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Old 2nd August 2011, 02:58 PM   #1
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Default Ceramic capacitor non-linearity

Linear Tech just introduced a new negative side regulator, the LT3015 -- now the positive regulators have a mate.

There's an interesting commentary on the bias non-linearity of certain ceramic capacitors in the LT3015 datasheet, plus a graph which had to have been done by the late Jim Williams on the piezo-electric effect of tapping a device with a ceramic cap.
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Old 2nd August 2011, 06:04 PM   #2
AndrewT is offline AndrewT  Scotland
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Why would one use a ceramic cap to vibrate a device?
I would find a little toffee hammer, it's sure to do a better job of finding resonances !

Ceramic caps do not remain "linear" when pulsed with varying voltage.
This applies to Low and Medium and High K versions. It seems the regularly recommended C0G and NP0 behave similarly when voltage varies.

When ceramics are used in decoupling duty, where voltage is held substantially constant, the ceramics have few peers.

Now to go and find that LT note and learn some more.
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Old 2nd August 2011, 06:19 PM   #3
jcx is offline jcx  United States
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Default nonlinear PS cap? - so what?

title misleading, Andrew's post just compounds the error of lumping "Ceramic" caps together


np0/c0g are better than some plastic film caps on all measures - they are not piezoelectric, have negligible Vcoeff - distortions can be measured but can be as low as -100 dB level


for power reservoir/bypass there are highly piezoelectric ceramic formulations - high K ceramics are not suitable for signal path use

whether their application in PS has a problem with piezo electric or nonlinear Vcoeff of high K dielectric ceramic caps is dependent on the circuit

the "non ideal" properties of the high K parts are a trade off for size, which reduces parasitic inductance - critical for sw mode supply bypass

so what is the point of the thread - what about nonlinear caps in PS?
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Old 2nd August 2011, 06:44 PM   #4
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I can imagine an unimaginary scenario in which some badly behaving ceramics are beating to the tune of the vibrations in a CD/DVD/SACD player.

We need someone with an HP35670 or SR785 and a bunch of accelerometers to see how badly behaving ceramics are influenced by mechanical and musical vibration, and how they may affect the quality of reproduction.
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Old 2nd August 2011, 06:56 PM   #5
DF96 is offline DF96  England
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The solution is simple: use appropriate components for the application. In the digital part of the player a ceramic cap may be the best RF decoupler, provided that there is also audio decoupling to short out any piezo voltage. Glitches from supply or ground bounce could upset the audio far more than piezoelectricity.
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Old 2nd August 2011, 07:05 PM   #6
jcx is offline jcx  United States
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I have seen DVD rotation frequency sidebands on a OPPO player's analog out with a single tone sine on test DVD I burned

common ground impedance coupling, inductive loop coupling are probably the 1st things to get right

decent PSRR in the analog circuit helps too

so far I haven't traced any problems to cap piezoelectric behavior in stain gage amps with gain up to 8000, 16 bit ADC, DSP, Ethernet or USB on one board with the analog front end
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Old 2nd August 2011, 08:10 PM   #7
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Y5V capacitors should not be used at all. Their high cap/size ratio is more or less marketing scam. This got big attention in a couple of recent years, as soon as capacitor vendors started to publish Cap/Voltage plots. They will have ~5x smaller capacitance in real application, compared to part value.
X5R and X7R should be OK to decouple voltage rails for analog path. Peso effects would be hardly noticeable if they are used together with electrolytic caps. Vibrations should have big amplitudes at low frequencies, where el caps and voltage regulator feedback is capable of suppressing piezo effects in ceramic caps. Just necessary to have in mind that most of linear regulators will become unstable with big low ESR cap at the output. ESR value should be taken into account. ESR of more than 30 mOhm is usually OK for stability. And ceramic will provide good decoupling at relatively high frequency.
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Old 2nd August 2011, 08:29 PM   #8
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Quote:
Originally Posted by jackinnj View Post
I can imagine an unimaginary scenario in which some badly behaving ceramics are beating to the tune of the vibrations in a CD/DVD/SACD player.

We need someone with an HP35670 or SR785 and a bunch of accelerometers to see how badly behaving ceramics are influenced by mechanical and musical vibration, and how they may affect the quality of reproduction.
Nah, you can just plug them into a guitar amp! Not quite as good a a real pickup but they do work!
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Old 2nd August 2011, 08:36 PM   #9
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In fact, characteristics like X5R or X7R are very generic and don't tell much about capacitor behavior. Most of caps are more or less prone to microphone effect. It does not have to be piezo effect. And cap vendors don't specify much, related to this.
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Old 2nd August 2011, 09:01 PM   #10
qusp is offline qusp  Australia
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Quote:
Originally Posted by jackinnj View Post
now the positive regulators have a mate.
not really, theres been lt1175 for quite some time and this, while marginally better, is still not the equal of the excellent 3A lt1764a or lt1963a, or even on the lower current low noise side, the 500ma lt1763. since the lt1033 is out of production (though still available around the place) there is only the lt317/337 that are really similar enough in dynamic performance to be used as a bipolar regulator and those that have been close have only been available in wildly different physical packages from each other.

at least its available in some of the same packages as the 1764a and 1963a. you are right though, the books at LT have been pretty badly weighted on the positive side, for lack of a compliment to the low noise ldos i ended up using the lt3032 bipolar reg chip and modifying the pcb pattern to allow it to be hand soldered.

still, all the same, imo they do not have any real competition in the IC regulator sector and the app notes and datasheets are so thorough its sublime
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