EEVblog #859 - Bypass Capacitor Tutorial

hi welcome to fundamentals Friday today we're going to answer the question why do you use multiple bypass capacitors you've probably seen this in many circuits that you've got your chip here you've got your power power rail and you might have more than one bypass capacitor just on that one chip or even just that one power rail on a chip that might have multiple power rails you but for example it's not that uncommon to find like a 1 micro farad cap a hundred nano farad cap a 10 nano fairy cab or 1 nano fairy cap can have 2 3 or 4 caps in parallel why what's going on here hmm let's answer it now I actually covered this very briefly back in episode 33 back when I was in the old lab but it was only like a minute or two explanation so I thought we'd go more in depth here and I've actually done a not video not that long ago on why you would use multiple electrolytic capacitors in parallel and I came up with a huge list of nine different reasons why you would actually put more than one electrolytic capacitor in parallel so click here if you haven't seen that video it goes in-depth and it does some art thermal testing as well to actually prove it now we're not talking about electrolytic capacitors here this is a different scenario we're talking about different value capacitors for in particular chip bypassing now there are very good technical reasons why you would actually want to put multiple capacitors for bypassing applications in parallel in particular different values and different types of capacitors but before we answer that we have to actually look at what is bypassing now in an ideal world you wouldn't actually need bypassing to be completely pointless because let's take a look at a chip like this ok it does whatever this chip happens to do we've got a battery or a power supply here it doesn't really matter and we've got a load so it's a consuming power inside the chip to do various switching and things like that and that's what I'm showing here with these two our MOSFETs in there let's assume it's a CMOS chip doesn't matter and so it's doing internal switching it's doing all its business and we got a like a totem pole output it's driving loads it's driving lines it's doing whatever chips normally do now let's assume that we had a five volt supply here let's go old school now this 3.3 volt rubbish and this five volts in an ideal circuit you're going to get five volts directly on the pin of this chip in here because there's no internal resistance in the power supply there's no internal resistance in the battery whatever you happen to be using there's no internal there's no resistance on the PCB traces that you're using there's no inductance there's no nothing it's just an ideal world and our ideal chip everything's hunky-dory you don't need bypass capacitors and every other chip on your PCB as well it's also going to get exactly 5 volts on that pin it never moves it's rock solid so you don't need any bypassing in an ideal world unfortunately we don't live in an ideal world in the real world unfortunately everything has resistance and everything has inductance everything has capacitance all these parasitic elements and take your power supply for example it's you can't get a perfect power supply it's going to have some equivalent series resistance AC a resistor in series with it your PCB traces going from your power supply like your power supply input connector on your board for example to you chip or to multiple chips the PCB traces they're going to have resistance they're going to have inductance every piece of wire has inductance no matter how small it's going to have capacitance down to ground but we won't look at that in this case so what's going to happen if we have no bypass capacitor on our VCC power pen of our chip our 5 volts if the chip is doing nothing and it's just static ok yes we will get a straight we will get just our 5 volt line on there but the chip is switching it's doing stuff there's lots of capacitance inside the chip capacitance takes switching currents and things like that so you're getting all these pulses of current so our waveform is not going to be straight like this on our VCC pin it's gonna it might jump up and down like this depending on the switching inside that thing and then we've got our load as well our load is powered through the VCC pin through that top transistor up there to actually drive the load whether or not a sinking current or sourcing current so that's going to contribute as well and hey depending on the value of these traces here it can you can actually get significant dips and it can drop below the operating voltage of the chip and start causing strange weird things this is sort of like a gross generalization but this is what sort of thing can happen if you've got no local bypassing on your chip but one of the big problems is not so much the resistance of the traces it's more to do with the inductance of the traces especially the higher frequency your chips get even in that high frequency can be a megahertz or so look at an old you know computer board from the 1970s or 80s you know with hundreds of chips on them they've all got a bypass cap next to each particular chip because of the inductance of all the power traces going there and remember we won't go into details but remember an inductor actually resists change in current so if you're a chip or your load suddenly decides it needs to switch your inductor goes oh no I can't change that quickly I can't do it so you're going to get these huge dips and problems and all sorts of stuff so it all just becomes really nasty and your five volt supply for your chip your power supply is not the solid power supply you're expecting so that's why we add in a bypass capacitor in here like this right at as close as possible to the pin of the chip because why does it have to be as close as possible because you're trying to avoid the inductance in the line here and every trace has inductance so the further away you put your bypass capacitor from the chip the greater the inductance and then causes all sorts of problems when your chip starts to switch at high frequencies so the goal of when you're bypassing is to try and produce a low impedance low inductive supply element remember capacitors store charge so they charge up and then when your chip switches and it requires a gulp of current it