[sdiy] C0G/NP0 caps for VCOs

[Andre Majorel]

On 2013-10-14 09:24 -0700, rsdio at sounds.wa.com wrote:
> At 06:13 AM 10/14/2013, Andre Majorel wrote:
> >I don't know what you folks have been using for VCOs lately but
> >these look like a pretty good substitute for polystyrene
> >capacitors :
>
> What makes you think they'd particularly be good substitutes for
> polystyrene?

The low tempco.

> I thought that the primary benefit of poly was the low
> self-absorption, so it seems like we'd need data on that to know.

Yes, I was wondering how important DA was. Intuitively, it would
seem that if the time constant of the dieletric absorption is
much shorter than 50 µs or much longer than 50 ms, linearity
over 20 Hz - 20 kHz is preserved. But I haven't done the math.
And of course, I don't know what the time constant(s) is (are).

The _Op Amp Applications Handbook_ says electrolytics, tantalum,
PC, paper-oil, silver mica, glass and high-K ceramics have DA >=
0.2% and PP, PS and PTFE have DA around 0.02% but says nothing
about low-K ceramics.

In _Understand Capacitor Soakage to Optimize Analog Systems_,
Bob Pease lists NP0 among the low-DA dielectrics. In figure 7,
it looks to be about on a par with PS. TI has helpfully 404'd
the original URL but the Wayback machine once again comes to the
rescue :

  http://web.archive.org/web/20120107045614/http://www.national.com/rap/Application/0,1570,28,00.html

Thanks for the report, Ian. That's good to know.

Neil, I had 100 nF stuck in my head. Don't know from where.
Perhaps a VCLFO.

-- 
André Majorel http://www.teaser.fr/~amajorel/