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 its 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 its >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 <time-nuts@febo.com> >> wrote: >> >> A good filter in the cable is highly recommended, 5 KOhm & 1000 uF cleans >> many things _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.