwiched inside the non-moving portion of the core is the Hall Effect sensor. The Hall Effect sensor is necessary to measure the DC portion of the signal since a transformer only works for AC. In its simplest form a Hall Effect sensor is a conducting (or semiconducting) plate with a bias current flowing along it with voltage measurement points placed perpendicular to the current path. As a magnetic field hits the plate, perpendicular to its center, the electrons flowing across the plate from the bias current tend to shift towards one side or the other, depending on the direction of the magnetic field, and create a voltage potential across the plate. The probe measures this voltage which is proportional to the strength of the magnetic field which of course is proportional to the current flowing throw the conductor being measured.

 

There are a couple of things to be aware of when using this type of probe. First, remember back to when you were studying magnetism and current flowing through a conductor and they would always say “assume an infinitely long, straight conductor”. Well, that’s important to keep in mind. It turns out that changing the shape and position of the wire passing through the probe can change the measurement a little and this creates a repeatability problem with this type of probe. It’s usually not a big deal if you are measuring amperes of current but if you are measuring milli-amperes then it can affect the measurement.  Another issue is the air gap between the two halves of the core. If you change that gap just the slightest amount, by adding side pressure or letting the probe hang on the wire you are measuring you will have repeatability issues when measuring smaller current. There is also the issue of mechanical stress on the Hall Effect element.  Thermally or physically induced stress on the Hall Effect element can change its resistance or induce a piezo electric effect which will cause measurement inaccuracies. It’s always best to let the current probe warm up for 20 minutes or so before use to minimize thermal stresses. Finally there is the issue of residual magnetism. Most probes have a zero/degauss function to address this. The proper procedure then for getting the best results from you clamp-on current probe is to let it warm up, clamp it onto the unenergized wire that you want to measure, zero and degauss the probe and then energize your circuit.

 

The Ohm’s law approach for measuring current is very popular for measuring current in sub-circuits or in targets that can’t tolerate the big long wires for the clamp-on probes (either due to size or because the long wire adds inductance and acts like an antenna picking up external noise). Traditionally folks use a pair of single-ended probes or a differential probe to measure the voltage across the current sense resistor. I am aware of one dedicated Ohm’s Law style current probe, the Keysight N2820A. This probe is a differential probe with two outputs. Each output has different gain for measuring different current ranges. The user can connect one output to the oscilloscope and choose normal or high sensitivity and the probe automatically switches which output goes to the scope. If you have a signal with a large dynamic range, like something that has a sleep state, then you hook both outputs to the oscilloscope and can view both the zoomed-in (high sensitivity) and zoomed-out (normal sensitivity) simultaneously. In this mode the probe has a dynamic range of 20,000:1. This allows simultaneous measurement of very small sleep currents and large current spikes associated with the active state. To use the probe you simply select the current sense resistor value appropriate for the current you want to measure.  The probe will work with anything from 001Ω--1MΩ. You tell the oscilloscope the value of the resistor being used and the scope automatically scales and displays the results in amperes. As an example, using a .050Ω resistor the probe can measure from 100uA to 24A—assuming the resistor is rated to handle the power of the large current.

Screen shot of the N2820A showing zoomed-in data (green) and zoomed-out data (yellow) simultaneously. This is from an IoT weather station.

 

The drawback of the Ohm’s Law approach is that you need to either design in sense resistors or cut the trace and solder them in place. You’ll also need access to some current sense resistors. I usually just get mine from one of the many online component distributors. They stock a wide range of values and power ratings for various current ranges. One great thing to note about using these application specific current sense resistors is that they have very small thermal parts per million resistance variation—the resistance changes very little as the resistor heats up.

 

If you want to dig a little deeper into what is out there just hit our website. You’ll see the full range of current probes we offer. They are of the two main types I described above, magnetic field sensing split-core AC/DC current probes and Ohms law current probes. The web pages show the important specs like maximum and minimum current, bandwidth, price and so on. You’ll also find some application notes, data sheets and videos. If you are making power measurement such as testing a power supply or measuring low power, like a battery powered device, then you should visit the Keysight oscilloscope power page.