comes from the capacitor instead of way way back on the other side of your PCB which has all these long inductances in series and all sorts of stuff it comes directly from the local capacitor so it minimizes the amount of duct inductance and resistance in series with it so that bypass capacitor can supply that little gulp of current that your chip suddenly needs without being affected by the rest of your PCB layout and all the other parasitics so let's take a quick look at what actually happens to an output pin here for example which is really important because it's driving other chips as well as part of your system so if you get issues on that output signal it can cause corruption the other chip may not read it properly all sorts of issues like that and you may have actually seen this let's take a look so what with drone is another waveform like this in of course the ideal world your output will switch from 0 volts up to 5 volts here it'll be absolutely perfect there'll be no ring in that we know overshoot now under shoot nothing but of course I use those key words their overshoot and undershoot and ringing what they're caused by is the inductance in the power supply here even if you've got local bypassing bypass capacitors there's going to be a little bit of inductance in the trace keys can't put it right on the pins there's going to be a little bit inside the chip with the bonding wires for example that actually you know because you died as like inside this they've got to have the little bonding wire which goes over inside the chip that's got a little bit of inductance and all that can actually lead to ring in on your signal like this and you've probably seen that before and then you can get some undershoot down here like this and causes issues like this it's all to do with bypassing and the higher frequency content you've got the more this becomes a problem and I'm not just talking about the signal frequency itself it could be you know 1 1 kilohertz or 1 kilohertz square wave for example not high frequency as you would measure it on a frequency counter but remember that a change in digital signal like this it's all it's not to do the fundamental frequency the time difference between here and here to do with how fast the rising and falling edges are the faster the edge you know if it's a really slow edge like that it's going to have low frequency content if it's a SuperDuper fast edge that switches in a nanosecond or something like that then it's going to have a really high frequency content that's your basic Fourier theory and all that sort of stuff so even a 1 kilohertz signal can actually have this real high frequency broadband content in there that causes all this ringing and when you've got a complex system with many chips and everything else well it can cause a major problem even within if you've only got a single chip solution like this if you don't bypass the caps and it's not going to clean power then internally - the chip is still going to get all this effective ringing and things like that due to the bonding wire inductance your PCB trace inductance and everything your ground inductance here it's not just your power line up here you're going to have some inductance in here you're going to have some inductance down here like this so that's why it's important to have your bypass cap directly on the pins of the chip as close as physically possible and of course if you actually probe your power supply you'll actually see this sort of stuff happening here okay you'll get your foot you might have your five volts but then you'll see that the ringing on the power slight yeah like that so you'll get all these little you'll see that if you actually probe correctly there's high frequency probing techniques you need to use and everything else but if your probe that you actually start seeing the switching on there and the if you have a no bypassing or not very effective bypassing your ringing can be very big and calls all sorts of problems so I know what you're saying Dave that's all great but why not just whack one big bypass capacitor on there that can handle the most amount of current that this thing is going to pulse current that this switching chip and the system is going to take why do you need to have multiple different values and different types of capacitors on there aha trap for young players this is where we have to get into what a capacitor actually does and it's impedance versus frequency or let's go so in a real capacitor which I've shown in the previous video on electrolytic capacitors if you're maybe want a bit more detail it's not just a capacitor inside a capacitor here it is a real capacitor has an equivalent series resistance which you might be familiar with the ESR which is a constant resistance value essentially in series with the actual capacitor itself but crucially also inside a capacitor is a little tiny bit of inductance as well lead inductance plus construction inductance and various things and that's called the ESL the equivalent series inductance so it's far from a just an ideal capacitor it's an RLC circuit what happens with RLC circuits well you can get resonances and you can get all sorts of funny things happening and as you should know from your basic component theory for capacitors and inductors they actually have an impedance or what's called a reactance or capacitive reactance and inductive reactance at a certain frequency they effectively have like an AC resistance so to speak and this is these are the standard formulas for your capacitive reactance and your inductive reactance and they change with frequency capacities is inverse with relation to frequency and the inductive reactance goes up with frequency and we're going to have a total impedance for the capacitor so a total a C resistance of the capacitor is actually going to be the ESR which is that constant fixed value in there plus the impedance of the capacitor at whatever frequency you're talking about plus the impedance of the inductor at whatever frequency you're talking about so if we go over here and have a look at this graph here we've got the impedance of the capacity bypass capacitor it's in ohms of course so the impedance in ohms versus the frequency here and you get this for a real bypass capacitor or a real capacitor we just happen to be using in a bypass situation real capacitor is going to have a response