The broadband thermal noise at a circuit point with a cap is always kT/c On Monday, 1 August 2016, Bob Camp <[email protected]> wrote:
> Hi > > If you wire up all the possible circuits and check them all out … the > answer is that big C / small R wins. Big R gets you into resistor noise > issues > and stray pickup. > > Bob > > > On Aug 1, 2016, at 4:16 PM, David <[email protected] <javascript:;>> > wrote: > > > > This duplicates the problems encountered when trying to quantify low > > frequency noise from a voltage reference; it is difficult to make an > > low frequency high pass filter with lower noise than the lowest noise > > references and the capacitor is the problem. > > > > In Linear Technology Application Note 124, Jim Williams discusses the > > problems with electrolytic capacitors for this type of application. I > > have read that you *can* get away with aluminum electrolytics if you > > grade them for low leakage and low noise. The dielectric absorption > > is also a problem unless you can wait hours for best performance. > > > > What about the alternative of buffering the signal with a low noise > > low input bias current operational amplifier so that a large film > > capacitor can be used instead? Is the low frequency noise of a good > > operational amplifier still too much? What about a chopper stabilized > > amplifier without suitable output filter? > > > > On Mon, 1 Aug 2016 11:46:51 -0400, you wrote: > > > >> Hi > >> > >> > > .. until you discover that you picked the *wrong* capacitor manufacturer > and you have > >> more noise from leakage in the cap than you did to start out with :) > In general “big C and > >> small R” is the better solution than “big R and small C”. > >> > >> The pesky part is that with electrolytic caps, the whole “noise > current” thing changes as > >> the voltage moves around. You go to measure things and by the time the > gear is set up, > >> the noise has dropped. Turn it all off, come back the next day and it’s > noisy again. > >> > >> An even more subtle issue can be capacitor temperature coefficient on > really long Tau filters. If C > >> changes (due to temperature fluxuations) faster than the settling time > of the filter, you get noise. Charge > >> is the same so delta C gives delta V. > >> > >> I *wish* I could tell you that was all purely theoretical. > Unfortunately it’s based on empirical data > >> collected in the “how could I be so stupid” fashion. > >> > >> Bob > >> > >>> On Aug 1, 2016, at 11:21 AM, KA2WEU--- via time-nuts < > [email protected] <javascript:;>> wrote: > >>> > >>> A good filter in the cable is highly recommended, 5 KOhm & 1000 uF > cleans > >>> many things > > _______________________________________________ > > time-nuts mailing list -- [email protected] <javascript:;> > > To unsubscribe, go to > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > > and follow the instructions there. > > _______________________________________________ > time-nuts mailing list -- [email protected] <javascript:;> > To unsubscribe, go to > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. > _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
