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RE: High-bandwidth Decoupling Caps?

Posted by Ugly (A) on June 10, 2007 at 11:30:27

In Reply to: High-bandwidth Decoupling Caps? posted by spindrifter on June 9, 2007 at 20:24:29:

You make an important distinction in your post that holds the key to understanding. I'm reffering to what self proclaimed designers and or DIYers who post on internet electronics tweaker forums might present as fact.

Your job is to try and take the 10,000 foot view and seperate the wheat from the chaff.

The real answer is that each situation is unique and the real requirements to achive the desired level of perfection is different for everybody and depend alot on what the rest of the design is made up of and taste too. This is why it is hard to answer your question.

The highest priority when choosing DC power supply decoupling caps in a design which already has a decent power supply design and decent PCB layout design is choosing the decoupling cap type which displays the proper amount of capacitance across the frequncy range necessary to pull off expected level of performance. In most of todays audio circuitry this means best high frequency performance and thus highest self resonant frequency. By doing this you have ensured that the part is still acting capacitively during local high frequency transients which is exactly why a decoupling cap is there in the first place.

Problem is this can often be harder than it sounds.

This frequency response and SRF aspect all capacitors display becomes important in selecting decoupling caps. What you realize is there has to be a tradeoff between actual capacitance and voltage ripple at the load due to this non ideal decoupling cap frequency response.

Limitations of todays commercially available packaging technology and material types means there are trends which rules of thumb can be formed by. Capacitor performance is directly related to material. Capacitor performance is also directly related to it's packaging configuration.

Honestly man if you really do need decoupling solutions Black Gates are not the answer, way too expensive to waste there and not ideal anyway. I like to think about it in terms of what fits the job: Black Gates are AEL meaning wrong material choice for optimum frequency response and they also come in in through hole cans which means to me they come in the wrong package for optimum frequency response. Granted the black gates are obviously an exceptionally good AEL as far as frequency response and ESR goes but....no where near what a true decoupling cap contender could offfer. Aluminum electrolytics are great for bulk capacitance needs so obviously the Black Gates will shine whenever there is a need for bulk AND linearity. In this app we need neither bulk nor linearity so Black Gates are the wrong choice.

It is kind of the same case with polyesters. Unless you go out of your way to find a surface mount version designed for good high frequency response chances are most of what you do find is through hole. As with the black gates the through hole polyesters good for what it is intended for ie low distortion on time varying fairly low frequency signals but due to the relatively large bulky through hole packages you find parts in typically pretty much worthless for real high frequency work like decoupling caps.

In summary neither choice is axactly stellar but I bet you'd have better luck with the polyesters.

It is extremely clear that in the job of effective DC power supply decoupling is best accomplished by surface mount ceramics for most audio circuitry I've seen but I won't recomend them.

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