Lee Ritchey FAQ Why Do Ferrite Beads Show Up in Applications Notes?
This is a good question to ask, since these ferrite beads often make PCB layout difficult around very high pin count BGAs, such as FPGAs.
It is worth exploring what ferrite beads are as a first step. In the context of EMI control and power supply engineering these components are not actually beads. They are, instead, surface mount components much like other chip components such as capacitors and resistors (See Figure 1). They are available in the same sizes as these other components.
Figure 1. A Typical Ferrite Bead Package Ferrite beads are made from a ferromagnetic material commonly referred to as a ferrite. This material behaves like an inductor made from a coil of wire. The feature that is attractive about such a component is that a relatively high inductance can be had in a small package. Typically, these components are not specified by the amount of inductance they have. Rather, their impedance at a particular frequency is listed. As can be seen in Figure 2, the impedance of a ferrite bead is a function of frequency, much like an inductor with the impedance being quite low at low frequencies, rising to a high point and then dropping off.
Why, then, do engineers put ferrite beads in series with the power leads of ICs? This practice dates back to the earliest days of EMI control, when the containment of EMI was done on a trial and error basis. A product failed EMI tests, so an EMI technician would explore where the EMI came from by using a near field RF probe.
In logic designs, the EMI nearly always came from a large component such as a custom part in a pin grid array (PGA) package. When a ferrite bead was inserted in the power lead of such a part, the EMI would go away. The reason given was that this "fast" part was making noise that was getting into the rest of the PCB. What was actually happening is that the body of the ASIC or IC was radiating the energy because it is a good antenna and the noise voltages related to switching were exciting the case itself.
GHz for most products. When the IC attempted to draw power at high frequencies from the power supply it was prevented from doing so by the impedance of the ferrite bead. As a result, there were no high frequencies on the IC package to cause an EMI problem. This is one of the two ways to control EMI; eliminate the source or eliminate the antenna. This technique works well as long as the IC or ASIC is not expected to operate with fast edges or fast clocks.
The effect of placing the ferrite bead in series with the power lead of the device is to degrade the performance of the power delivery system by increasing its output impedance. Remember, a power supply is expected to be a voltage source, meaning that no matter how much current is drawn from it, the output voltage remains the same and no matter at what frequency which that current is drawn, the output voltage remains the same. Said another way, power sources are expected to have zero or very low output impedance at all frequencies in order to do their job properly.
Sure enough, the speed of ICs increased to the point that this ferrite bead prevented them from operating properly. Again, the reason is that the power delivery system output impedance was too high. The proposed solution was to add a capacitor after the inductor as shown in Figure 4. This solved the operating problem, but brought back the EMI problem. More about the EMI problem later. Now, the method recommended for implementing this circuit was to cut an island in the Vdd plane as shown in Figure 5.
Notice that in Figure 4 the capacitor is called a "bypass capacitor" with quotes around it. The reason for the quotes is to call attention to the fact that this capacitor is not bypassing noise, it is serving a source of high frequency charge, so that the ASIC can again switch rapidly. A much better name for these capacitors is "coulomb buckets" as they are functioning as local charge storage devices. Also note that the inductor and the capacitor form a low pass filter, preventing high frequency noise from getting to the ASIC from the power subsystem side of the system. This is the reason given in most current application notes for placing ferrite beads in series with the power leads of phase locked loops and other "analog" type circuits, including high speed serdes such as Rocket I/O.
The technique shown in Figure 5 worked as a way to keep noise from the power system out of the power leads of the ASIC circuit until two things happened. The first is this noise became high enough in frequency that the ferrite no longer represented a high impedance (see Figure 2) and the capacitor no longer functioned well as a coulomb bucket (See the chapters in "Right The First Time, A Practical Handbook on High Speed PCB and System Design" on capacitor limitations for this.) When this occurred, the noise was no longer blocked and the function of the circuit in the ASIC was degraded due to a lack of a low impedance source of switching current.
That is the reason for the large red X through the drawing in Figure 5. With the speeds of modern logic this is no longer a safe solution to either noise or EMI.