I hope to impart on you a bit of wisdom I have learned from my years of travel and talking with well over 1,000 oscilloscope users. If you do yourself the favor of reading through this you will have gained enough insight to not shoot yourself in the foot like so many of the scope users I have visited. They are not to be judged, they were doing the best they knew how at the time, and we all make mistakes or could do better—I know this to be true for me. Once I pointed out the mistake and the solution to these users they usually all had the same reaction—“oh, that makes sense”, followed by the classic palm slap to the forehead.

These users I speak of all made the same mistake. They spent valuable time and money selecting the best oscilloscope to buy or use for their measurement task. Then they connected a high quality probe to their scope. In some cases the probe was the nice passive probe that came with their scope, other times they had sprung for a snazzy active probe (smart move going for the active probe upgrade, more on this in another blog post). Then, and this is the crux of the matter, they put a bunch of long, dangly connection accessories onto the end of the probe. Maybe it was something innocent looking like a nice convenient long ground lead or one of those super helpful looking long red input wires that make it easy to connect the probe to a grabber that they could clip onto a part on their board.  In the end, the result was the same, the signal on screen “looked bad” or the device they were testing started to misbehave. This is usually when they grabbed me and said “Hey, you designed this probe. It’s not working right”.

The Weakest Link

What these users were experiencing was what I like to call the “weakest link” phenomenon. There are three links in the typical oscilloscope measurement chain—the scope, the probe and the physical connection to the target. You can have the best scope and probe that money can buy but if you put some crazy long wires on the end of the probe to make the connection to the target easier you have limited the performance of the measurement system to be equal to the performance of those crazy long wires. The connection accessories are the weakest link. They will limit the measurement bandwidth and they can excessively load your target.

 

Think of those long connection accessories as inductors that are being placed in series with the probe. If they are connected to the signal pin of the probe they are going to limit the bandwidth of the signal that can pass through to the probe because an inductor’s impedance increases proportional to frequency. Additionally, since there is an impedance mismatch between the long inductive connection accessory and the probe input, the signal traveling up the wire will create a reflection that will show up on the scope.  If that nice long ground wire is connected to the probe similar results will follow. The long inductive ground creates a higher impedance path for the ground return currents flowing on the shield of the cable. This will also limit the bandwidth of the probe.  Additionally, the impedance resulting from the inductance of the long ground wire can create a voltage potential between the ground on the target and the ground point at the tip of the probe resulting in measurement error and poor common mode rejection. If all that isn’t bad enough, those nice long connection accessories act as an antenna and can pick up noise from your surroundings and couple that noise into your measurement. Finally, there is loading. These long wires that are touching your circuit are now part of your circuit and their inductance and capacitance can change the way your circuit behaves. We call this probe loading.

Shorter is Better

At this point I can almost hear you saying “if those connection accessories are ‘bad’ why do you include them with your probes?” We include those accessories for convenience. The idea is that you use those accessories for qualitative measurements, things like “is the clock toggling”, is there “data on the bus”, “is the 5V up”. They make it easy to poke around your circuit quickly to check for functionality. If you want to make quantitative measurements like rise-time, over-shoot, noise levels, et cetera, then we intend for you to remove the convenience accessories and use the shortest connection possible. That’s it, that’s the punch-line, shorter is better.

Consider this example. I take my fancy 2 GHz active probe and I configure it three different ways, long wires connected to grabbers, long wires only and short input pin and ground contact. Notice how the bandwidth increases as the length of accessory in front of the probe gets shorter. By the way, we try to make it easy for you and we publish these bandwidth limitations in the product manuals.

