There’s a common pattern of constructing electronics smaller and with extra performance. This interprets into: “Cram it in there,” so naturally designers look to search out smaller and smaller elements.
What was on paper no less than a 0.1 µF, 1206 sized capacitor, as we speak may be purchased in a 0402 measurement. However is it actually the equal capacitor? We are going to check out a number of the points right here.
As engineers, the perfect studying that we ever get is once we do one thing that doesn’t work out correctly after which we uncover the basis trigger. Hopefully, this occurs lengthy earlier than we get a product into manufacturing. A lot of what’s mentioned here’s a lengthy unhappy story of engineering “on the job coaching,” hopefully, in the event you aren’t conscious of all of those points, you’ll be forewarned and know what to look out for.
Ceramic Capacitor Specs
A number of us thought that the widespread ceramic capacitor nomenclature of C0G, X7R, B5X, and Z5U, et al., was a bodily dielectric specification. It didn’t assist that some producers would say issues like “X7R dielectric.” However these three letter designations should not a bodily dielectric, they’re a efficiency ranking system, and the producer can use any dielectric formulation that they select so long as they meet the three letter efficiency specification. Ceramic capacitors of kind designation C0G or NP0 (it’s a “zero,” not an “Oh” within the designation) are of kind Class 1, there are very steady capacitors having very low temperature drift of lower than 30 ppm per levels Celsius. C0G varieties even have the bottom volumetric effectivity.
Dimension is the place the Class II capacitors come into play. The three-letter numbering system is detailed in Desk 1.
Desk 1 The three letter class II capacitor labeling system. Essentially the most generally used varieties in electronics are the X7R and B5X. This designation doesn’t name out a particular dielectric, however moderately it’s a specification. The producer can use any building that they need so long as they meet the specification. For example, an “X7R” has the traits of: A -55 oC to +125oC working temperature vary and a +/-10% capacitance change most.
Life (and our circuits) could be so a lot better if we simply all used C0G specified capacitors in every single place. The difficulty is they’re too massive to be used in every single place in most fashionable purposes. Something apart from a C0G kind is the place “capacitors go unhealthy” as we’ll see beneath.
When ceramic capacitors go unhealthy—DC bias
My “unhealthy expertise” with a ceramic capacitor was once I first determined to go along with a largely ceramic energy distribution bus in a brand new software-defined radio. The radio labored simply wonderful however the energy bus, which was a 5.5 V bulk provide feeding a big community of 5 V low dropout regulators (LDOs), oscillated. The supposed 0.22 µF, 0402 capacitors that I had used, when biased to five.4 V merely misplaced a big portion of their capacitance and the bus didn’t have sufficient bulk capacitance to buffer out a low-frequency interplay that developed due to the LDOs. Oscillation ensued!
Fortunately, this was simple to repair. I simply piggybacked 2.2 µF capacitors on the present capacitors and carried on with testing the primary board iteration. This bought me wanting into the basis trigger and punctiliously finding out the capacitor knowledge sheets.
Earlier than this product required an excessive measurement discount, I had at all times used 0603 and 0805 sized capacitors often rated at 50 V with no less than one tantalum capacitor for the facility provide rails. Since my energy provide rails have been 5 V or beneath 80% of the time, the capacitors exhibited no noticeable unhealthy conduct.
As was expertly detailed by Mark Fortunato [1] and Istvan Novak [2] many people have had related experiences. What labored for years all of the sudden didn’t work with a measurement discount.
As proven in Determine 1, the well-known discount in capacitance versus bias for a number of capacitors.
Determine 1 Typical curves displaying the capacitance discount of two 0.01 µF, 50 V, X7R capacitors in comparison with a C0G capacitor versus DC bias (A), and two X7R capacitors versus AC excitation Stage (B). There may be massive variations between seemingly related producers of X7R capacitors.
Regardless that I used X7R specification capacitors for my bypassing, I noticed an enormous discount within the capacitance worth as a result of I additionally decreased the voltage ranking of the capacitors concurrently, getting a “double whammy,” which is an impact I name: “whammy squared.”
Novak acknowledged in his paper: “To assist the customers, all main distributors as we speak provide no less than DC bias data with their MLCC elements,” [2] however recently I discover that is now not true. Within the final a number of years, this data has gone lacking from numerous knowledge sheets, and now it’s important to go particularly on the lookout for it if you could find it in any respect.
