Re: Noise in tantalum SMT caps?

On Fri, 18 Jan 2008 21:51:26 -0500, Spehro Pefhany
<speffSNIP@xxxxxxxxxxxxxxxxxxxxxxx> wrote:

On Fri, 18 Jan 2008 18:12:34 -0800, the renowned John Larkin
<jjlarkin@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote:

On Fri, 18 Jan 2008 16:29:19 -0500, Spehro Pefhany
<speffSNIP@xxxxxxxxxxxxxxxxxxxxxxx> wrote:

Any idea what noise I'd see in a tantalum 150uF cap with 1.25V across
it?


Best regards,
Spehro Pefhany

I'd guess just the thermal noise of the esr, which is only a couple
tenths of an ohm maybe.

John

No sub-10Hz voltage noise as aluminum electrolytics under bias are
supposed to have?

Dunno. I recently tested some standard alums and some tants, posted
elsewhere, and the alums had serious chemical things going on inside;
the tants and polymer aluminums much, much less.


I'm looking at a design (not my own) where there's an RC on the output
of a voltage reference feeding a Sigma-Delta 24-bit ADC. The R is
causing a full scale gain error of, oh, 17bits. 8-( Over 1%.

Is the reference input current constant with input voltage?
Temperature? I doubt it. 8-(


It certainly varies as a function of adc activity, which can vary in
some systems, even for delta-sigmas.


Well, the cap-versus-temp bahavior of a tantalum isn't great. So if
there were standing DC on it, as there is here, minute temperature
fluctuations will modulate C and, since CV is conserved, V would
wobble around. The same thing would happen on hi-K ceramics.

The R is maybe a bad idea, if unbuffered by an opamp.

John


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