Re: X7R ceramics for DC blocking ?

Eeyore <rabbitsfriendsandrelations@xxxxxxxxxxx> wrote in
news:469E44C3.48245DE2@xxxxxxxxxxx:



Bob Quintal wrote:

Eeyore wrote
Bob Quintal wrote:

From what I understand of the distortion creation
mechanism, the increase in voltage causes a change in the
capacitance.

The dielectric constant isn't errrr... constant !


This change is most pronounced at low voltages especially
for Y5V caps. Since without bias, the change in voltage is
equal to the peak-peak voltage, that represents a large
change in the impedance through the cap.

The bias reduces the percentage change significantly.

No. Not so. Coupling caps *do not* see the signal voltage
across them. They wouldn't be doing their job if they did.

Every capacitor has some series impedance. In a properly
designed circuit, a coupling cap should be chosen to have a
very low impedance compared to the rest of the circuit, In
the case of a ceramic cap, the impedance may be 1% of the
load impedance it sees, but if the capacitance changes, that
impedance will change. All I'm saying is that if it changes
from 1% when the signal is at negative peak to 2%, when the
signal is at positive peak, that will introduce a certain
amount of distortion.

Let's suppose the signal voltage is 1V. 1% of that (per your
example) of 1V is 10mV. That's the kind of signal that'll be
*across* the cap.

Your statement "the change in voltage is equal to the
peak-peak voltage" is simply incorrect. Perhaps you could
simply have accepted that without seeming to obfuscate ?

FYI I typically use a 10uF (electrolytic) coupling cap for
inputs. That has an impedance of -j795 ohms @ 20 Hz. With a
1V rms signal and a typical 10k input impedance the voltage
*across* the cap in those conditions is ~ 30mV. In this case
that 30mV is well below the threshold at which electrolytic
rectification takes place and the signal is not distorted. I
have performed measurements to confirm this btw.


Graham

Graham, The thread is about ceramic capacitors used for
coupling. I usually use a 6.8mF cap for coupling a microphone
input,and 22mF for interstage . It's a fact that ceramic caps do
have a capacitance/voltage sensitivity. If the capacitance
changes then the series resistance (the real part of impedance)
will change too. It doesn't happen with electrolytics, so you
don't have to worry unless you change your designs. The Original
Poster was asking about cercaps because of their smaller
physical dimensions per unit capacitance. Using those as
coupling caps requires additional considerations be entertained
over yours. Not everybody remains stuck with a 1960s mentality.


--
Bob Quintal

PA is y I've altered my email address.

--
Posted via a free Usenet account from http://www.teranews.com

.

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