 

Notice too how the probe loading (how the physical presence of the probe changes the way your circuit functions) decreases as the length of the connection accessory decreases. In this example the original circuit is producing a rising edge with a rise-time of 1.1 ns (the green trace). Connecting the probe to the circuit using the long wires and grabbers loads the circuit and the rise-time changes to 1.7 ns. When I remove the grabbers and just use the long wires the rise-time gets better, 1.5 ns, though you can see the connection accessories are still affecting the circuit. Finally, I remove all the wires and go with the shortest connections for this probe and the circuit rise-time is back to its original 1.1 ns.

 

I Hope This Was Helpful

Don’t feel bad if you’ve been making the mistake of using long connection accessories when making important measurements. You’re in good company, a lot of oscilloscope users have made this mistake and to be honest, I have too. Just remember, it’s ok to use those long, convenient connection accessories for a quick peek but if the signal looks strange or you are not getting the answer you expect, you’d do best take them off and go with the shortest connection possible. Shorter is better.

See all of the Keysight Oscilloscope probes.

Being an oscilloscope probe design engineer I get the chance to get out of the cave several times a year and talk with our users so that I can better understand the measurements they want to make and what they need from us to make their lives easier.  In a typical conversation we would be discussing the next type of DDR memory or CPU (or whatever) that the users need to probe/measure and inevitably the question would come up—you got anything to measure ripple and noise on my power supplies? Initially the answer was no. These users wanted to measure mV ripple and noise riding on top of their 1.8 V, 3.3 V…24 V supplies. This is kind of a specialized measurement. To turn that answer into a yes I would need to design a specialized probe. But before I could do that I had to understand the application and measurement need better. Here is what I learned and what we came up with.

Thanks to Moore’s law, the doubling of gates on an IC every 18-24 months, the electronics that we encounter everyday are packed with more functionality in ever smaller, denser packaging. Consider your mobile phone. It wasn’t that long ago they were a brick that performed a single function—they made a phone call, and now they are elegantly thin machines that can give turn-by-turn directions, shoot high-definition video, monitor some of my biological functions and respond to my voice control. With this increased functional density comes some power related problems—power density and power supply induced jitter (PSIJ—power supply induced jitter can be the single biggest cause of clock and data jitter in a digital system). Folks learned that if they put tighter tolerances on the ripple and noise on their supplies and reduced the supply voltages where possible, they could reduce their power and jitter issues. It is not uncommon today to see supplies with tolerances of 1-3%. I saw an LPDDR with a 0.6 V supply with a 1% tolerance—this means measuring 6 mVpp ripple and noise.

Based on what our users shared with me I distilled the measurement challenges down to this: the need to measure a small AC signal riding on top of a large DC signal. If the AC signal exceeds the tolerance, the design failed to meet its requirements. This illuminated the challenges we needed to overcome. Users needed a very low noise probe & oscilloscope combo so that their ripple and noise were not overshadowed by the noise of the measurement system. They also needed a way to remove the large DC offset so they could put the signal in the center of the screen and zoom-in on it (get down to 1 mv/div if needed). The measurement tools also had to have enough bandwidth to capture the high frequency noise caused by the switching of the digital circuits. Since this is a function of clock speeds many users needed up to 2 GHz of bandwidth (frequencies above this are attenuated quickly by the circuit board fairly close to the noise source). And in addition, they had to have a probe that would not load the supply. For example, some users would connect a 50 Ω cable to the 50 Ω input of the scope to measure ripple and noise—for a 3.3 V supply the scope would sink 66 mA which can change the behavior of the supply.

Here is what we came up with, the Keysight N7020A Power Rail probe. The first and only probe designed specifically for making ripple and noise measurements on supplies. The probe has 1:1 attenuation ratio so that full size signals make it to the oscilloscope. This creates a very favorable signal:noise ratio. There is ±24 V of probe offset. This means the probe can remove up to 24 V of DC content from the signal so that signal can be placed in the center of the screen and be scrutinized at high sensitivity settings. 

