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Thread: [EE] 24-bit A/D. Are We in the Twilite Zone Here?
www.piclist.com/techref/io/atod.htm?key=a%2Fd
picon face BY : Russell McMahon email (remove spam text)



Dan,


1.    It's vital that your op-amps have good gain at the common mode
frequency.
ie - even if you are amplifying a dc or low frequency signal, if you have
digital (eg clock, uP data, SPI signals etc) appearing as common mode
signals then your op-amps MUST be able to reject these actively . ie you
should be using amps with a 10MHz plus gain bandwidth and really the more
the better! A low gbw opamp will pass the common mode signals as noise in an
uncontrolled way.

2.    Consider using parts like the AD7730 or the newer AD7731 with internal
gain switching. I haven't used these personally but they are very well
spoken of by friends of mine with much experience in this area.

3.    If I was doing this (and I'm glad I'm not just at present :-)) I would
consider building a minimum component configuration just to see how good I
could make it with the IC used.
eg

- No gain switching 4052
- No potentially noisy adjacent 766x adjacent.
- Power rails sourced from some rather external stable source with copious
local filtering.
- SPI control chip farish away with decoupled well shielded control lines.
- House in ferrous box.(need not be pretty or fancy.*
- etc

ie get rid of everything possible

Once you are sure of how well you can make the simplest possible circuit
perform you can then extend it and see where it starts to degrade.


* For "proof of principle" in suspect environments I have used light "steel"
element covers intended to make range tops look prettier when elements are
not in use. These make a nice flat steel sheet with a turned up edge and 2
can be nested (bottom one upside down) to make an easy open enclosure. Cut
holes, use feed throughs etc as required You do have a complete annular open
ring when these are nested but it is vanishingly small - especially if you
put a weight on top. Aint pretty but it helped me.

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     Russell McMahon
_____________________________

>From other worlds - http://www.easttimor.com
                               http://www.sudan.com

What can one man* do?
Help the hungry at no cost to yourself!
at  http://www.thehungersite.com/

(* - or woman, child or internet enabled intelligent entity :-))


{Original Message removed}
<013001bfaf58$0c0fad00$633661cb@mkbs> 7bit

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