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I'm assembling a capacitor decade box. It will be used to tune circuits both in power & high frequency electronics (but, probably not GHz RF; with all the parasitics).

So, I need it to be as low tolerance as possible (high accuracy), whilst having low esr (for ideal behavior), low leakage (for DC), works at high frequencies (for AC), high tolerance to ripple current (so to not be aged too quickly) & lastly, a long lifespan (that the the characteristics do not change too appreciably given years & years).

Maybe also a capacitor type that does not have polarity or not too sensitive to over/reverse voltage would be good as well.

Anybody knows the best capacitor that fits this unicorn ideal capacitor description?

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    \$\begingroup\$ Film capacitors, probably. Hand-selected to get tighter binning than the tolerance rating suggests. \$\endgroup\$
    – Hearth
    Dec 17, 2022 at 20:59
  • \$\begingroup\$ (For small signals and frequencies, you may be able to get by using a capacitance multiplier.) \$\endgroup\$
    – greybeard
    Dec 17, 2022 at 21:15
  • \$\begingroup\$ Polypropylene film and foil capacitors is my choice. \$\endgroup\$
    – Andy aka
    Dec 17, 2022 at 21:29
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    \$\begingroup\$ would be important to actually spell out 1. capacitance range you want to cover, 2. frequency range, 3. amplitude range, 4. DC offset range, if any. \$\endgroup\$ Dec 17, 2022 at 21:31
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    \$\begingroup\$ How long leads will you have into your capacitor decade box? The inductance of the leads would certainly make any capacitance useless for high frequencies. \$\endgroup\$
    – Justme
    Dec 17, 2022 at 21:48

2 Answers 2

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For pF up to about 100nF and 50V or 100V (1uF if you are willing to put some in parallel or pay through the nose), NP0/COG ceramic capacitors.

For (somewhat) higher capacitances, stacked film such as PPS (Polyphenylene Sulfide) is pretty good in terms of tempco and dissipation constant.

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Stack ceramic caps in parallel, it will lower ESR since resistors in parallel get very small and caps increase in parallel, same as ESL (eevblog, why do we stack caps). I also recomend capacitance multiplier circuit explained on eevblog, you could work from there and play with gain for diffrent capacitance, but I don't know the complications that comes with such circuit for your applications.

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  • \$\begingroup\$ Good idea but will it help if there is e.g. 10 cm of ESL from wiring and selector switch? \$\endgroup\$
    – Justme
    Dec 17, 2022 at 22:14
  • \$\begingroup\$ hm, ceramic caps have nonlinearity. This can be corrected out if this is used for e.g. single-tone tests (low-pass filter out harmonics caused by the nonlinearity, apply correction curve for derated "linear" capacitance), but depending on the amplitude range of the signals, this can be a dealbreaker for a measurement standard. \$\endgroup\$ Dec 17, 2022 at 22:40
  • \$\begingroup\$ @Justme what for ? the ESL will get lowered as well, BTW I got this idea from eevblog why do we stack caps in parallel episode \$\endgroup\$
    – AhmedH2O
    Jan 8, 2023 at 11:44
  • \$\begingroup\$ @AhmedH2O The added ESR and ESL from wires between caps and device will surely nullify or negate the effect of putting caps in parallel at the end of wires. \$\endgroup\$
    – Justme
    Jan 8, 2023 at 12:05

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