[HN Gopher] The basics of decoupling capacitors
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The basics of decoupling capacitors
Author : zdw
Score : 157 points
Date : 2023-04-16 22:34 UTC (1 days ago)
(HTM) web link (lcamtuf.substack.com)
(TXT) w3m dump (lcamtuf.substack.com)
| magicalhippo wrote:
| Another issue to keep in mind is how different capacitors are
| affected by DC bias. Some lose a lot of effective capacitance
| close to the rated voltage.
|
| Here's a shortish video I found useful on this:
| https://www.youtube.com/watch?v=k7aPb585Y6k&t=189s
| exmadscientist wrote:
| Some comments on a great article from an EE who has many axes to
| grind about stupid old decoupling myths (which this article gets
| mostly right!):
|
| > A related trick is to put ferrite beads on MCU output lines
|
| You must be extremely, _extremely_ careful with this. In general,
| ferrite beads are hard to apply correctly and great care is
| required if you want them to work out. Do it wrong and you 'll
| probably be making things worse, instead of helping. See for
| example:
|
| https://incompliancemag.com/article/ferrites-to-kill-ringing...
|
| https://incompliancemag.com/article/all-ferrite-beads-are-no...
|
| >The datasheets ... The manufacturer is
|
| Honestly, the datasheets are usually just wrong. The writer is an
| intern and the material is cargo-cult copy-pasted. If it works
| it's by accident, or because it wasn't critical in the first
| place. Read the datasheet, understand what it's _trying_ to do,
| then go ahead and achieve that end in the most sensible manner.
|
| > It is true that at very high frequencies -- hundreds of
| megahertz -- the capacitor's residual inductance becomes a
| limiting factor. At that point, combining multiple different
| capacitors can offer somewhat better wideband noise suppression.
|
| Nope. It's still wrong and dumb to do this with MLCCs unless you
| have simulated the hell out of the whole thing. You should
| practically never parallel different values of MLCC. Instead
| follow EMC wizard Henry Ott's advice: pick the smallest package
| you're willing to deal with, then the largest capacitor you're
| willing to pay for in that package, and just use that
| _everywhere_. This is called "big V" decoupling by Ott and
| decoupling master Bruce Archambeault and it is not the best way
| to do things, but every better way is much, much, much harder to
| do. "Big V" will work for everything that doesn't involve
| underfilled BGAs, and even most of those.
|
| > tantalum polymer
|
| I actually kind of hate these guys, I don't find much use for
| them outside DC-DC converter output capacitors. They have too
| little ESR to damp things that need it, too much ESL for high
| frequency use, and are just too expensive for general use.
| They're not bad or anything, they just don't really have a sweet
| spot. MLCCs plus a few cheap high-ESR aluminum electrolytics
| (often found these days as the high-temp long-life parts) is a
| really effective combination. Maybe a few solid tantalums for
| intermediate bypass if appropriate.
|
| > Y5V
|
| Thank the heavens that these are basically extinct. Good
| riddance.
|
| Okay, enough comments, you might then ask, how the hell do I
| decouple things in production designs?
|
| First, put one bulk capacitor, minimum, on every rail. Aluminum
| electrolytic if the rail leaves the board ever, maybe tantalum if
| it doesn't (or maybe not, solid tantalums have... reputations).
| My go-to series is Rubycon YXM or YXJ for through-hole
| electrolytics, Nichicon UCB/UCW or Chemi-Con MLE/MLF for surface-
| mount, and AVX TAJ for tantalums. These can physically be located
| anywhere.
|
| Then sprinkle down one 1uF 0402 per part for the small parts, or
| one per power pin for the big digital chips. Place these _at the
| power pins, no exceptions_. Things like MCU analog rails usually
| don 't need ferrites but might get pi filter type structures. It
| depends on how important they are, really. If it's a big or dense
| board, toss in another tantalum or two physically near the chips
| or chip clusters to help keep the electrolytics honest.
