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- Posted on
- Christoph Loew
April 8, 2005, 9:18 am
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I've been using Murata's GRM series capacitors with X7R dielectric
for power supply bypassing in digital circuits and am now looking
into using the same capacitors in a Sallen-Key bandpass filter for
The TI application report "Analysis of the Sallen-Key Architecture"
suggest using NP0 and avoiding Z5U while stating "X7R is OK in a
pinch". If I understand the matter correctly, the X75 and Z5U are
worse than NP0/C0G when it comes to aging, stability of capacitance
over temperature and changes of capacitance depending on applied
voltage since the X75 and Z5U are ferroelectric.
The last one is supposed to make X75 and Z5U a bad idea for audio
circuitry sice they will add "a lot" of distortion to the signal
(I've seen a diagram showing 100 times the TDH when measured against
C0G in a simple RC highpass).
So my questions are :
Is the a recommended book that covers these details of ceramic
capacitors (physics and chemistry of the dielectric and the effect
on component behaviour)?
What effect is responsible for the distortion of a sine wave
applied to or passing through a capacitor ? Is it only the change of
capacitance with voltage ? Again, is there a recommended book that
covers this ?
How can the adding of distortion be modeled in e.g. LTSpice - the
default capacitor model only covers ESR,ESI,EPR and EPC ?
any hints appreciated,
Re: dielectrics and distortion; modeling of ceramic capacitors
Hi K ceramics achieve their performance by doping with piezoelectric
ceramics. It is common to hear them click if hit with a high level
rectangular pulse at an acoustic rate.
NPO are now available up to 1U or so by means of very thin dilectric
layers but expensive.
The feed material for the best plastic is being discontinued by its
German chemicals co.
A great series of articles over the last 3 years in electronics world
giving circuits to measure and compare capacitor distortion.in Hi Fi