[jeffreyj]

I had forgotten about this great little document from Texas Instruments about selecting decoupling (aka bypass) capacitors, but when I was looking for some info on resonant flyback controllers it popped up in the search results. It should help in replacing some of the mythology (blind faith? voodoo? black magic?) that seems to characterize decoupling capacitor selection..

How to Select Decoupling Capacitors

 

[guzzler]

thats pretty hardcore stuff jeff! unfortunately, most of it applies to MHz or even GHz levels; we're really concerned in the realm of 100kHz at the most, but i suppose the maths and methods must stand! good stuff as you say...

well written though for a technical document, some are absolute slogs!

g

 

[jeffreyj]

Yeah, it's pretty rough, but contrary to what you might suppose, one should be concerned with the MHz stuff. The problem is that people here love to use ultra-fast op-amps that were meant to push MHz DSL signals down a couple kilometers of twisted pair for their headphone amplifiers. Since 99% don't have 100+ MHZ oscilloscopes, they have no idea what their circuits are really doing. Poor decoupling capacitor selection, board layout and circuit design can all lead to parasitic oscillation, and if the op-amp is fast, the oscillation might be in the dozens to hundreds of MHz range.

When someone says, "don't use a ceramic capacitor because they are microphonic," I want to cringe, because the knee-jerk assumption has been made that occasional microphonic disturbances are more objectionable than op-amp destroying, grain/grit-inducing parasitic oscillation. Furthermore, only the high-k dielectrics - X7R, to some extent, and Z5U to a much greater one - are actually microphonic (piezoelectric, to be precise) whereas NPO/C0G is definitely not.

Quite simply, then, anyone who says you shouldn't use ceramic capacitors has already displayed their lack of knowledge of capacitors. Saying NPO's and Z5U's are the same is like saying the Dodge Viper and the Dodge Omni are the same...

But I'm rambling on a rant... I've been reading too many threads at diyAudio.

 

[ppl]

As nice and conveinent as Ceramics can be because thay simply do the Job is not IMHO what obtaining good sound is about. You will find all Kinds if low fi stuff using Ceramic Bypass cap's but not in the High End. To extract all the nuances within a recording possible one must consider PPM and this is wher the Pizo effect of ceramics are real bad. With normal Precision Op-Amps like The AD-8610 at a lowly 20+ MHz GBP polypropylene capacitors perform quite well in these ranges with prober selection and matched to the qualties of the Larger Electrolytic, Cleen and stable Pulse Response can be obtained through out the Amps useable operating range and beyound. so for me wether to user or not to use Ceramics in a circuit is not bound by constraints that one would have to apply with those 100 Plus GBP Video Opamps some folks like. However I know Tomo built one with a TI opamp on a TI Eval board and works fine and i am sure he probed his for nasties i belive he also may have used some sort of SMT film Bypass caps but i could be wrong on this part. I recomend this link
http://www.capacitors.com/pickcap/pickcap.htm


from that artical>
Ceramic Capacitor Tests
In a second series of tests, the distortion produced by a common ceramic disc capacitor was studied. Data in the form of the THD vs. frequency for this test are shown in Fig. 4. The first circuit used is a simple low-pass (LP) filter, with the capacitor under test as the shunt C arm. The values chosen for the test were R = 1K and C = 0.1 uF. A 100-V type was used for the capacitor.


As the LP data show, distortion is produced well below the corner frequency, which in this case is 1800 Hz. The data shown are corrected, so the THD 100 percent set level follows the LP roll-off. Even as such, however, the higher harmonics are attenuated, and this data may be a pessimistic representation. An IM test might show even worse performance for this LP filter. Figure 5 shows the nature of distortion in 5a; as can be noted, it is third harmonic. By contrast, a polyester type inserted into the circuit shows no discernible distortion (5b).

By placing the same ceramic capacitor in an HP filter circuit, the roll-off of harmonics can be circumvented. In this type of use, the voltage across the capacitor is highest at low frequencies. Thus nonlinearities will show up as higher harmonics, which are readily passed by the filter.

The data for the HP test show much stronger distortion at the lower frequencies, where the voltage is highest. We are not sure what should be interpreted as the common distortion-producing source in these two tests. One thing seems quite clear, however, and that is the simple fact that you cannot "work around" the distortion problem in ceramics. Our feeling is that they should simply be avoided anywhere near an audio signal path and probably just avoided altogether for audio. For example, some listening tests have indicated that they can produce audible distortion when used as supply bypasses, let alone coupling!

One obvious implication which emerges from the above is that a capacitor is not just a capacitor by any means. Of course, what we've discussed here are only two types of capacitors, and we really ought to make some general recommendations as to desirable types. This leads us more deeply into just what a capacitor is, and how this knowledge relates to audio.

 

[aos]

Quote:

"don't use a ceramic capacitor because they are microphonic," I want to cringe, because the knee-jerk assumption has been made that occasional microphonic disturbances are more objectionable than op-amp destroying,

If you're using a ceramic as a noise reduction capacitor for an ultralow noise voltage regulator, then microphony will bite you.

But for wide BW opamps I would agree with you (as well as with need to keep opamps in proper operation and that most of us has no access to proper instrument, or training on how to use them), as you really don't have much choice. The only other choice I know of - and that's what I use - are PPS SMD film caps. Well, not true, as the effects of ground planes will kick in too. In fact there is another article that shows that at sufficently high frequencies - like those that are shown in your article (which was pointed to last year on Headwize I think) - effects of ground planes dominate the contribution of any kind of bypass capacitor.

Also, the technique in that article aims to create resonance at frequencies that circuit operates on so that they can be eliminated from rails. While more often than not the bypassing in audio use is trying to minimize impedance of rails over a wide range of frequency. Completely different requirement.

 

[jeffreyj]

The NPO dielectric is so different from the ferroelectric types that it should be in a completely separate category of capacitor, which was my point, really. The NPO types produce little or no piezoelectricity and have zero to miniscule temperature and voltage coefficients of capacitance change whereas the ferroelectric types (barium titanate, etc.) sacrifice everything to achieve a high capacitance per unit volume.

Attempting to use a capacitor's series resonant frequency to precisely null out ringing/oscillation is, admittedly, a tad beyond the scope of this forum's pursuit

. However, the document you referenced, ppl, seems to be a bit Spartan in the details about just what ceramic capacitors were tested - not to mention that the part numbers of the other capacitors tested indicate that the data are a little outdated (polycarbonate, for example, is no longer being produced as a film for capacitor dielectrics - what is available today is what is left of a very finite supply). So, perhaps they tested Z5U ceramics, in which case, yes - they suck for pretty much any use, even bypassing, in audio. But X7R's are a bit better, and NPO's aren't even in the same league. Of course, the low k of NPO dielectric means you won't often find capacitance values of more than 1000pF, but part of the point of the TI document was that more capacitance does not necessarily improve the decoupling of an op-amp's power supply.

Furthermore, while it is important to know how much distortion a capacitor introduces to a signal if it is used for coupling, it is not at all relevant when used for decoupling. The ESR and ESL of a capacitor are pretty much the only relevant parameters w/r/t decoupling. In this respect, then, NPO ceramic reigns supreme. PolyPhenylene Sulfide capacitors look pretty good, too, but I don't know enough about this dielectric yet (aos - isn't this dielectric one that you favor for bypass applications?)

Quote:

If you're using a ceramic as a noise reduction capacitor for an ultralow noise voltage regulator, then microphony will bite you.

aos - what kind of ceramic? I would think that the amount of capacitance required in this application would preclude the use of NPO... Correct?