|
| You can decouple 500MHz processors and pass radiated EMC at Class
| B with this approach. It's not hard to do, it's cheap (but not
| cheapest, this isn't going to get you to Shenzhen-special COGS),
| and it _works_ great.
| dragontamer wrote:
| >> tantalum polymer
|
| > I actually kind of hate these guys, I don't find much use for
| them outside DC-DC converter output capacitors
|
| Bulk capacitance in extreme battery constrained scenarios?
|
| MLCC has difficulty going above 1uF at reasonable costs...
| especially when you consider voltage derating. Aluminum is
| relatively leaky, IIRC like 20uA, or 20x more leakage than MLCC
| or Tanalums.
|
| Think like bulk capacitance for a CR2032-cell (which has issues
| serving more than 10mA). Serving an ESP32's 100mA+ current draw
| for a second or two (and then ESP32 sleeps) kinda thing.
|
| ----------
|
| But not really a "decoupling cap" in this case. Just a bulk cap
| where Aluminum is disqualified.
| exmadscientist wrote:
| I mean, yeah, they have uses. I just struggle to find them.
| Something else usually wins out. Height-constrained layouts
| are usually a decent bet, tantalum shines if aluminum can't
| fit!
|
| They do leak pretty badly though. I think it's better than
| other electrolytics, but it's still not great. Of course,
| they do tend to do better than spec... until they don't. It's
| been a while since I've done micropower stuff though, so what
| do I know.
| vanchor3 wrote:
| Have you used hybrid polymer electrolytics at all? I tried
| using a few just for fun when replacing capacitors on an old
| computer but I haven't seen them used or mentioned much before.
| exmadscientist wrote:
| Availability isn't great (supply chain is A Thing again), so
| not really. I've also been unsuccessful in finding a good
| description of what they actually _are_ , which would be kind
| of helpful, but I admit to not having tried too hard. Yet.
| They still look interesting!
| RicoElectrico wrote:
| > You should practically never parallel different values of
| MLCC
|
| Can you then say why paralleling capacitors to cover higher
| frequency range is repeated often in literature? Was it true
| before MLCC? Why is it not valid with MLCC?
| CarVac wrote:
| Based on jmwilson's article above, it seems you can end up
| with nasty resonances. With other capacitor types that have
| more ESR, they dampen out the resonance peaks.
| exmadscientist wrote:
| Technically these are antiresonances. But, yeah, exactly
| that.
| fps-hero wrote:
| Wonderful example Cunningham's law, thank you.
|
| To summarise your point about decoupling capacitors. Use
| physically smallest capacitor you can, but with the largest
| capacitance (limited by the knee in capacitor price). High
| frequency response is dominated by parasitics, but you will get
| lower impedance for the vast majority of the frequency range
| until you hit the SRF. A lower capacitance will be better high
| frequency around the SRF, but worse at every other frequency.
|
| Don't place ferrites down blindly. Ferrites vary wildly,
| impedance at 100Mhz more or less a useless specification. You
| need impedance graphs, and you really need to know when a
| ferrite lossy and not simply inductive, and when it loses
| effectiveness and becomes a capacitor.
|
| If you are going to put them down blindly use the exact same
| part number and manufacturer as on the development board.
|
| If you think you might need a ferrite, put a zero ohm down and
| measure later. Sprinkling ferrites blindly without a spectrum
| analyser is at best a placebo, doing it incorrectly is almost
| always worse than not doing it. Examples of what not to do,
| splitting a plane to add a ferrite, adding impedance to power
| pins, adding inductance to IO lines or filter network and
| creating resonance.
|
| As an addendum: Power delivery network, power plane stack up,
| and component placement matter far the than the precise value
| of decoupling capacitors. Thinking in terms of current loop
| area is vital. The PCB is a decoupling component, and the power
| planes may be your only decoupling at frequencies higher than
| your capacitors SRF.
