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

Thanks to Sean, Don, Craig who also responded here. Some
additional thoughts regarding my noise problem:

Grif wrote:
Have you tried moving the first op amp into a separated, shielded box, with
 Feed thru's on the
power into the box,,, and signal in and out of the box. and a separate
regulator, make sure you don't let any left over noise on the input side to
the regulator couple to the ground or output of the regulator.

Thanks, for the many suggestions. I think I have already incorporated
most of them. However, thermal control - not on this thing!! Also, the
front-end amp isn't separately shielded, but is as far from the digital
cktry as possible, has gnd planing under all the chips/Rs/etc, has the
EMI-shielded ABS box overhead, consists of 2 inverting amps in series,
has separate V regs and busses, low-Z 2nd amp, etc.

The one thing I might try on the new layout is returning the bypass
caps on the analog V-reg hi-sides straight back to the power entry
point. They return to the analog gnd buss now. Might help.

Wagner wrote:
>The biggest problem I found years ago with low voltage is really noise
>in voltage reference generators.
>the twilight zone, doomed, alone, in total fear, no support, no
>experience around, no books, no fancy "do it by yourself multicolored
>animated gif playing at your screen"... just plain despair!
>PS: Just try and turn off the PC and that fluorescent lamp over your
>bench, and see if the 2mV disappear... :)

This was good info you provided on your experiences with V-references.
[And I'm gonna ignore all that "total fear" & "plain despair" talk.
Sun's out today. I need to get < 1 mv noise, before worrying about
10 uV. :-)].

Considering I am using the LTC1400 with builtin Vref, there isn't much
I can do in this arena, except external bypassing, heavy traces, etc,
which I do now. I would think the 1400's Vref would be commensurate
with 12-bit ops.

Is it your experience, Wagner, that onboard Vref is worse than
external? After all, the A/D's 6.4 Mhz internal SPI cktry is only
microns away.

I did discover the PC sends piles of hash out the RS-232 line, and
of course, the monitor is a great broadcaster. I did some controls for
this, added a ferrite and shielded cable to the RS-232 line, and started
using EMI-shielded ABS cases [huge improvement, as noted before].

I probably should go thru another round of testing, to try to localize
the noise source(s) better. It is pretty broadband, but thinking about
it, I should do more scope/triggered/source_correlation measurements,
since end-to-end measurements suffer from aliasing in the A/D.

Anyone have any ideas regards the great noise reduction I see
when the EMI case is put on?

I do think this is shielding LOCALLY-generated noise, rather than
external. I presume "radiated" E-noise, because conducted and
H-noise would not be affected as much by the shield. Correct?

I do know that the shielding greatly cuts radiated E-noise from
the 7662 and local VCO chip [those sources were already dealt
with, BTW, via gnd planing and separating gnds, and easy to
measure since can be correlatd]. I do not believe I have any
serious inductive loops in the "digital" section - the power
and gnd busses and signals all run pretty much in close
parallel formation there.

Please don't anyone suggest "multi-layer" pcb for this project.
I'll have to accept 2 mV noise over that option.

- Dan Michaels
Oricom Technologies


See also: www.piclist.com/techref/io/atod.htm?key=a%2Fd
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