Russell McMahon wrote:
.....
>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.
.......
>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.
>

Russell,

Thanks for going the extra distance and talking to your associate about
my problem. This entire thread has offered some really good advice for
successful application of 24-bit A/Ds. Unfortunately, the AD7730 won't
help me very much, since my system requires 1 Mhz BW, and the BW of
the 24-bit converters is a little too low - 10-50 hz. If I ever do get
into the 24-bit business, I'm going to come back and find this thread.
================
......
>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
>drift.
......

I find this observation pretty interesting. The front-ends of the
successive-approx A/Ds, like I use, tend to be quite wideband - the
LT1400 has a 400 Khz max samplg rate but 4 Mhz BW. So I think
aliasing of hi-freq pickup may be a problem - and I'm trying to
figure out how to deal with this other than just low-passing the
signal in. I don't have a drift problem, like your associate,
but hi-freq noise can still wrap around into the pass-band.

I found an article dealing with some of these issues, and am
perusing it:

"Attack the Noise Gremlins That Plague High-Speed ADCs"
http://www.elecdesign.com/magazine/1999/dec1799/analog/1217ao3.shtml

Best regards and thanks,
- Dan Michaels