curve something like this and this is sort of like an industry standard way to show it it is not a straight line like that because of course a capacitor will actually have infinite impedance at down at DC here so it'll taper up like this now if we didn't if this capacitor didn't have any inductance in it at all of course this line would not be here and you just get a slope going down like that which changes with frequency and you can plot that yourself put the formula into Excel and you can do it yourself at standard basic component theory but as I said crucially that little inductor in there it's tiny it could be like Pico Henry's or something like that but at a particular frequency it's going to start to matter now the capacitive reactance operates like this but at some particular frequency here which is the resonant frequency of this RLC circuit using a standard resonance formula that's where the capacitive reactance and the inductive reactance are equal and that is going to be the resonance point at that point then the impedance of the reactance of the inductor starts to dominate instead of the impedance of the capacitor so hence white reverses and the resistance starts to go back up and that's a very undesirable thing to happen you don't want this thing to go back up at higher frequencies you want it to be down like this why because as we talked about before about the series effectively this series resistance the series impedance you want the energy to come directly from the capacitor with no effect whatsoever with no impedance in the paths no inductance in the paths when you start adding this real inductance either inside the capacitor itself or outside of the capacitor with your PCB traces you're in inside the chip with a little bonding wires everything else then this can be a real problem you impedance starts to rise and your bypass capacitor isn't acting like a good bypass capacitor anymore at these higher frequencies and of course it's a these higher frequencies in modern devices say for an FPGA for example which take here have huge densities in a huge amount of switching huge amount of logic and multiple rails and they take huge amounts of current and everything else and they operate in extremely high frequencies like you know hundreds of mega they can switch at hundreds of megahertz but the edges are even faster and you can get our frequency components into the gigahertz range fairly easily and if your reactance of your bypass the impedance in your bypass capacitor starts to rise at these really high frequencies up here at hundreds of megahertz or a gig or whatever and you're going to be seen serious trouble your bypass capacitor may as well not even be there at these higher frequencies because yet the capacitance is still there it's still got you know one microfarad or whatever it is lot of capacity you can have a lot of energy stored in that one micro farad capacitor but it's no good it can't get into the chip if there's this massive series impedance in series with the capacitor it just can't deliver the energy when that when your IC actually requires it give me a big pulse of energy not can't do it now I think I mentioned before that not only do you have different values here but you have different packages as well because the package actually makes a difference as a general rule of thumb the smaller your package gets the lower inductance it's going to have the lower your internal inductance here so it let's assume that this one is a oh six oh three for example you know a SMD package then if you've got an OE a five it's going to look something like that it's going to have a higher value so that could be a 805 and then you could have and you guess that Oh 402 package looking something like that they're actually going to have different values for the different packages so it's actually better for higher frequency stuff to use the smaller packages but of course the big question is why do they use different values well different values have remarkably different frequency characteristics as you'd expect the bigger value capacitors in this case say 1 micro farad for example is going to have a reson appoint at a much lower frequency so it's going to cover the lower frequency range is going to have a lower impedance at a lower frequency once again it's not this v-shape it's you know it's going to be something like this right so it's going to actually cover a much broader range at a lower frequency right down here but yeah work with me okay and then you're going to have different values for assuming like this all the same package for example hundred n is going to be high in frequency and then a ten in again and then a one nano farad then 100 puffs if you want to is going to be much lower so what you get and the answer to the question why do you use multiple bypass capacitors it's so you get the lowest impedance across the largest frequency range possible so if you've got all three of these values in here your final curve is going to look like this tada so you've got a much broader lower impedance so you've got a more effective bypass capacitance over a bigger frequency range and that's why you do it so there you have it that was a bit longer explanation than what I intended what was it 20 minutes or something to explain how bypassing works and why you use multiple bypass capacitors I could have just jumped straight to this and said this is why which is what I did back in episode 33 or whatever but there's good background information there to explain exactly what's happening here so I thought I hope you found that interesting but hey I think we might be able to reproduce this on the bench and actually show you could be a little bit tricky but now let's give it a go now ideally to measure this we would use a network analyzer big expensive bit of kit which I don't actually have here in the lab I need to get myself one but hey we can use our red Pattaya which you've got seen in a previous video and now even I'm now powering it from an external plug pack to amp plug pack by the way over via the USB which seems to have solved the rebooting issues I was getting before even though before in the previous video I was actually powering it from a USB 3.0 port which is supposed to be capable of supplying two amps but now I don't know anyway so it's working a bit more reliably now but I'm still having a few issues with the