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

Tom Handley wrote:
>    Dan, I'm currently working on a PIC-based DAQ system that uses the
> LTC2400. Others have mentioned noise problems and techniques with
> low-voltage measurement. I think the key here is that the LTC2400 family are
> extremely accurate converters that can provide very accurate measurements at
> lower resolutions. I'm using 16-18 Bits for display while averaging all the
> Bits. The software provides for calibration.
>    Wagner mentioned reference noise which is a good point that is sometimes
> overlooked. I'm using a MAX6341 4.096V reference with a 1ppm/C Tempco, 0.02%
> initial accuracy, and around 2.5uV of noise. It provides a pin for an
> external cap which augments wide-band noise reduction. With 24 Bits the
> resolution is ~244nV. As is pointed out in one of Analog Devices'
> applications books; 600nV is the Johnson Noise in a 10KHz BW of a 2.2K
> Resistor @ 25C. So, yes, this really is the "Twilight Zone" ;-)

Yes, I forgot to say, using a high voltage Vref, like 4.096, the silicon
noise is the same from the smaller ones, so the relation Vref/noise is
increased.  I did it once, even using 0.1% 10ppm matched pair resistors
(If I remember well) to reduce the Vref it worths, the final noise is
also reduced.

The Max62xx/63xx units are very steady ones, of course they need to
justify the cost above $10 :)

<3905E086.11BDFCBC@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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