[EE] 24-bit A/D. Are We in the Twilite Zone Here?
Russell McMahon email (remove spam text)
I've attached some hopefully helpful comments from my colleagues. who are
expert in this area. As you will read, they are achieving results that would
be very acceptable to you.
>From other worlds - http://www.easttimor.com
What can one man* do?
Help the hungry at no cost to yourself!
(* - or woman, child or internet enabled intelligent entity :-))
Further to our recent discussion re high-resolution ADC's, we have for some
time been manufacturing a universal digitiser based around an AD7730
sigma-delta ADC which achieves much better noise performance than your
PIClist colleage appears be observing.
Our digitiser operates as an addressable node on a four-wire (comms + power)
RS-485 bus running at 0.75Mbit/s and has digital I/O capable of switching up
to 400mA. A single LM2940 5V analog regulator takes the bus power in and
provides +5V out for digital and analog power and transducer excitation.
The on-board 8051-derived microcontroller runs at 24MHz. In addition to
accommodating mV-level bridge transducers, an on-board single-ended to
differential converter allows bipolar single-ended inputs up to 10V full
scale. The unit is constucted on a 2-layer PTH PCB (120x60mm) with a
ground-plane occupying most of the component side of the board.
For reasons I won't go into the ADC is run quite fast with a (pk-pk
noise-defined) resolution of about 15 bits and then additional filtering is
done by the microcontroller which improves this to about 18 bits. The
filtered 24-bit result is updated internally at about 400Hz but is normally
only read over the bus at about 20Hz. Input bandwidth is around 10Hz under
small-signal conditions, but large-signal transitions are tracked much more
rapidly using the FASTStep feature of the AD7730 and supporting features in
our own digital filtering.
With a good quality 350-ohm loadcell connected in a typical industrial
environment (most of these are used in the dairy industry) and using the
most sensitive range (10mV full scale) we typically achieve an
input-referred resolution (defined by pk-pk noise) of well under 0.1uV.
Note that this resolution is achieved even in the presence of substantial
mechanical vibration (a primary design requirement). Using a 120kg-rated
loadcell we get better than 1 gram usable resolution in the presence of
floor vibration which generates acceleration forces on the loadcell (when
weighing a 25kg bag of milk powder) equivalent to about 2.5% of the loadcell
capacity (about 3kg pk-pk).
The digitised result is impervious to significant local electrical noise, is
unaffected by the use of a cellphone within 1 metre, and shows no
perturbations when the digital outputs switch inductive loads (pneumatics
solenoids). The entire system (typically comprising 50 nodes) is powered
from a off-line switching supply.
Interestingly, during development, while we did not have any real problem
with noise limiting usable resolution, we did have difficulty approaching
the claimed drift figures for the AD7730 device. The problem turned out to
be due to high-frequency noise injection into the ADC front-end being
aliased to very low frequencies which were indistinguishable from offset
The point I am making is that it can be done (though in our case development
took about 4x longer to get it right that we originally budgeted for). I
don't know why your colleague might be having problems with noise (I haven't
used the LT sigma-delta converters myself), but has he taken the obvious
step of trying a second device to see if it behaves the same. ESD damage
will often manifest itself as poor noise perfomance in analog devices. Also
what happens if he shorts the input of the ADC (directly at the pins) ? Is
the reference noisy and does the input-referred noise level vary if the
reference voltage is varied ? Is the noise really analog (could the digital
comms with the ADC instead have a high bit-error rate). What is he using as
a signal source - could it be noisy ?
Hope my comments help.
See also: www.piclist.com/techref/io/atod.htm?key=a%2Fd
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