| jeffbee wrote:
| > This is called "big V" decoupling by Ott and decoupling
| master Bruce Archambeault
|
| I assume this refers to the shape of the impedance plot? I mean
| sure you are going to try to get the deepest, widest "V" you
| can but it's a game where you must optimize the cost, usually.
|
| My favorite newish technology for this is the "reverse"
| footprint surface-mount capacitors, where "reverse" means the
| leads are on the long side of the package where they should
| have been in the first place, instead of the normal style where
| the leads are small and far apart. E.g. a backwards 0612
| package has ~10x less impedance at 1GHz than a 1206. But the
| price is 2-3x.
| exmadscientist wrote:
| Yep, that's the origin of the name. Cost is not usually the
| constraint being optimized, it's design time. This approach
| is simple to implement, relatively cost-effective, and works.
| That's useful!
|
| The wide parts are great. But they're also overkill. You
| _can_ clean your kitchen sink with a pressure washer, but why
| would you? You don 't need to. And so you don't need 0306s
| for normal designs.
| mastax wrote:
| The worst part about tantalums is their tendency to catch fire
| if you look at them wrong.
| spacedcowboy wrote:
| > Then sprinkle down one 1uF 0402 per part for the small parts,
| or one per power pin for the big digital chips. Place these at
| the power pins, no exceptions
|
| How does this work for something like an FPGA ? Generally there
| will be several power rails (Vdd, Vio, Vpll, maybe Vusb,
| others) in an area far too tiny to put even 0102's let alone
| 0204's. Or maybe I'm just rubbish at placement :)
|
| I've been using double-sided placement underneath the FPGA as
| far as I can, but that also restricts the egress of the signals
| when you have as many power pins as an FPGA has...
|
| Any hints for power-pin-dense applications ? Enquiring minds
| want to know :)
| 15155 wrote:
| How much do you want to spend? How many pins? How much unused
| I/O? What FPGA family? What ball pitch? What trace width?
|
| Blind and buried vias open up a whole world of space if
| you're willing to spend for them.
|
| You may just need to add more layers.
| InitialLastName wrote:
| for BGA's, via-in-pad (plated over) makes decoupling caps
| almost too easy; do a VIPPO in each BGA pad; for the power
| pads, you can place the cap directly on the other side of
| the board. In many cases, the power pads are arranged next
| to corresponding ground pads and you can select a capacitor
| whose footprint exactly bridges those two vias.
| spacedcowboy wrote:
| I'm just a hobbyist, so regarding spending, the ever-
| popular "as little as possible" is close to my heart, given
| that each penny comes from my own pocket...
|
| I try to keep my BGAs limited to 256 pins if I can, but
| sometimes it will go higher - I'm looking at using an
| I.MXRT1176 for example, and that's 289 balls at 0.8mm
| pitch. I've also used Efinix FPGAs at 0.8mm pitch/256
| balls. I've gone as low as 0.65mm pitch, but that is
| pushing the limits of where I want to be...
|
| It's fairly common to see 3.5mil as space/trace minimums
| nowadays (again in the cheap(er) Chinese PCB houses).
| That's still not _quite_ enough to escape two traces
| between each outer ball on a 0.8mm grid - though reducing
| the solder pad and ignoring the warnings from the board-
| house has worked before :)
|
| I've never done blind/buried vias, they always seem to be a
| lot more expensive than the 6- or 8-layer boards I can get
| done for cheap at nextpcb or jlpcb. Maybe I ought to ask
| again...
|
| Things like FPGAs, DDR/HyperRAM ram, etc. used to be
| outside the province of hobbyists, but given the packaging
| for those high-frequency-capable pins, BGAs are ever-more
| common, and with them come the layout issues.
| fest wrote:
| Isn't the volumetric energy density higher for tantalum
| capacitors? I haven't done detailed comparison, but I recall
| having to reach for tantalum capacitors in some space-
| constrained applications requiring 100-200uF at 20-30V.
| mrWiz wrote:
| Yeah, that's the appeal. Occasional combustion (though less
| frequent now than in the past, I think) is the drawback.
