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Power supply bypass capacitor types
Power supply bypass capacitor types
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Old 13th November 2013, 06:24 PM   #11
fpitas is offline fpitas  United States
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Op-amp power bypass is a broadband thing, not for one specific frequency.
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Old 13th November 2013, 06:38 PM   #12
tinitus is offline tinitus  Europe
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yes, and from what I understand, snubber is better for this (resistor/cap)
its now well known that what may appear like a simple regulator might tend to oscillating, if not careful

actually I thought it was better to use cap with not too low ESR, hence the snubber resistor on film caps

or maybe I have misunderstood what it is about
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Old 13th November 2013, 06:54 PM   #13
fpitas is offline fpitas  United States
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A snubber, with a series resistor, for bypass? First I've heard of it. I'll let someone else try that
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Old 13th November 2013, 06:55 PM   #14
AndrewT is offline AndrewT  Scotland
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Tin,
the resistor view is valid.

It;s the resistance that does the snubbing.
Similarly it's the ESR that does the snubbing, if there is the correct amount of it.
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Old 13th November 2013, 06:58 PM   #15
fpitas is offline fpitas  United States
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The main problem people run into with low ESR caps is oscillatory behavior at the resonant frequency. The small series resistance from the supply rail I mentioned earlier tends to damp that sort of thing, while adding isolation from supply noise.
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Old 13th November 2013, 07:14 PM   #16
fpitas is offline fpitas  United States
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http://www.analog.com/static/importe...als/MT-101.pdf
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Old 14th November 2013, 09:54 AM   #17
Monte McGuire is offline Monte McGuire
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Power supply bypass capacitor types
The problem that can result when using several scaled bypass caps is that, above resonance, the larger capacitor can look like an inductor, and can form a tank circuit with the smaller capacitor that is placed in parallel. So, the combination can ring at some magic frequency depending upon the effective inductance of the larger cap and the value of the smaller cap.

If the larger cap is somewhat lossy, as a tantalum or aluminum electrolytic would be, this resonance is damped. A film cap would offer nearly no losses at all, so it would tend to ring much more easily. A series resistor from the supply to the pair of caps will not damp the oscillation currents flowing between the caps, but will help to keep them out of the supply rail.

Again, as has been stated earlier, it really depends on what you're 'bypassing'… if it's a digital circuit with a fixed clock frequency, you're asking it to couple regular transient pulses to ground, so specific resonant frequencies could be a problem. If it's just a 'keep the rails low impedance so the circuit remains stable' bypass, then specific resonant frequencies may not be important.
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Old 14th November 2013, 11:35 AM   #18
fpitas is offline fpitas  United States
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I've found that with reasonably closely spaced surface mount decoupling caps, if the paralleled values are in the ratio of 10:1 or greater, the resonance between caps is minimal. I didn't invent that, I got it from an old app note somewhere, but it does work in practice.
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Old 14th November 2013, 11:55 AM   #19
marce is offline marce  United Kingdom
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MLCC SMD small as possible case for a particular value, COG for the very small values X7R for larger values, inductance is the killer here.
Just laying out a design with some AD8620ARZ, LMH6553MR and ADS8471 amongst other devices and there used on this design and basically every other high spec design I have worked on uses mainly X7R's with occasional COGs for crystals and RF sections.
A decoupling capacitor provides the required power at switching, so low values nearest the pins larger values further away, via from power planes to small caps to IC pins, minimal track length, wide as possible track.
Through hole are not a lot of use these days lead length is to long.
In fact with MLCC you can get a low value and a 10u reservoir next to the op-amp pins great for high Mhz analogue layouts.

Last edited by marce; 14th November 2013 at 11:59 AM.
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Old 14th November 2013, 11:59 AM   #20
fpitas is offline fpitas  United States
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Quote:
Originally Posted by marce View Post
MLCC SMD small as possible case for a particular value, COG for the very small values X7R for larger values, inductance is the killer here.
Just laying out a design with some AD8620ARZ, LMH6553MR and ADS8471 amongst other devices and there used on this design and basically every other high spec design I have worked on uses mainly X7R's with occasional COGs for crystals and RF sections.
A decoupling capacitor provides the required power at switching, so low values nearest the pins larger values further away, via from power planes to small caps to IC pins, minimal track length, wide as possible track.
Through hole are not a lot of use these days lead length is to long.
Yes; that's the elephant in the room here: leaded parts are very inferior to SMT for bypass of high speed parts. My boss would think I had gone mad if I tried to use them.
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