Back to the original question, why do ferrites beads appear so often in applications notes? It is my experience that there are two reasons for this. The first reason is the least defendable. The answer given when the author of the applications note is asked, the reason for the ferrite bead is more often than not, "we've always done it this way and if you don't follow our application note, we won't guarantee that the circuit will work correctly." When the question is turned around by asking if the vendor will guarantee that the circuit will work if the applications note is followed exactly, the answer is still no! What kind of technical advice is that? The second reason given is that the ferrite bead is there to block noise in the power subsystem from getting into the sensitive circuit. I have seen examples of this in actual test circuits. The noise is blocked, but the circuit performance is likely to be degraded due to poor power delivery to the circuit being "protected." Figure 6 shows the output waveform of a 3.125 GB/S serial link with a ferrite bead in the power lead of the output stage. Figure 7 is the same output with the ferrite bead removed and the power lead connected directly to Vdd. Inserting the ferrite bead actually made the circuit perform worse than with no ferrite bead. The circuit for Figure 6 was recommended by the supplier of the part without first checking to see if the advice was sound. These waveforms were actually taken from an evaluation board supplied by the vendor. How could such incomplete engineering be done on such important matters? Good question.
When the second reason is given, namely to block noise from the power subsystem, this is treating a symptom, not dealing with the problem. The problem is that there is noise in the power subsystem because it was not designed correctly. Chapters 32 to 37 in the above mentioned book cover how to do a good power delivery system design. Several of the references listed below also lend insight into how to do this.
My experience has been that the use of ferrite beads is either a knee jerk reaction or a band aid. In 30+ years of designing high speed computing systems and networking products, I have never used a ferrite bead in the power lead of a device, whether it is a phase locked loop or an "analog" circuit. Instead, I have determined what the "ripple" requirements of a circuit are and designed the power delivery system to meet this requirement.
What should the vendor have done to insure that its application note correctly advised its customers?
The first thing an IC vendor needs to do is understand the power delivery needs of each IC. This includes maximum delta I that the circuit may demand of the power delivery system as well as at what frequencies and the maximum allowable delta V (ripple). Without this, it is not possible to design a power delivery system.
There is a problem that can be solved by the use of a ferrite bead.
The ferrite bead does not cause a new problem such as that illustrated in Figure 6.
My experience is that, after step 1 & 2, ferrite beads are eliminated from the design.
What should an engineer do when he or she encounters an application note recommending ferrite beads?
Whenever I encounter an applications note that recommends the use of ferrite beads I call the author and ask that the four steps above be demonstrated. In no case have I found one where going through these steps results in agreement that the ferrite bead was a good choice.
If the vendor still insists that the ferrite beads are required, insist on seeing a test circuit in which the component is used exactly the way it is intended to be used in the new design. If no test circuit is available it is good to be suspicious. In one case when I was having trouble getting a microprocessor to work properly I asked to see the test circuit used to arrive at the application notes and specifications for the part. I was told there was none and never had been one. To this I responded, how do find out if the part works correctly? The response was, "we give them to our customers and they tell us if they work"!
If there is no test circuit, be suspicious.
Why are there so many applications notes that contain ferrite beads if they are not a good solution?
As mentioned earlier, many times ferrite beads are included in applications notes "because we have always done it that way". Why, then, haven't they caused problems? The answer is that prior to 130 nanometer ICs coming onto the market, most circuits ran slow enough that there wasn't the need for a very low impedance power source at high frequencies. This is a case of succeeding in spite of bad habits rather than because of good engineering practices. The same thing is true of the use of bypass capacitors.
Because users have succeeded in spite of poorly engineered applications notes there had been little incentive to put any resources into making sure they are technically correct. As with FPGA packages, it is only after a number of customers fail and start to complain, or worse, switch to the competition that resources are applied to creating applications notes that correctly characterize parts and how to use them.
130 nanometer IC processes have resulted in ICs that are as fast as or faster than the high speed logic know as ECL which all engineers seem to know requires good high speed design practices. Fortunately, there is an abundance of good design information available to make this transition.
EMI containment is a major reason for ferrite beads in power leads of ICs. As mentioned earlier, this solves the EMI problem by preventing the part from switching fast. Again, this is fine as long as the part does not need to switch fast. Virtually all modern ICs need to switch fast to do their job. Placing ferrite beads in their power leads stops the fast switching and the EMI. The same result can be achieved by turning off the power. A solution is required that does not preventing proper function. Again, there are two ways to contain EMI, turn off the source or eliminate the radiating surface or antenna. For most products turning off the source is not a choice. All that remains is to avoid making unwanted antennas.
What things function as antennas?
To deal with these antennas, see the EMI article in Volume 3 Page 2 published by Speeding Edge.
Ritchey & Zasio, "Right the First Time, A Practical Handbook on High Speed PCB and System Design", Speeding Edge, 2003.