| magicalhippo wrote:
| IIRC the trick to not having the tants go up in flames is
| to make absolutely sure they will never ever see a voltage
| beyond their rating. Apparently they _really_ don 't like
| that.
|
| I prefer just to avoid that headache, but I'm just a
| hobbyist so...
| CarVac wrote:
| Ah, so ESD protection should cover that case?
| exmadscientist wrote:
| Surge current is potentially an even bigger issue. I use
| a lot of tantalums (more than many engineers), and my
| personal rule is basically that they are great for
| anything that never leaves my board and very, very
| sketchy for anything that does. This works out pretty
| well!
| magicalhippo wrote:
| Ah I probably misremembered a bit. I did some digging and
| found this[1] paper which goes into some detail. It
| contains this interesting section:
|
| _Experiments show that solid tantalum capacitors can
| tolerate discharge currents at much higher levels of
| voltage (typically, close to the scintillation breakdown)
| than the charge currents, so current spikes are much more
| "dangerous" in combination with the increasing voltage
| that happens during charging. This indicates that a fast
| voltage increase to sufficiently high level is critical
| for surge current failures, and high current spikes are
| byproducts of the fast voltage increase rather than the
| prime cause of failure._
|
| [1]: https://nepp.nasa.gov/files/24745/2013_n240_Teverovs
| ky_ESTEC...
| dekhn wrote:
| Some things I've learned in electronics: switches are noisy. They
| bounce. People will advice all sorts of things, like decoupling
| caps. I've applied these (artisanally) and they never make a
| difference (for switches) but schmitt triggers do. It was only
| after I looked at a switch with an oscope that it made sense, the
| noise is almost never due to coupling, and almost always bounce
| that causes the problem (assuming you're already pulling up/down
| the switch).
| dfox wrote:
| Putting caps or even somewhat more complex networks in parralel
| to switches or around switches is a solution when you cannot
| change the rest of the system.
|
| Correct solution for this issue is debouncing logic (be it in
| HW or SW). But if for your application an SPDT switch costs
| same as SPST switch, driving an RS flipflop from that works
| better (less latency) and component-wise is cheaper than
| deboucing in HW (same amount of logic, less passives).
| compumike wrote:
| You may also be interested in the book "High Speed Digital
| Design: A Handbook of Black Magic" which, despite "digital" in
| the title, is really about the analog details of getting signals
| cleanly from A to B. http://www.sigcon.com/books/bookHSDD.html
| Eduard wrote:
| How come ICs don't integrate decoupling capacitors themselves?
| dfox wrote:
| There is a no sane way to manufacture large enough decoupling
| capacitors in the semiconductor processess involved. Some ICs
| (including most of socketed CPUs) include some decoupling MLCCs
| in the module package, but still the bulk has to be outside.
| jmwilson wrote:
| I wrote an article last month about testing the received wisdom
| about decoupling capacitors: https://jmw.name/projects/exploring-
| pdns/
|
| A lot of design guidelines and advice boil down to cargo-cult
| rules like "sprinkle 100 nF caps everywhere," and many people
| don't have don't have the tools to measure PDNs and decide if
| that rule is actually good enough.
|
| I made a custom PCB to emulate different configurations of
| capacitors of various types and layouts. With a spectrum analyzer
| and tracking generator, it's possible to measure and visualize
| several points that are important: what does "physically close"
| mean, the effect of DC bias on capacitors, and how does
| parallelizing capacitors together affect the circuit. The author
| of this is using an oscilloscope to look at time domain behavior,
| but generally I think of these things in the frequency domain.
| hinkley wrote:
| I'm experiencing minor flashbacks to the EE intro class where
| they made us calculate RC circuits. Objectively I know a .1uF
| capacitor doesn't affect circuit voltage that much but my inner
| 19 year old is screaming.
| polishdude20 wrote:
| Id love to see a practical test of how we'll certain IC's work
| with different capacitor combinations. Because in the end, the
| IC operation is the important part.
