[sdiy] integrator / capacitor leakage

[Ian Fritz]

JH --

Ah, what we don't know about passive components and their nonidealities!

There used to be an audiophile-type site with good cap info, but it is 
either gone or moved.  Caps have leakage, yes, but also anomalies from 
time-dependent and memory effects in the dielectric material.  So there is 
dispersion, absorption, soakage, hysteresis, etc. to worry about.  I've 
never seen good information from manufacturers on all these properties, but 
you could try to check with them.

I would imagine that the answer to your question also depends on the 
voltage used relative to the spec'ed maximum, i.e., the actual E field in 
the dielectric. I would not assume that nonidealities are linear.

You could try to characterize yourself whatever caps you are thinking 
about, but that is a tricky business and is best done with fancy lab equipment.

Your best option might be just to try both configurations with different 
caps and see what works best.

The only really good caps that I have seen are glass.  Unfortunately they 
are usually made for high-voltage applications and are not too practical 
for sdiy work.

Please let us know what you find out about this question.

   Ian


At 05:56 AM 12/2/05, jhaible at debitel.net wrote:

>When I'm building an integrator for a triangle or sine wave oscillator 
>with an
>amplitude of 20Vpp, I have two options:
>
>a) run the integrator at 20Vpp, or
>b) run the integrator at a lower voltage, and amplify the signal
>    with an extra amp.
>
>Now, for a certain current into the integrator, the integration capacitor
>in case (a) will be much smaller. (Larger voltage to pass in the same time
>at th esame current.)
>
>What is better, in terms of precision / leakage?
>
>At first glance, I'd say leakage is mostly leakage _currents_, so
>it will be the same in both cases.
>At second glance, I'd say if the leakage currents are not entirely
>independent with voltage, case (a) will be worse.
>
>Is this right? Are there other things to consider?
>
>Background: VCO which not only will run from 0.03 Hz to 20kHz,
>but which will also produce very precise waveforms over that
>whole range.
>
>Any ideas / hints welcome.
>
>JH.
>
>
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