[EE] 24-bit A/D. Are We in the Twilite Zone Here?
Dan Michaels email (remove spam text)
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.
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
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
"Attack the Noise Gremlins That Plague High-Speed ADCs"
Best regards and thanks,
- Dan Michaels
See also: www.piclist.com/techref/io/atod.htm?key=a%2Fd
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