Desperate users had been making use of DC blocks/AC coupling/DC reject to remove the DC content. They told us they disliked this because if filters the signal. A DC block is a ‘big’ capacitor so it also blocks low frequency supply drift and supply compression from being seen on the scope. Since the probe is active it has a DC input impedance of 50 kΩ which means it won’t change the behavior of the supply when it is attached. Finally the probe has 2 GHz bandwidth so that users can capture high frequency digitally induced noise on their supplies. Not everyone needs this much bandwidth so for those that don’t we recommend using the oscilloscope’s built-in bandwidth limiting capabilities so as to minimize oscilloscope/probe noise. And, I almost forgot, the probe comes with a lot of cool connection accessories so that you can easily probe a variety of locations on your target.

If you are curious to learn more, there is a great teardown video of the probe at The Signal Path: https://www.youtube.com/watch?v=d_Ybe6xnMIg.

Why, you may ask, would I want to spend money on an active probe for my oscilloscope when it came with free passive probes? As an oscilloscope probe designer I’m going to share with you a reason why you should consider upgrading to an active probe—and it’s not about bandwidth. I think a lot of folks who upgrade to an active probe do it because they need more bandwidth. Most passive probes top out at about 500 MHz so if you need more bandwidth than that you’ll need to buy an active probe. An active probe offers other benefits that should be considered even if you only feel you need a 100 MHz of so of bandwidth. I’m going to point out one that I think is most often overlooked.

Consider this, an active probe will provide significantly less probe loading than a passive probe. Probe loading is the effect that the probe has on your circuit when it comes in physical contact with it. Excessive probe loading will change the behavior of the signal being probed. With excessive probe loading, the signal that you see on the scope will be an accurate image of the signal being probed but it won’t be an accurate image of the real signal—the signal when the probe is removed. Here is how it works. When you look at the probe you are using, the label will say something like 10 MΩ:10 pf for a passive probe and 1 MΩ:1 pf for a general purpose active probe. What this is describing is a simplified circuit model for what the probe will look like to your circuit when the probe is connected to it. When in contact with your circuit the probe will appear as a resistor and capacitor, in parallel, connected to ground. It is easy to focus on the resistor value and overlook the contribution of the capacitance of the probe to probe loading. Considering only the resistor would lead one to conclude that a passive probe, with its 10 MΩ impedance will have much less loading than the active probe with its 1 MΩ impedance. Remember though that the impedance of a capacitor is inversely proportional to frequency. This means that the liability of the passive probe lies in its large input capacitance. Comparing the two probes, their input impedance (the impedance to ground when connected to your circuit) will be equal at 10 kHz. Therefore the active probe will produce less loading to any signal you are probing that has frequency content above 10 kHz. This is shown in figure above.

Now we will put this to the test. I’ve got a circuit that contains a 1.1 ns edge. Traditional guidance would suggest that I need about 300 MHz of bandwidth in my measurement system (scope and probe) to measure this signal. This is well within the capabilities of our free passive probe. I first probe the signal with my active probe and I measure the rise time. Just like I expected, 1.1ns. Now I remove my active probe and probe the signal with my passive probe. Oops, I’m measuring 1.5 ns. Is my measurement wrong? No, my measurement is correct. That is what the edge speed of my target signal has become due to the loading of my passive probe. The large capacitance to ground of the passive probe is creating a low impedance path to ground for the higher frequency content of my signal and my target cannot drive this load so the actual signal on my target is distorted.

What you can conclude is that a passive probe is good for making qualitative measurements and an active probe is good for making quantitative measurements. Qualitative measurements are things like: is the patient’s heart beating, is the 5V up, is the clock toggling..? Quantitative measurements are things like: what is the patient’s heart rate, how much ripple/noise is on the 5V supply, what is the rise time..? Do yourself a favor, next time you get a chance, splurge and buy an active probe.