| jeffbee wrote:
| Very cool and thanks for sharing. From practice and hearsay I
| had reason to expect that resonance at ~900MHz from standard
| FR4 stackup, but seeing it on the graph really clarifies.
| chongli wrote:
| Your article says "VNAs are very expensive." Does this mean
| something like the NanoVNA [1] is inadequate for this
| measurement?
|
| [1] https://nanovna.com
| jmwilson wrote:
| I do mention the NanoVNA as a measurement option that has
| better accuracy since it can do phase measurement. It's an
| amazing tool, though its lower limit for frequency is 50 kHz
| and I'm not sure how its dynamic range holds up there.
| Ideally you want the option to go very low for this
| measurement. Like most VNAs sold today, the NanoVNA is really
| designed for looking at RF and microwave circuits.
|
| I only know two commercial VNAs in-production that are made
| specifically for this kind of work: the Keysight E5061B (5 Hz
| - 3 GHz, $50000+ when optioned out appropriately) and the
| Omicron Bode 100 (1 Hz - 50 MHz, I think $5k-10k).
| e-_pusher wrote:
| I think the Omicron Bode 100 is the best bang for buck for
| a proper VNA these days.
| nunuvit wrote:
| I assume you don't actually need all of the transmission
| line s-parameters at low frequency. In which case, you can
| use a frequency response analyzer (FRA), of which there are
| a few in production. The gain and phase measurements are
| enough to plot transfer functions, impedance, Bode, and
| Nyquist.
|
| The FRAs span the same price range as the VNAs you listed.
| Having a low-cost version of each can extend the frequency
| range over which you can make good measurements compared to
| just one tool of similar total price.
|
| The Bode 100 sacrifices some low and high frequency range
| and performance to be a 2-in-1.
| magicalhippo wrote:
| Nice article! For the decade test, did you try just four 10uF?
| I've seen others perform similar tests where four equal values
| were from at least as good to much better.
| jmwilson wrote:
| I have data collected for 3x MLCC 10 uF + 1x 100 nF but need
| to make the charts and update the article. Short answer is
| it's basically just as good and has fewer resonance peaks.
| I'd also like to do the test with some tantalum 10 uFs to
| throw in some ESR and see if that tames the impedance spikes
| even more.
| GeorgeTirebiter wrote:
| Thanks James, nice article. (I had not seen the TLA "PDN"
| before but now I know you mean "Power Distribution Network".)
|
| VNAs have become remarkably cheap; here's one that < $ 60
| https://www.amazon.com/50KHz-900MHz-Analyzer-Measuring-Param...
| I have one of these, it works very well.
|
| Fancier ones are of course available. This one goes to 6.3 GHz
| https://www.amazon.com/LiteVNA-64-Analyzer-50KHz-6-3GHz-Port...
| and is $ 180.
|
| I have this 'fancy' one:
| https://www.tindie.com/products/hcxqsgroup/nanorfe-vna6000/
|
| One trick I learned from the RF guys is: put two same value
| capacitors in parallel; it doubles the capacitance, and also
| reduces by about half the ESL (inductors in parallel rule).
| Clever.
| orangecrust wrote:
| This is a neat tool for designing PDNs http://app.pdntool.com/
| Workaccount2 wrote:
| My approach to decoupling my circuits is to take out my 100nF
| salt shaker and to sprinkle liberally over the board.
| rvense wrote:
| That's how you get a capacitor shortage :)
| tylerag wrote:
| When an 0805 is 2 cents in bulk, I can't be bothered to do it
| any other way.
| GeorgeTirebiter wrote:
| 0805 ! Get thee some 0201s and a good microscope.... ;-) If
| you are working at DC / audio, sure, 0805 probably fine, tho.
| Truth be told, I generally use leaded parts because they're
| easier to work with. If you understand what is going on (a
| leaded Cap is actually a cap in series with some noticeable
| inductance), and if the self-resonant freq is 'high enough'
| --- it's fine.
