[EE] 24-bit A/D. Are We in the Twilite Zone Here?
Dan Michaels email (remove spam text)
Craig Lee wrote:
>You could also put little RCs at the source of digital signals to
>attenuate the high frequency components that are more likely to
>radiate. Requires that you know your maximum frequency to select
>the RC. (has this been said already?) ie. LSB changes fastest...
Yeah, good point. Thanks. I was actually thinking about this
after I mentioned yesterday that I have 2 DC control signals
going from the PIC over to the '4052 that select the 4 gains
in the analog section. The lines are bypassed at the '4052,
but a series R would probably help too, to bandlimit possible
hash. [what the heck - adds only a few more components to the
200 already on the board].
Sean Breheny wrote:
>I think you went past the BW consideration too quickly. There is a BIG
>difference between getting uV resolution in a 3HZ BW and in a 1 MHz BW.
>got 0.05 deg/sec noise floor in about a 50Hz BW (amp noise + gyro noise)
>which corresponds to 32uV of noise.
>final noise of 0.066 deg/sec for the whole system, which corresponds to 42
>uV. I obtained this on the first try, by just following the manufacturers
>recommendations and using careful layout and limiting BW when I could (I
>think this is key). I have an (AVR ;-) ) micro running on the same board,
16-bit, 42 uV, AVR inches away. "first try". Shoot, that's just
too easy!! How can you learn anything when it's so easy? :-).
Yeah, I realize BW = 50 hz vs 1 Mhz is a major factor. Noise voltage
is related as what, sqrt[BW1/BW2]. Yes ?????
So, 2mV * sqrt[50/1,000,000] = 2mV * 0.007 = 14 uV. Sounds good, but
something is still off. My 100 Mhz Hitachi scope has < 1 mV of
noise with the input dead shorted.
Tom Handley wrote:
> 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" ;-)
The largest resistor in my analog ckt is the 1M input on the 1st
stage, so Johnson noise for it at 1 Mhz = 0.12 mV. Well below my
2 mV measured noise. So, that's not a problem here.
Tom, from your experience, do you think the internal Vref ckts
found in [some of] the A/D converters are as noise-free/stable/etc
as the external Vref ckts you've been using???
Well, all in all, it's sure good to find out I'm the only guy
around here who can't get his stuff to work down into the uV
region. !!!! Hmmm, guess it's just a matter of finding that magic
configuration, afterwards which all is golden. Back to the lab.
Cheers and thanks for all the input,
- Dan Michaels
See also: www.piclist.com/techref/io/atod.htm?key=a%2Fd
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