impedance analyzer app which we're going to use today so I'm going to use three channels here and here's a diagram of how it's actually hooked up we've basically got a 10 ohm shunt resistor in there and then the device under test now the reason I'm getting this convoluted arrangement here with the bit of Erebor and the wires and everything else is that you're probing in this sort of thing and your wiring test cabling is actually quite critical if I actually ran coax is off here and stuff we'd find that we'd be getting all sorts of issues in our impedance plot the higher up in frequency we go so yeah I'm often just dangling wires like that can be better so I'm converting my SMA to B and C then converting B and C to a banana and a binding post here so I can just hook that up and should be right it's a little bit you know it's a bit crude but hey we should be able to show the concept at least now the good thing about the very board here is that it has two convenient strips like this that allow us to put multiple capacitors in parallel so I've got a cap in here I've just been testing the thing to make sure it all works and we've got our 10 ohm shunt resistor there something just put as many caps in here as we want but with something like this we're dealing with you know high frequency we're going to go up to 60 megahertz today I sweep it all the way up to that frequency so what we want to do first is actually replace the capacitor with a shunt resistor in there because we've got a 10 ohm sorry I've got a 10 ohm shunt resistor replace the capacitor with a resistor so that we can actually check to see our frequency response responses flat and there we're not getting any weird effects caused by a cable in all the test setup so just like we discussed what we want to get is an impedance versus frequency graph so basically anything that goes up to and megahertz should we should be able to see something like this this thing's 125 makes samples per second you know analog bandwidth 50 60 megahertz something like that that'll be good enough to see various arc capacitors in parallel hopefully now you'll have to forgive me for not doing this live so to speak but not only does it save time but trust me I spent a lot of time around with this thing actually trying to get a result because the test setup is actually quite crude had a lot of issues with the red pitaya there's software and things like that and the test feature with even with you know the short wires that I'm using the BNC s make a difference the adapters all that sort of stuff all comes into play so you know I didn't really engineer a proper setup for this so I was actually lucky to actually get a usable result out of this but I should be able to show you something here so what we've got here is an impedance response graph just like we saw on the white board their impedance in ohms versus frequency there in this case we're sweeping from 100 kilohertz up to 60 megahertz on a logarithmic axes there I tried to set it to start at a higher frequency but I just wouldn't let me I'm not sure what's wrong with the app anyway um you can see that started off at 100 kilohertz there and right down a DC of course it started off as a nice perfect 10 ohms exactly what you'd expect so that just verifies that the system is working but of course now the parasitics of our test setup come into this you can start see it around about two megahertz there or so you know it starts to roll off and you know it's usable up to say you know 20 megahertz might be usable it's down to measuring you know 7 1/2 ohms or something like they're you know good enough a ballpark but at the higher frequencies of course then it becomes you know they're all the parasitics of the véra board and everything test fix should come into play you can see a bit of noise right at the high frequency that's because there's not much out signal-to-noise ratio there but in this case that in accuracy at the you know greater than 20 megahertz range isn't that bad because some of the impedances as you'll see actually go up to you know hundreds of ohms and things like that so you know it it's kind of usable so I'll sweep it to 60 megahertz but just keep that in mind that yeah it's a little bit off up there and I'll start out by showing you some large value capacitors this is a 10 micro farad 1206 packaged ceramic capacitor very typical large value bypass cap and as you can see it does have that characteristic v-shape response as I said before quite you know much broader than what we saw on the white board what's there you can see there's a resonant point about one megahertz there and then it tapers back up now here's a 10 micro farad are tantalum capacitor and you can see it's actually higher in value goes up to like 1.75 ohms at you know 60 megahertz or something like that because see it's got a similar shape similar sort of resonant frequency around 1.5 megahertz and now this is a just as a curveball 47 microfarad electrolytic capacitor you can see it resonates about you know 8 9 megahertz or something tapers back up and obviously that big tail down at the end is Judas and parasitics on the vero board now here's a very typical hundred nano farad Oh 805 bypass capacitor you'll find in practically every product and you'll see notice that the impedance scale has now gone up it's auto scaling and this red potato software was a little bit annoying that I couldn't actually manually a scale the thing to see the data but it's changed significantly we're talking about hundreds of millions before but now down at 100 kilohertz we're talking like up over 15 ohms quite large valued no good for low frequencies and you'll notice that the resonant point is now up to you know around about five or six megahertz so higher than it was with that larger value capacitance and now we'll take a look at a a 10 nano farad capacitor same Oh a 805 up package but you'll notice that the Y scale has changed even again by an order of magnitude down at 100 kilohertz but let's up up over 150 ohms or they're about 10 times more than what it was before and you'll notice that the now the resonant frequency is right up near 40 50 megahertz or something like that in fact this test setup isn't good enough because we're talking about you know much higher frequencies here so but as you can they are actually quite broadband you know tens of megahertz for these values like