Hubing, Todd H. etal, "Power Bus Decoupling on Multilayer Printed Circuit Boards" IEEE Transactions on Electromagnetic Compatibility, Vol. 37, NO 2, May 1995.
Greim, Michael C. High-end Digital Systems Give a Thumbs Down to Rules of Thumb" EDN, June 5, 2000.
Smith, etal, "Power Distribution System Design Methodology and Capacitor Selection for Modern CMOS Technology" Published by Sun Microsystems, 1999.
Ritchey, Lee W. "What is EMI, Where does it come from and How Can It be Controlled", Current Source, Volume1, Issue No 3, Summer 2005, available from Speeding Edge, www.speedingedge.com.


The Hook: NEW RELEASE - "CLEOPATRA'S NEEDLE" is here!
I'm super-honored to be bringing you this awesome novella by the bestselling author of THE READING LESSONS - CAROLE LANHAM.
CLEOPATRA'S NEEDLE A story of Welsh witchcraft, love, and murder.
Wishing Carole all the best of success!
Thanks so much for this lovely blog post, Louise! It's very much appreciated.


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Freedom and Flourishing: Difficult questions Part IV: Do people suffering from DIP have identity issues?
Difficult questions Part IV: Do people suffering from DIP have identity issues?
In the preceding post I suggested that identity economics may help us to understand teenage drug use. Ruth, a nurse who has worked in psych wards has responded with some encouraging comments about the potential for identity economics to help in exploring the drug-using phenomenon.
In my experience there are different aspects involved.
Some use drugs to escape their thoughts. (I suspect this is the largest group.) Those thoughts invariably include memory (what went before) and fantasy (what may come). The nature of those thoughts are hugely varied and may or not be based on shared experiences. They are not the same as delusions. Instead they are the result of a person trying to explain where he fits in his world, within the (limited) knowledge he has accrued in his life thus far. And the results are a distorted view of what's so, of where they fit, of what they mean to others, of how the world around them works, of the possibilities still awaiting them in life. The younger kids are when they start taking drugs, the more limited this knowledge is likely to be. The thoughts these people experience are particularly painful and cannot be mitigated easily through the usual counselling techniques.
A second group identifies their personalities as predominantly risk taking and therefore actually experience the need to arouse angst in those closest to them. This provides the sense of being cared about by those people. The more they upset the people around them the more evidence they have that they are loved - which of course sets up the adrenaline response very frequently (with every associated thought). Adrenaline in itself is a highly addictive drug - one that many very healthy non drug users like me are quite unashamedly addicted to.
Another group simply start experimenting with 'soft drugs' and end up with physical addictions requiring servicing at every opportunity. These people are the easiest to help as they are generally most motivated at the emotional level.
I think it's easy to get mistaken between the view looking in and the view looking out. Those close to the problem emotionally don't see through the same lens as those with an objective (professional) filter. The greatest mistake I see day in and day out is people - sufferers, family, researchers, medics, friends, observers - categorising the problem and therefore the sufferer.
The real answer - in my experience - is to take one person at a time and simply listen to them for quite some time before even attempting to think or consider what to do to help. The person themself inevitably can reveal the true cause of the problem and only then can a useful - long term effective - solution be proposed.
and side effects of drugs must of course be dealt with. But it is in the listening that the true cause of the problems are found. And listening is such an underrated skill; it hardly features amongst the more 'sophisticated' skills.
True therapeutic listening puts the practitioner in a place of nothingness, conscious only what is occurring in the room in each moment as it transpires. As the person speaks, the truly listening 'other' feels the person's psyche and is able to communicate in such a way that the person actually experiences a healing feeling without any recommendations or solutions or questions being offered. This is the beginning point for the journey to wellbeing for everyone. It is especially important for kids using drugs.
Ruth obviously feels passionately about therapeutic listening. Her views on this seem to me to make a lot a sense (but I can't claim any expertise in that area). I would like to round off this discussion by pointing to possible implications of Ruth's observations for use of identity economics to understand teenage drug use. The important point is that the people who end up in hospital as a result of drug taking do seem to have some particular identity characteristics that may help to explain why they got involved with drug taking in the first place. Ruth sees people making mistakes when they look in from the outside and attempt to categorize individuals. This suggests to me that there may be a need for better research instruments that will enable researchers to get a better understanding of individual behaviour by learning how individuals categorize themselves. In other words, if we are to understand the choices that the person makes it might help to know why the person perceives himself or herself as the kind of person who would obtain satisfaction from that kind of behaviour.