AVX, Murata and KEMET to call a number of, have web sites that present all kinds of parameters and efficiency curves for practically all their capacitors [3], however sadly this data shouldn’t be usually transferable to a different producer’s half. For example, a 0.1 µF, 10 V, X7R from one producer will lose -4% of its capacitance with a 5 VDC bias, and one other may be -35% decrease capacitance with the identical situations. This exhibits that you just can not assume that one producer’s capacitor efficiency is equal to another producer’s capacitor.
Moreover, you could recall that in 2017 there was a worldwide scarcity of ceramic capacitors of all sorts. At the moment, producers scrambled to get sufficient elements to promote. I bumped into a number of production-related points with not solely finish customers making untested elements adjustments, however producers additionally making adjustments to their elements that prompted them to have totally different DC bias attribute curves.
Trying round for present-day capacitance versus DC bias curves and you will see producer data that’s a long time previous, and I start to surprise if it even carefully matches actuality anymore. It merely isn’t being up to date, and extra just lately appears to be eliminated as an alternative of up to date.
When Ceramic Capacitors go unhealthy—distortion
One other challenge I see is distortion. That capacitance change with DC bias could trigger a problem with any use of capacitors within the analog sign path. I’ve seen far too many engineers with an area crunch, merely choose capacitors based mostly on the dimensions and temperature ranking. This will get them to design all kinds of what quantity to “digital solely” capacitors, with disastrous outcomes to their analog sign processing.
To indicate and measure this distortion impact, I used a part of my BlasterAmp undertaking [4], specifically the audio output of a Sound Blaster USB dongle, and software program for creating very low distortion audio tones, a custom-built 18 bit, FFT analyzer to measure distortion [5], together with the circuit of Determine 2.
Determine 2 NE5534 op-amp circuit for testing distortion. The check capacitor is the place I solder my numerous capacitors for testing. This circuit, when related to a Sound Blaster USB Dongle and a few {custom} software program, produces a distortion free sign to the 16 bit stage (> 95 dBc distortion).
The setup in Determine 2 is restricted to the 16-bit distortion of the DAC within the Sound Blaster. I measured all kinds of 0.01 µF, C0G ceramic, and stacked movie capacitors as anticipated. They added no measurable distortion to the output.
The enjoyable half got here once I measured the X7R capacitors. Usually, I exploit X7R capacitors in bypassing circuits solely, however I’ve let one or two slip into the sign processing path throughout my provider for certain. Happily for me, they prompted no points, as a result of they have been nearly at all times 50 V rated and that was properly beneath the sign swings getting used.
Measurements of two, seeming equivalent 0.01 µF, 50 V, 0603, X7R varieties with a 20 volt peak-to-peak sign swing are proven in Determine 3. As may be seen, these capacitors have very totally different distortion signatures on the FFT plot. Desk 2 is a greater comparability of the distortion merchandise. One of many ‘seemingly equivalent’ capacitors has a 2:1 higher distortion than the opposite!
Determine 3 The FFT distortion signature for 2 seemingly equivalent 50 V, X7R, 0603 sized capacitors. As may be seen one has considerably worse distortion traits.
Desk 2 Tabular knowledge for the totally different capacitors measured in Determine 3. One of many capacitors has twice the proportion whole harmonic distortion of the opposite. Neither has efficiency even on the 8 bit stage (-49 dBc) for the third harmonic!
I additionally measured some X7R, 0805, 50 V capacitors, and even a 0402, 10 V rated capacitor with related distortion to the above. The 0402 ought to have been a lot worse due to the higher swing relative to its rated most working voltage, nevertheless it wasn’t. That is what makes me assume that the datasheet curves on a number of these elements are very previous and don’t match actuality anymore. I additionally biased the 0402 capacitor as much as 50 V with no measurable enhance in leakage present, so maybe it’s constructed on a 50 V capacitor course of in any case? I don’t know, however based mostly on the basic drop in capacitance versus working voltage curve, it ought to have been a lot worse than it was.
When rummaging round in my elements for capacitors to measure, I additionally ran throughout a 45-year-old Z5U disk capacitor. I believed it might exhibit very unhealthy distortion, nevertheless it turned out to be not so unhealthy in any respect, about the identical as a contemporary X7R! (See Determine 4).
Determine 4 A 50-year-old Z5U capacitor measured only for enjoyable. Its distortion wasn’t as unhealthy as anticipated.
