Digitizing a signal with small and much larger voltages

[Nicholas Kinar]

Thank you so much for your response, Fred.

Well, we still don't know what your signal is and what you want to do
with it.

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.

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.

Processing in the frequency domain is a good idea.

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.

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.

 

[John Devereux]

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.

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".

 

[Jamie]

Here's maybe some other aspects of the required circuit:

(1) Bandwidth = 30 kHz (maximum)
(2) Sampling rate: appropriate sampling rate required to resolve arrival
time down to 1.6 microseconds
(3) Amplitudes: 5V (p-p) to 0.5 microvolt

I would wonder if there's a good ADC or other type of circuit to be able
to do these types of processing.

AD7760 ?