[time-nuts] Thunderbolt? (re simple gpsdo.) >> capacitors

Azelio Boriani azelio.boriani at screen.it
Fri Jan 6 18:07:08 UTC 2012


OK, note taken. Usually large capacitors are not available with tantalum
dielectric, it seems that the last is a 1000uF 6.3V.

On Fri, Jan 6, 2012 at 6:54 PM, Bob Camp <lists at rtty.us> wrote:

> Hi
>
> Oddly enough, if you use a tantalum electrolytic as C2, it's not very happy
> in that circuit. Their leakage with low voltages on them can be pretty
> nasty.
>
> Bob
>
> -----Original Message-----
> From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
> Behalf Of Poul-Henning Kamp
> Sent: Thursday, January 05, 2012 5:50 PM
> To: Discussion of precise time and frequency measurement
> Subject: Re: [time-nuts] Thunderbolt? (re simple gpsdo.) >> capacitors
>
> In message
> <CAL8XPmO76XuTETZC=33_v2YWuJGcw8gCvtTDHyae6E4MFb18=g at mail.gmail.com>
> , Azelio Boriani writes:
>
> >I have googled extensively trying to find something about the dual
> >capacitor method of reducing the leakage current... nothing found. Please,
> >can you indicate anything for me to learn more?
>
> It is very simple:
>
> R1 charges C1 to the DC potential and therefore C2 sees
> (almost) no DC voltage, which means (almost) no leakage
> current.  C2 is still a capacitor for any AC or dV component.
>
> I belive I picked this trick up from a datasheet or app-note relating
> to precision voltage references.
>
> Poul-Henning
>
> >> [Some op-amp]  >-+-----R2-----+-->
> >>                  |            |
> >>                  |          ----- C2
> >>                  |          -----
> >>                  |    ____    |
> >>                  +---|____|---+
> >>                        R1     |
> >>                               |
> >>                             -----  C1
> >>                             -----
> >>                               |
> >>                              GND
>
> --
> Poul-Henning Kamp       | UNIX since Zilog Zeus 3.20
> phk at FreeBSD.ORG         | TCP/IP since RFC 956
> FreeBSD committer       | BSD since 4.3-tahoe
> Never attribute to malice what can adequately be explained by incompetence.
>
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