Re: Noise in tantalum SMT caps?



Spehro Pefhany 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?

There most certainly is and it has been characterized as 1/f^2 down to the millihertz region. From what I gather from the research papers available on the web, most of this noise is due to thermal instability around dielectric microleakage areas which self-heal via a chemical transformation of MnO2->Mn2O3. These are characterized by a gradual buildup of noise to a peak and then a substantial drop once the healing has occurred. Also the very construction of the things makes them a hotbed of tunneling and current trapping mechanisms as well as possessing a nearly shot noise effect due to a metal-insulator-semiconductor conduction mechanism from anode to cathode. And all of these things are not even close to stationary, varying all the time with a very slow trend into deterioration with aging. This is all transparent to the majority of applications requiring bulk capacitance at low ESR in small packages, but entirely visible as a reference for a 24-bit A/D conversion application.


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-(



.

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