you know quite low impedance now if we actually combine a 10 microfarad ceramic with a hundred n ceramic and a 10 n ceramic you can see that we have look that rise around about 8 megahertz there so it very similar to the combined peak response we got on the whiteboard now here's an interesting little trap for young players which we didn't discuss before but what happens in reality now you can see on the left-hand side the same graph we had before of the combined 10 microfarads plus the hundred in there and you'll notice the big lump in there in the middle that around about 8 megahertz or so now this is actually undesirable because look at the one on the right as we saw way before this is just the 10 microfarad cap on its own and you'll notice the y axes are very similar it's actually a better result just to have the 10 micro farad capacitor there in this particular case with this particular these particular values on this particular very board with all its particular parasitics and everything and the values and the hole and the test setup and the whole works it can actually be detrimental in some cases to put capacitors in parallel you can form these resonant peaks there and sometimes it might interact with your hardware in ways that you didn't intend so you know it's not just magic you can't just you know put Y 10 different values and whack them all in you know you could actually get an issue with resonances between caps so it's a potential pitfall just watch out for in this case is not particularly bad but look just the 10 microfarads on its own would technically be better in this particular case now here's a better response if we actually combined four caps a 10 mic a 1 mic 100 N and a 10 M once again all SMD ceramics in various size cases and you can see that that 8 mega Hertz peak has gone away it's you know still at where we can argue that this is a bit better than the original our 10 microfarads just on its own but yeah it's hard to see this because the higher frequency ones really need a higher frequency response test system which we don't have here and here's my for ceramics in parallel here 10 microfarads 1 microfarad 100 N and 10 in various different package sizes and the package sizes are going to make a big difference in terms of the SR and the impedance response of the individual capacitor it's not just the capacitance value package plays a big part so I couldn't really get lots of visually good results with just the SMD ceramic capacitors they're just too good so I got like a really poor axial sorry radial lead at 47 microfarad electrolytic capacitor and put that in parallel with a 10 nano farad ceramic on there and you can see that you know peak around you know 15 16 megahertz or something like that but the extra 10 different ceramic brings the impedance of that way back down again at the higher frequencies which is desirable of course and that little Matteo back in up after 40 megahertz is just due to the test system as we saw right back at the start the 10 nano farad's would allow much better high frequency performance into the hundreds of megahertz and things like that that the 47 microfarad electrolytic on its own it just keep going up and up and up and there'd be hundreds of ohms at there free it would just be way off the scale at that frequency and you may as well not have it at all so that's a reasonable example visual example of how combining those two I caps actually can you know get a reasonably smooth response over a very broad range from 100 kilohertz right up to you know maybe a few hundred megahertz or something like that but we can't see it but yeah it would be quite decent performance over that big entire range so you use the 47 micro for decoupling big heavy current bursts and the 10 nano farad for all the high frequency switching now here's a little interesting aside you may have seen weird looking service mount caps like this in a wide package like this well why you're not you and you may not have thought anything of it well these are actually special low E inductive capacitors designed specifically for this application now if we have a look at this little snippet from an AV X app note on these low inductance or the evolution of ceramic capacitors here you can see that say a 1206 you standard 1206 one has about 1200 pica Henry's or there abouts of inductance right but if you take that exact same sized chip the 1206 and you put the caps on the sides the conductive caps on the sides instead of the ends same size cap but a hundred and seventy puffs and if we have a look at this TDK datasheet for their RC series their specific low ESL equivalent series inductance that we've been talking about they're called reverse geometry and they just put the conductive end end caps actually on the side of the capacitor instead of on the ends and it makes the world of difference and if you're designing in our high frequency switch mode power supply or something you might see you know real performance-critical are stuff where the you know the bypassing is really going to matter then you might typically find these low ESL caps in there so there you go I hope you enjoyed that rather lengthy look at how bypass capacitors work and why you put multiple values and types in parallel there's some real good reasons for it and sorry I couldn't really you know comprehensively show this this test setup is pretty crude it's not the best thing you really need you know a really high frequency high performance system and carefully lay it out test setup and everything else but hey you know just with this we were able to see so it did actually take quite a lot of mucking around and trial and error just trying different caps and different sizes and packages and values and things like that just to try and get a response and I probably you know ultimately could get a more realistic sample of what I showed on the whiteboard there but I hope you get I hope that was good enough and you really get an idea of how it can really make a difference especially at really high frequencies you can imagine just you know extrapolate those graphs right out and assume we've got a perfect test system and can make one heck of a difference anyway if you liked the video please give it a big thumbs up and all that sort of jazz you know we discuss it and links down below for data sheets and other uploads and things catch you next time you

