Re: "zero" biased polarized coupling caps

On Fri, 19 May 2006 20:22:00 +1000, the renowned Adam S
<not.valid@nosuchaddress> wrote:


I've have some cascaded op-amp stages, each providing +20dB, and worst
case DC offset is +-30mV at the output of each stage.

I want to AC couple each amplifier stage with polarized capacitors. I
cannot find any documentation recommending the use of polarized
capacitors for continuous reversed bias, even when the bias is very
small (< 50mV).

However there is the following PDF from kyocera,
http://www.kyocera.co.jp/prdct/electro/pdf/technical/revtant.pdf

Hmm. AVX (nee Aerovox) is owned by Kyocera (Kyoto Ceramics) eh?
Did not know that.

basically saying tantalums should never be designed in circuits where a
continuous reverse bias exists and since they fail to present a
definition of revers biased I find it hard to believe a microvolt is
going to do harm them. So whats the truth about using tantalum's in such
applications ?

I've used aluminum electrolytics in such applications with no
problems, in volume applications over many years. Tantalums- I have a
bad translation from Japanese that seems to say (not what it says, but
it doesn't make sense the way they wrote it) that for "short periods"
(not defined) 10% of rated voltage or 0.5V max is okay. Which fits
with AVX pdf.

I'd guess (which is worth what you paid for it) that -50 or -100mV
should be okay over temperature and time, and -30mV should be quite
safe.


Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
speff@xxxxxxxxxxxx Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com
.

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