[EE] 24-bit A/D. Are We in the Twilite Zone Here?
Dan Michaels email (remove spam text)
>I guess the next step is start breadboarding the thing one part at a
>time. Do the front end,,, no digital stuff at all, battery powered,
>running clean. Then start adding stuff.
Yeah, I do this with my s.w., when I have something I can't fix any
other way. With h.w., it's so much harder. I have done a lot of
testing in the past, mostly on the finished pcbs, removing other
chips, adding caps here and there, etc.
I think this is more a "systems-level" problem and has to be fixed
in place, so I've been going the route via first principles.
That is, I read everything I can find on low-noise analog and
high-speed digital design, and then incorporate those ideas into
the pcb design, etc, and then test and work with that. I think
sometimes the sum is greater than the parts [gee, sounds like
something *you* would say - yes?].
>Just a question: THe 2mV readed at the A/D output is a jumping noise or
>a steady noise, that can be confused with a/c or d/c A/D input offset?
>Another point is that I never use a 16 bits ADC to read 16 bits
>signals... I just ignore last 3 or 4 bits, so I pay more for a steady
Not an offset issue, I think, rather fuzz that overwhelms the lowest
2 LSBs. And for a 12-bit A/D I was hoping to get better than 9.5 bits.
>> The Vref on my LTC1400 A/D is inside the chip,
>> bypass caps outside. Noise measured via A/D binary
>> output, with front-end amp tied hard to gnd.
>What about short to ground the LTC1400 inputs? not just the front end
>amp inputs... You need to isolate and make sure where is the origin of
Yeah, I need to go back and do some more testing. Try to isolate
and correlate noise with possible sources - exactly. All the older
pcbs are gone now, and I am finishing a new layout, and so will
do more tests once I get those. Also, see above answer.
Thanks for the suggestions,
- Dan Michaels
See also: www.piclist.com/techref/io/atod.htm?key=a%2Fd
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