I took one of many capacitors from Determine 3 and ran the peak-to-peak voltage throughout it down till the distortion merchandise have been within the noise flooring after which plotted that knowledge in Determine 5.
Determine 5 Third harmonic distortion versus peak-to-peak voltage utilized on a 0.1 µF, 50 V, X7R, 0603 from Determine 3. The measured distortion adopted a fitted logarithmic pattern line as anticipated.
A pattern line plotted on the information in Determine 5 exhibits match to a logarithmic curve, this implies that even in the event you should use an X7R capacitor within the sign path, in case your sign stage is low sufficient you could not discover any distortion points.
No producer lists distortion as a specification and, as proven above, the change of capacitance versus DC bias knowledge on seeming related capacitors varies wildly.
All you are able to do is steer clear of something apart from a C0G kind or use a movie capacitor the place distortion could also be a problem. Even testing fastidiously could not guarantee success, as you by no means know when the design or building of the half could change inflicting manufacturing points. Sure, that signifies that measurement could change into an issue, however design tradeoffs generally must be made.
When ceramic capacitors go unhealthy—piezoelectric results
I discovered of this once I labored with some educated part lock loop (PLL) designers; they informed me that something apart from C0G or X7R capacitors could be a problem. That “challenge” is that any dielectric apart from what’s used to make C0G capacitors use a fabric that’s naturally piezoelectric and, when deformed, will trigger a voltage to develop throughout the half. I feel the PLL designers first discovered this challenge when a design confirmed RF sidebands on the cooling fan rotation frequency. The fan vibrated the PCB, and this vibration prompted the capacitors in query to generate adequate piezoelectric voltage to modulate the oscillator tune line of the PLL, inflicting the sidebands. Altering the capacitor to a C0G kind made the difficulty go away.
The capacitor trade is aware of about this, and so they name it: “singing capacitors,” as most individuals find out about this piezoelectric phenomenon in a type of reverse solution to my expertise. If an AC voltage is impressed on one in all these capacitors, then they flex themselves and, if the frequency, voltage, and mounting are excellent, it turns the PCB right into a speaker, producing audible noise.
Studying the critiques of many laptop computer computer systems generally describes an audible whine that may be heard beneath sure load situations on sure laptops. They often describe this as “coil whine,” however it’s most likely actually a “singing capacitor.”
A number of producers have modified the design of their capacitors to mitigate the difficulty and decrease the potential acoustic noise [6].
There was some good work to exhibit and measure these points prior to now and there’s no level in me repeating them right here, see Reference [7].
As I discussed based mostly on these different designers’ experiences, I’ve stayed away from something apart from C0G and X7R capacitors in my analog designs, solely utilizing the upper density capacitors the place it’s important to, like between the facility pads of an FPGA, and so on., or in strictly digital designs. Though, in the event you use one in all these very piezoelectric capacitors in a clock line, who’s to say that some piezoelectric potential can not trigger a switching threshold jitter thereby inflicting undesirable clock jitter farther down the road? Watch out!
The belief that I’ve at all times had is that X7R capacitors are protected from the piezoelectric challenge, which is just not true as X7R capacitors use dielectric supplies which might be inherently piezoelectric additionally. It has simply been that to this point, the extent of piezoelectric cost has been too low to trigger me any points, your mileage could range as they are saying. This example could change at any second, as my experiences with the 2017 nice capacitor scarcity confirmed, so it’s best to be very cautious. Or use one of many specifically designed low acoustic noise capacitors the place acceptable.
When ceramic capacitors go unhealthy—cracking points
Ceramic capacitors are very fragile. Who hasn’t cracked one or had the tip caps fall off? This fragility may be exasperated through the use of massive ceramic capacitors on a skinny PCB the place the flexing may cause many capacitors to crack (Determine 6). My expertise has been that I get scared utilizing something bigger than a 1206 sized half on a regular 0.032- or 0.062-inch-thick PCB. I’ve even gone as far as mounting the capacitors upright in an I-beam vogue to scale back the potential stresses. Many massive ceramic capacitor arrays are even mounted in stress-relieving frames to scale back the potential cracking stress.
Determine 6 If power is utilized to any accomplished PCB meeting, inflicting any bending (purple strains). Then any elements mounted alongside the bend will expertise power at their mounting factors (black strains). Ceramic capacitors, being very brittle, often endure from the bending power(s) first and crack on the mounting factors.
