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Old 9th March 2004, 08:59 AM   #1
borges is offline borges  Norway
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Default Do electrolytes dream of ceramic caps?

Hi,

What's the status on good decoupling electrolytes these days? Is there still a need for ceramics and 'lytts in paralell?

What I'm decoupling are CS8414 s/pdif receiver, PCM1704 and PCM1794 DACs, and opamps like 2134/2227/5534/2604.

The perfect decoupling, I guess, would be a cheap and compact SMD 'lytt that goes well into the MHz range. Any personal experiences you care to share with me on this one?

Greetings,
BÝrge
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Old 9th March 2004, 09:30 AM   #2
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The CS8414 datasheet specifies an electrolytic and a ceramic in parallel for decoupling.
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Old 9th March 2004, 11:19 AM   #3
borges is offline borges  Norway
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I know, but why is the ceramic specified in the first place? If the electrolyte alone is able to produce high-frequency supply current at a low impedance, is there a need for the ceramic at all?

One good electrolyte means less routing and thus less parasitic inductance between decoupling and power pin.
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Old 11th March 2004, 10:46 PM   #4
EC8010 is offline EC8010  United Kingdom
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All capacitors have series inductance causing them to have a rising impedance above their self-resonant frequency. Unfortunately, an electrolytic's resonant frequency could be as low as a few tens of kilohertz. Bypassing the electrolytic with another capacitor is the traditional method of more nearly approximating to an ideal capacitor. Ceramic capacitors are usually specified by (American) IC manufacturers because the USA has a large ceramic capacitor industry. Plastic capacitors are equally useable as bypasses.
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Old 11th March 2004, 11:09 PM   #5
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This thread may be of some interest:
Optimum Decoupling of Digital ICs
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Old 11th March 2004, 11:58 PM   #6
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Question Most ludicrous post of the week?

"Ceramic capacitors are usually specified by (American) IC manufacturers because the USA has a large ceramic capacitor industry. Plastic capacitors are equally useable as bypasses. "

Do you really believe this? Does this include decoupling for digital circuits? Doesn't the US make film caps as well, and the Japanese ceramics? Are you just trying to be funny or are you really that confused? I think 20 minutes with a search engine could help you learn the difference between ceramic and film caps........ it might be time well spent. I will even give you a big head start.

http://www.faradnet.com/
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Old 12th March 2004, 07:37 AM   #7
Raka is offline Raka  Europe
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Default Re: Most ludicrous post of the week?

Quote:
Originally posted by Fred Dieckmann
"Ceramic capacitors are usually specified by (American) IC manufacturers because the USA has a large ceramic capacitor industry. Plastic capacitors are equally useable as bypasses. "

Do you really believe this? Does this include decoupling for digital circuits? Doesn't the US make film caps as well, and the Japanese ceramics? Are you just trying to be funny or are you really that confused? I think 20 minutes with a search engine could help you learn the difference between ceramic and film caps........ it might be time well spent. I will even give you a big head start.

http://www.faradnet.com/

You are so kind, Fred... I like your emails a lot.
BTW, thanks for the link, it's a interesting source.
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Old 12th March 2004, 11:59 AM   #8
EC8010 is offline EC8010  United Kingdom
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Fred, if you disagree with a post, it is sufficient simply to give reasons why. Personal abuse is unnecessary.

The source of information that led to my remark was:

"There is an interesting geographical factor involved, though. US manufacturers are stronger in ceramics than in film, whereas in Europe the situation is reversed. Therefore, US design engineers will go for film. So far so good, but the majority of digital ICs sold throughout the world are of US manufacture, and their application notes are written by US applications engineers. These, quite naturally, specify ceramic capacitors when it comes to recommended practice for IC decoupling. As a result, the suppliers of monolithic ceramics are riding on the back of an enormous hidden subsidy as designers worldwide take the easy route of sticking recommended parts in their circuit designs."

This quote was taken from a discussion of ceramic versus film capacitors on p79 of: "The Circuit Designer's Companion" Tim Williams. Butterworth-Heinemann. (1991) ISBN 0 7506 1756 X
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Old 12th March 2004, 12:22 PM   #9
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Well, while film is indeed usable as decoupling, it will never be as cheap, nor as small. And since decoupling caps are a large portion of any pcb -in count and space- it makes sense to use the cheapest/smallest that do the job.
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Old 12th March 2004, 12:41 PM   #10
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Agreed, on all counts.
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