Hi The answer is (as always) "no free lunch". An op amp simulating a capacitor is always going to be performance limited. Different limitations come in with different circuits, but they all suffer from noise / drift / leakage. Put another way - for a long time constant, you are just as good off using the op amp as a buffer and dropping in a giant resistor.
Bob On Jan 1, 2012, at 4:22 PM, David wrote: > Gyrators are usually used to create impractical inductances or > frequency dependant negative resistances but I suppose you could. I > do not think you would gain anything though since you would be trading > one set of non-ideal behaviors for a different set. This is > especially the case since the non-ideal behavior of inductors is > almost always worse than the non-ideal behavior of capacitors. > > For example, you can sometimes avoid large feedback or input > resistances by substituting a T-network but offset voltages and > voltage noise will be multiplied accordingly. > > On Sun, 1 Jan 2012 13:52:27 -0700 (MST), "Don Latham" > <d...@montana.com> wrote: > >> Aren't there op-amp circuits that create a large capacitance? The gyrator? >> Don >> >> David >>> Jim Williams did this in one of his designs for measuring low >>> frequency reference noise. The large value low leakage wet tantalum >>> capacitor he used was like $400 and it took 24 hours for the >>> dielectric absorption to settle: >>> >>> http://www.linear.com/docs/28585 >>> >>> You can get the necessary time constant using a good 1uF film >>> capacitor with good design and construction in this case. >>> >>> On Sun, 1 Jan 2012 15:11:04 -0500, Bob Camp <li...@rtty.us> wrote: >>> >>>> Any real world capacitor will have a dielecric with an associated >>>> insulation resistance. It's a "more money gets better performance" sort >>>> of thing, but there are indeed limits. A 1000 uF cap that has a "good" >>>> insulation resistance number might cost you more than some new cars…. >>>> >>>> On Dec 31, 2011, at 11:54 PM, Chris Albertson wrote: >>>> >>>>> On Sat, Dec 31, 2011 at 5:56 PM, Hal Murray <hmur...@megapathdsl.net> >>>>> wrote: >>>>> >>>>>> I think the main problem in this area is building a low pass filter >>>>>> with a >>>>>> long time constant. >>>>>> >>>>>> The time constant of the filter has to be: >>>>>> long relative to the noise from the phase detector >>>>>> short relative to aging of the oscillator >>>>>> short relative to environmental changes >>>>>> (so the osc can track temperature and voltage >>>>>> those changes may be in the PLL system rather than the osc) >>>>>> >>>>>> If we are starting with PPS (rather than 10KHz), the filter time >>>>>> constant >>>>>> needs to be 10s or 100s of seconds. How do I build an analog filter >>>>>> with a >>>>>> time constant that long? >>>>>> >>>>> >>>>> Time constant is just R*C. If you have a 1000uF cap and a 1K >>>>> resistor you >>>>> have 1 second. In theory you could build 100s just by using a 100K >>>>> resistor but I think real world components are not perfect enough. >>> >>> _______________________________________________ >>> 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. >>> >>> > > _______________________________________________ > 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. _______________________________________________ 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.
