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I see in capacitor datasheets that the impedance of a ceramic capacitor changes with frequency, but I see this as the ESR and ESL of the part playing a factor.

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- Thread starter DragonPetter
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I see in capacitor datasheets that the impedance of a ceramic capacitor changes with frequency, but I see this as the ESR and ESL of the part playing a factor.

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- #3

sophiecentaur

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Eventually, the L and C will resonate (XL + XC =0) and, above this resonant frequency, the C 'becomes' an L!!!

It's a real problem to make a high value Capacitor that will operate exactly as you expected at UHF and above.

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A similar effect that I'm trying to compare to is that in electrolytic capacitors, as the voltage is increased very close to its maximum voltage rating, its effective capacitance is actually decreased because of the dielectric properties. So in this case, its some property of the capacitor aside from its capacitance, inductance, resistance parasitics that is affecting its effective capacitance.

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This app note could help

http://www.n4iqt.com/BillRiley/multi/esr-and-bypass-caps.pdf

http://www.n4iqt.com/BillRiley/multi/esr-and-bypass-caps.pdf

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If you use low voltage ceramic caps near their rated voltage they can loose more than half their capacitance. A 1uF ceramic cap rated for 10 volts is way less than 1uF if charged to 9 volts. The same cap if rated for 50 volts will still be 1uF. Nobody ever seems to talk about this.

I never use any ceramic caps at more than 50% of their rated voltage.

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Holy, I did not know this.

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Holy, I did not know this.

Glad I could help Yungman. I know you've been designing small low voltage mixed signal circuits. That's just where you will run into it.

"For example, the capacitors may undergo a change of capacitance (Y5V ceramic capacitors will loose 80% of the initial capacitance under rated input voltage). Also, the ESR of input capacitors will depend on the rise time of the waveform."

From Page two: http://cds.linear.com/docs/Application%20Note/an88f.pdf [Broken]

Edit: here's the definitive reference:

http://www.enpirion.com/bcf35454-46a3-4835-8ded-2ebf9fb53c27/resource-center-product-literature-delivery.htm [Broken]

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Thanks for the info. I just never thought of this.

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A similar effect that I'm trying to compare to is that in electrolytic capacitors, as the voltage is increased very close to its maximum voltage rating, its effective capacitance is actually decreased because of the dielectric properties. So in this case, its some property of the capacitor aside from its capacitance, inductance, resistance parasitics that is affecting its effective capacitance.

http://www.ami.ac.uk/courses/topics/0184_dp/index.html

http://en.wikipedia.org/wiki/Dielectric#Dielectric_relaxation

- #12

sophiecentaur

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Voltage dependence - High dielectric ceramic caps such as Y5U have less capacitance with increasing voltage.

Soakage - Capacitors are subject to having little "nooks and crannies" of capacitance that slowly become acclimated to a change in voltage. Thus, you can start with zero volts, apply 1, come back, and it will have say, .9 volts. High dielectric ceramics and electrolytics are the worst offenders, while some polypropylene film show very little of the effect. Note that this effect can be slooooow.

Frequency dependence of electrolytics - Electrolytics typically show less ESR and an improved looking capacitance as you go up into the 100 kHz range.

Equivalent inductance - As you go up in frequency, every cap will eventually suffer from equivalent series inductance. Thus, it stops appearing to be a cap, becomes a resistor, and then becomes an inductor as the frequency goes up. Chip capacitors can serve with very little inductance, hence many ceramics are well suited to high frequency use. The shorted and wider these caps are, the less inducatnce they exhibit, so these types are preferred for bypassing ultra fast processors.

Mike

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