| dfox wrote:
| For most of the embedded designs today the power planes are
| the bulk of low-ESR/ESL decoupling (even on 4 layer boards
| with powerplanes in the middle layers), it is a cap of
| ridiculously small capacity, but with essentially zero
| ESL/ESR.
| jjoonathan wrote:
| Yeah, and this is a case where a well cultivated sense of
| "good enough" is actually quite important because
| optimizing decoupling capacitors for high capacity, small
| size, and low price is a recipe for picking an exotic
| ceramic with a wacky tempco and then having a device fail
| intermittently when hot or cold:
|
| https://www.allaboutcircuits.com/technical-
| articles/x7r-x5r-...
| bsder wrote:
| I find that 0402s are plenty fine, thanks, and I can still
| see them with the naked eye to grab them with tweezers.
|
| In my experience, 0402s are the breakpoint on the curve.
|
| 0603s are large enough that there are a couple of cases
| where they're kinda marginal due to size (inductance) and
| the size gain isn't quite enough for things like QFN
| packages. 0402s almost never have the issue. 0201s aren't
| enough better that they are worth the extra grief to deal
| with unless I'm on a _really_ constrained board, and I 'm
| only letting an assembler do the board (nothing by hand
| from me).
| systems_glitch wrote:
| Most of what they're describing is _bypass_ which is the shunting
| of high frequency noise to ground via capacitors. The capacitors
| appear as a lower impedance path for the HF noise to ground,
| rather than return thru the power supply, whatever that may be.
| rzzzt wrote:
| You can have perfectly smooth supply voltage by using eg.
| alkaline batteries and still see noise on the power rails each
| time a digital IC switches a lot of outputs at once. The same
| caps can also act as reservoirs that work against these
| bounces.
| ansible wrote:
| > _A related trick is to put ferrite beads on MCU output lines;
| this takes the edge off fast-rising square wave signals, and can
| reduce needless inrush currents when operating slower buses such
| as I2C or SPI._
|
| As /u/exmadscientist mentions, you probably shouldn't do this
| unless you know what you are doing.
|
| I also want to mention here that the fancier MCUs and SoCs will
| have configuration registers for drive strength, slew rate
| control and more for each output pin. If you're having a problem
| with ringing or whatever, you should definitely look to these
| settings, and see if you can fix this way. You might not need to
| make a PCB design change, which is always nice.
| bsder wrote:
| If you genuinely need ferrite beads to solve your problems, the
| real solution is _use Ethernet_. Ethernet has already taken
| care of all the signal integrity issues as long as you follow
| the design guidelines from the application notes.
|
| You don't do this kind of thing with ferrite beads unless you
| need genuine high frequency isolation--generally because you
| are using a high precision ADC/DAC. A small resistor is more
| than good enough to round this off on an I/O pad.
|
| I always put a small (47 Ohm, 33 Ohm, or 22 Ohm) resistor in
| series with any digital "clock" line nowadays (SPI-SCLK, I2C--
| SCL, etc.). Modern chip processes can launch edges with GHz
| components onto clocks that are operating at KHz. I have had to
| debug quite a few "double clocking" faults and invariably just
| putting a small series resistor does a nice job of completely
| avoiding the problem.
| dfox wrote:
| If you are driving something that really should not be a
| transmission line for your application from fast enough
| driver that makes it look like a transmission line then
| source termination (or just placing random resistor there as
| who the hell knows what the characteristic impedance of the
| thing really is) is the main thing that everybody should get
| from Handbook of Black Magic. Ferrite beads are for when you
| are driving external cables that should conform to some kind
| of interface standard and you have slew-limited driver.
| wtfmcgrill wrote:
| My immediate thought when I saw Handbook of Black Magic was
| "wonder what book has his nickname". "High Speed Digital
| Design: A Handbook of Black Magic" well I'll be damned,
| wish I had known about this a year ago.
| phkahler wrote:
| Doesn't inductance go up with narrower traces? So keep the ones
| between the pin and the decoupling capacitor wider if there is
| room.
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