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

Bob Camp lists at rtty.us
Fri Jan 6 17:54:23 UTC 2012


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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