Channel: EEVblog

Views: 746,040

Keywords: bypass capacitor, tutorial, why, how it works, how to, measurement, impedance analyser, red pitaya, pcb, chip, ic, inductance, impedance, reactance, capacitance, reverse geometry, surface mount, network analyser

Id: BcJ6UdDx1vg

Length: 33min 28sec (2008 seconds)

Published: Thu Mar 10 2016

Reddit Comments

[u/Enlightenment777]

See graphs on page 6 for why you should use different package sizes:

• http://intersil.com/data/an/an1325.pdf

For hand-soldered home projects with a microcontroller on the PCB, I use the following as a starting point goal on my new schematics, each capacitor 100x (2 magnitudes) apart. Different physical sizes and different material chemistry for each one.

• Ceramic, 1206 package, 10uF, 25V, X5R

• Ceramic, 0805 package, 0.1uF, 50V, X7R

• Ceramic, 0603 package, 0.001uF, 50V, C0G

For the largest cap, if I have the room, I put down a 1210 footprint for the largest part, which allows me to easily change to higher capacitance values and more package choices: 1206 / 1210 / Tantalum A / Tantalum T / Tantalum B. This is not recommended for automated production, but hand-soldered home projects the flexibility is far more important.

If I have the room, I put down a radial capacitor footprint on the PCB to provide me an option to easily add an electrolytic capacitors later.

As I stated earlier, these are goals as a starting point of what I prefer to use, but depending on the PCB, I may put more or fewer capacitors depending on the amount of free PCB space I have available, so if I don't have enough room then I'll make some sacrifices.