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A friend told me that doing something like this might be a good idea since I sometimes just seemingly disappear for days on without blogging at all.
It's really ambitious to try to keep this particular feature running every week, I feel. But I'll try.
I also think it's best to keep things brief. Maybe around three hundred words.
This week I was thinking of expanding The Lily Garden. Maybe set up a Patreon. Set up a Buy Me a Coffee. Try making videos. We'll see. Currently feeling a bit weird as seen here. I'll spare you all the melodrama by not going into detail. It's definitely affecting me right now, though.
Maybe it's a bad idea to do these things when I'm lacking direction and drive. Why would people want to invest in me if I can't guarantee what they want? But I'm telling myself these feelings will fade and that I just need to keep myself busy. And if I continue to deliver content, then I might get somewhere eventually. That's the plan, anyways.
I'm having dinner with my family tonight. Considering how often I eat out to avoid them, this should be considered a unusual occurrence.
Some of my online friends have private Twitters and they all seem to find that having confidential accounts provides cathartic release. I've considered following suit, but I don't know. I already feel conflicted about offering exclusive content to Patreon patrons (wow, I'm talking as if I've already set up a Patreon). I also don't really want to be selective and potentially cause hurt feelings. Therefore I'll continue to just be an open book (to a reasonable extent), I think. Wearing my heart on my sleeve is how I've always rolled.
So please forgive me if I get too melodramatic on Twitter. I'll try to keep that to a minimum in 2018.
More visual novels to play/read. More anime to binge. Need to catch up on people's blog posts. Still unemployed. It's a never ending journey.
Oops, 340+ words. That's more than I thought I would write. Sorry.
Belated happy new year! I thought I hadn't seen you around much recently; sorry to hear you're having a bit of a turbulent period in the mental and emotional wellbeing department. I can definitely relate to that, particularly the bad places being unemployed can send your brain.
It may sound like trite, obvious advice, but… do what makes you happy, and don't worry about other people; if they're true friends and supporters (which most of the people in this wonderful little community are, I feel!) then they'll understand whatever you decide to do.
Personally speaking, I find that putting out regular content (with a certain degree of structure to my weekly activities) helps give me something to focus on each day as well as something to look forward to after the daily grind at work. Even keeping the fairly rigorous schedule I do on my site, writing is still a pleasure, which is probably a good sign!
This doesn't necessarily work for everyone, of course; sometimes everyone finds they need a bit of a break, and if you start feeling a greater sense of obligation than you do pleasure from your writing, it's probably a good sign to pause, take stock of the situation and get things in perspective. Your readers will still be waiting for you when you come back refreshed.
As dear old Michel has already noted above, you have a unique and distinctive style to your writing, and your passion and enthusiasm for what you write about always shines through, along with your sincere desire to be part of a community. I wish you all the best, and hope you feel a bit better about things soon.
Happy New Year to you, too, Pete! Ah, yeah, I disappeared for a bit. Thanks for being understanding and showing that you get it. I'll take your advice. The people in this community do genuinely seem kind and it's great!
Ahh, more words of wisdom. I'll keep them in mind, too. A schedule sounds like an excellent idea.
Thank you. That means a lot. I also wish you all the best, too, as you continue to write engaging and enlightening articles.
Whatever you decide to do, I'm sure things will work out. Your blog is fantastic and you produce great content.
Thanks, Karandi. I'll try to keep doing my best!
Because they have more confidence in you than you do in yourself?
You make a good point. Can't say it's wrong.
Thank you, raistlin. Well, what I want to please others. That's what it boils down to. Maybe I need to develop my own sense of self in 2018.
But thank you for the kind words. You and DerekL have been very nice in assuring me that I have a unique style and it means the world to me. Thanks for the encouragement once again.
But Remy: think of it this way. Maybe you are already pleasing others with everything that you are writing?
Follow your heart and take your brain with you!!
Thank you, haha. That really is a cute gif.
Well, you said it yourself on the last line nii-san!
I'll try to believe what I'm saying, then.
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Lauren and Matt were married at Holy Trinity Church in Georgetown. This is the church that President Kennedy attended while in office. It was a gorgeous Fall day and this made for some amazing backgrounds. After the ceremony we walked the streets of Georgetown and got some great shots. The reception was held at Washington Golf and Country Club in Arlington. This is one of the oldest clubs in the area and can count many past presidents as members. A wonderful day.
Beautiful work, love love love Washington, D.C. streets!
The photos turned out beautiful! Great job Rob!
Mary and John in Georgetown The Replogle Wedding, Washington, D.C.