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Thread: [EE] 24-bit A/D. Are We in the Twilite Zone Here?
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face BY : Wagner Lipnharski email (remove spam text)



Yes, I thought the same, did several different kind of passive filters.
Used from pico to microFarad capacitors, different capacitors, diferent
resistors. Looks like the microVolts noise increase with some
capacitors... leaking?  Used a filter like this, no change at all.

      1k       1k                   1k       470
+5V----R---o----R-----o-----o----o---R---o-----R----o-----o--->
          |          |     |    |       |          |     |
         ===        ===   ---  ---     ---/       ===   ---
         ---        ---   ---  ---      A   1.2V  ---   ---
          |          |     |    |       |          |     |
         Gnd        Gnd   Gnd  Gnd     Gnd        Gnd   Gnd

       100uF      220uF  0.1uF  82pF            330uF  0.1uF

When you read the documentation you will understand why they offer it in
different noise levels, still the same unit, cost 3 or 4 times more.
The noise can reach low frequency, in some rare situations I noticed it
as low as 0.5Hz.  National website pdf file about the LM385
http://www.national.com/ds/LM/LM185-1.2.pdf state at page #4, two graphs
about noise, the second show a single pole low pass filter where the
noise is reduced from 60uV to 55uV at 100kHz noise, with a sharp cutoff
filter it is reduced to around 35uV... so no game, noise still there.
The low voltage noise is difficult to remove.

Wagner

Sean Breheny wrote:
{Quote hidden}

<3905087E.C3A495B0@ustr.net> 7bit

See also: www.piclist.com/techref/io/atod.htm?key=a%2Fd
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Subject (change) [EE] 24-bit A/D. Are We in the Twilite Zone Here?

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