Interesting, Thanks. I'm doing mostly 'air' core inductors, or caps with opamps. A tuned circuit seemed like the easiest way to compare the dissipation in the caps. (at that frequency)
George H.
I guess I don't see it. The DC bias is even worse, in this case. The voltage across a single cap is only significant at the corners, often outside of the interesting band. If the center is grounded, the bias is not only the average voltage but is asymmetrical.
It depends on the situation. If the midpoint is biased halfway between the two ends, and the capacitors are identical, all the even-order distortion products cancel out exactly. Departing from that condition makes things worse fairly fast, but it won't be as bad as 3 dB unless the situation is very asymmetrical.
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net
But that's exactly what you have with a single supply and a grounded bias in the middle of the capacitors. With a single supply, the capacitor's midpoint would have to be at Vcc/2 (or some such). For capacitors between supply domains, there would have to be an additional vref, half way between those references.
No, it doesn't have to be at Vcc/2, but it does have to be more than the signal swing.
Cheers
Phil Hobbs
I tried *hard* to find a source for a polystyrene cap a couple years ago, for use in a picoamp integrate-and-dump.
The answer came back "No mas." Several sources said the film was no longer being made; and only a few cap-makers were still producing from life-time buys of the raw material. Digikey shows none, today.
That led to comparing dielectrics. It was tempting to try NP0. The dielectric absorption and leakage looked low, but there wasn't time. We found a PS supplier, ultimately, and a polypropylene part for back up.
Cheers, James Arthur
Of course they're no longer offered by the major players, but you can still get 63V, 160V and 630V rated parts below 10,000pf in both axial and radial form from distribution (RS, Rapid, ~Mouser~).
They are about as common as commercial tuned circuits....
I see newer offerings in the audio lunatic fringe as well.
The older power components that used to use polystyrene are generally replaced by films with better temperature limits....
RL
I meant by Vcc/2, centered on the audio signal (the local reference). However, it really should be the average of the two references? I guess I'm not seeing how this helps if the capacitance error isn't split between the two caps. Perhaps I should figure out how to get an accurate capacitor model and simulate this. I'm not seeing the magic.
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