It does matter to some extent what the capacitor building is, however all ceramic capacitors are liable to be cracked by flex stresses. It’s good to maintain this in thoughts and to make use of an acceptable thickness PCB for the dimensions of the elements to scale back the potential flexing. A 0.090- or a 0.120-inch-thick PCB is far stiffer than the usual 0.032- or 0.062-inch-thick materials and could also be sufficient to unravel any potential points.
Do not forget that flex stress not solely occurs once you bodily deform the board. Temperature biking the finished meeting may also trigger sufficient stress to crack ceramic capacitors.
Some producers produce capacitors with particular versatile or gentle terminations that enable the solder joint to truly flex some, which might vastly assist mitigate the issue.
One other actual challenge with any element smaller than 0603 is once you deal with the board or attempt to place the assembled board right into a stiff/faraday-shielded metallized anti-static bag. Most of these metallized baggage are fairly stiff and might shear small elements off the board very simply. Putting the meeting right into a extra compliant or padded anti-static bag earlier than inserting the meeting within the stiff Faraday shielded bag can forestall a number of harm points.
You don’t know what you don’t know
All these capacitor points are well-known and documented, however nonetheless not well known by the rank-and-file engineering neighborhood. I nonetheless see designs as we speak that attempt to use the smallest potential capacitor in an analog sign path. These designers are sadly about to be taught in regards to the drop in capacitance, distortion, and piezoelectric points firsthand. That’s unlucky, as it’s a lot much less painful to be guided across the pitfalls as I’ve been, so I haven’t suffered as a lot as I might have if I needed to hit upon all these issues personally.
Not too long ago, after the nice capacitor scarcity, I’ve re-evaluated how a lot extrapolation I could make on the piezoelectric rule of thumb that I’ve assumed, specifically that X7R capacitors are by some means resistant to the difficulty. I’m not assuming that blindly anymore and am rather more cautious. Because the TDK software notice [7] states:
“The engineer can not make common assumptions based mostly on … {out of circuit} … measurements alone.”
Fastidiously check the half you wish to use in circuit, however don’t attempt to extrapolate to different related elements. They could be completely totally different, and even worse, they could change subsequent week when the subsequent half scarcity arrives.
Bonus: Capacitors aren’t the one potential downside
When Linear Know-how began to supply 18- and 20-bit ADCs a while again, they found that even the smaller SMT resistors may cause distortion [8]. It seems that the 12- and 14-bit decision was fairly simple, and something higher than 16 bits as we speak takes a number of cautious design the place each single half must be scrutinized for nonlinearities. Together with “tapping” on the finished board to search for piezoelectric results!
References:
[1] Mark Fortunato, Maxim Semiconductor, 2012 https://pdfserv.maximintegrated.com/en/an/TUT5527.pdf
[2] Istvan Novak, et.al., Oracle-America Inc. http://electrical-integrity.com/Paper_download_files/DCE11_200.pdf
[3] AVX, Murata and Kemit On-line Selector / Simulation Instruments,
[4] Hageman, Steve, “Simplify testing of embedded analog-to-digital converters”, EDN, June 23, 2022 https://www.edn.com/simplify-testing-of-embedded-analog-to-digital-converters/
[5] DMT9000, 18 Bit, FFT Analyzer shall be detailed in a future article.
[6] TDK Company, “Singing Capacitors (Piezoelectric Impact)”, December 2006.
[7] Cadwell, John, Texas Devices, “Stress-induced outbursts: Microphonics in ceramic capacitors” December 2014, Components 1 and a couple of,
[8] Hutchison, Tyler, “Matched Resistor Networks for Precision Amplifier Purposes”, Linear Know-how Design Observe 502 https://www.analog.com/media/en/reference-design-documentation/design-notes/dn502f.pdf
Steve Hageman has been a confirmed “Analog-Loopy” since in regards to the fifth grade. He has had the pleasure of designing op-amps, switched-mode energy provides, gigahertz-sampling oscilloscopes, lock-in amplifiers, radio receivers, RF circuits as much as 50 GHz, and check gear for digital wi-fi merchandise. He is aware of that every one fashionable designs can’t be achieved with Rs, Ls, and Cs, so he dabbles with programming PCs and embedded methods simply sufficient to get the job achieved.
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