Digitizing a signal with small and much larger voltages

30KHz is very high, tough to design high performance with 24bit d-s, but if you don't need the full performance range then much easier and possible. Performance of d-s converters drops drastically as the bandwidth increases. Analog Devices do a couple of suitable bandwidth 24bit converters I believe. They also do competing seismic chipsets to Cirrus Logic, but they ain't as good (last time I evalulated them). Seismic specific market parts are optimised for 2KHz and under, and more importantly for extremely low current consumption, due to the need to have several thousand of them stringed together in serial cable systems.

Looks fairly suitable, although you have to go into a lot of detail with these things. Front end amp design is also another rather exotic area you have to get right.

Dave.

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Thank you once again for your reply, Dave. BTW, I'm also thinking about using another similar ADC, but with a lower sampling rate. The AD7766 from Analog Devices is a 24-bit ADC with a 109.5 dB dynamic range at 128 kSPS, which may also be okay for my application:

IMHO, apparently the best way to deal with front end amp design is by studying the evaluation kit that the company puts out. In general, if the company doesn't have an evaluation kit for the ADC (most do), then it probably isn't a good idea to use the part.

Probably the same way autoranging voltmeters do it. Sit with the system at high gain when nothing much is going on so you can digitise the noise floor with good resolution and decrease the gain by a factor of ten each time it (nearly) clips and vice versa when the signal drops below 1/10 of fullscale. You will lose a few samples in the switchover.

You probably do not want any funny log or A-law amplifiers in the signal path if there will be a spectrum of frequencies present with some high level fundamentals and low level components also of interest. If this is the raw data for some scientific experiment ask the experimenters what sort of signal they want and what if any conditioning is acceptable.

Linear time domain data is a lot easier to analyse for faint frequency components.

Regards, Martin Brown

Nicholas Kinar a écrit :

Well, we still don't know what your signal is and what you want to do with it. Do you want to process it in the time domain or frequency domain? That's not the same thing. For one, in frequency domain, the total noise is spread over the whole bandwidth, and detecting/measuring 0.5uV spectral lines is much more easier, since it'll be way over the noise floor.

Now, one other 'small' thing is you say you have at the same time, 0.5uV and 5V signals. That means you need better than -140dB THD for the full processing chain. At a full 30kHz BW? Hmmm,... still doable, but this requires being increasingly cautious, and if you're to ask all those questions, you're in some trouble there.

I think it's time for you to think more carefully about your signals, your needs and what you want to do with this. Engineering has to do with the limits of physics and just piling up requirements generally won't give you any sensible answer.

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Thanks,
Fred.

Thahks, Martin. I'm going to have to take a look at what is required. The autoranging voltmeter idea may be very useful.

Nicholas

Thank you so much for your response, Fred.

The challenge with doing this for scientific experiments is that sometimes you don't know all of the signal properties. It's a bit of trial and error. The signal is from a proposed experimental acoustics sensor which is designed to measure extremely low SPL. Just how low the sensor can go is a matter of debate. All that I have to go on at this time is a mathematical model. I'm trying to scope out what is required and perhaps find some clearer paths to travel.

Processing in the frequency domain is a good idea.

Of course, and asking this question on the newsgroup has indeed allowed me to think more clearly about what I am going to do, and how I am going to do it. I now know that it is possible to measure such signals, and I understand a little bit better the limits of current technology.

Or, have multiple ADC circuits, each one say 10x the gain of the last.

Dynamically select the one with the highest non-railed output (in software). That way you don't lose any samples and there is no switching hardware. Need to be careful with saturation recovery times, but should work OK at "audio".

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John Devereux

AD7760 ?