Digitizing a signal with small and much larger voltages

Hello--

I'm working with an experimental acoustics sensor which will produce voltages ranging between 0.5 microvolt (minimum) and 5 V (peak to peak). What is the best way to digitize this signal? At first blush, it appears that a 24-bit ADC would work quite well for this type of application, since (5V)/(2^24) = 3E-7 Volts. However, would it be better to do the following?

(1) Convert the voltage signal into a current using a transconductance amplifier.

(2) Use a logarithmic amp to find the logarithm of the current. The AD8304 from Analog Devices is a 160dB (100 pA to 10 mA)) logarithmic amp used as a photo-diode detector:

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(3) Digitize the voltage output of the AD8304 log amp using an ADC.

BUT how do I determine the number of bits of my ADC? The logarithmic amp will take the logarithm of the signal, but how do I choose the number of bits of the ADC so that I can adequately measure the range of voltages?

OR is it best to simply use a 24-bit ADC?

Nicholas

Reply to
Nicholas Kinar
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Take a look at Cirrus Logic's Geophysical products, designed for ultimate performance in the acoustic arena:

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Their 24bit Delta-Sigma converters are the ducks guts. Performance can be had beyond the datasheet typical values too.

What's your bandwidth?

Dave.

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Reply to
David L. Jones

What is the bandwidth?

What kind of accuracy are you looking for?

None of the 24-bit converters deliver true 24 bit performance.

The true dynamic range of 140dB requires very carefull design. Amateur approach is not feasible.

Vladimir Vassilevsky DSP and Mixed Signal Design Consultant

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Reply to
Vladimir Vassilevsky

Great stuff, Dave. These ADCs seem to have excellent performance. I can think of a number of applications for these particular ADCs which would be extremely useful in environmental sensing applications and environmental physics.

I've calculated that the maximum bandwidth for my particular sensor would be 30 kHz. However, for time-of-arrival estimates and digital filtering, the sample rate of the ADC must be much greater than Nyquist. I would wonder if this ADC would fit the bill:

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Perhaps the 625 kSPS rate is too much, but I like the fact that the ADC has an SPI bus, and that the maximum sampling rate is at least 5x to 10x greater than what might be required.

Nicholas

Reply to
Nicholas Kinar

I've calculated a maximum bandwidth of at least 30 kHz.

At least 16-bit accuracy. But how would I deal with the range of signals? Perhaps a log amp would be worthwhile to use?

Of course, since there will always be noise. However, by oversampling and decimation, it may be possible to obtain close to 24-bit accuracy. This implies that I would need to choose a much larger sampling rate.

Nicholas

Reply to
Nicholas Kinar

Achieving the 140dB of the true dynamic range in the 30kHz of BW is rather non-trivial if feasible at all.

Log amp -> the accuracy of 1% or so.

If you can split the 140dB into 2..3 gear shifts with the different gains, that simplifies the problem immensely.

Nope. The performance of the ADCs gets worse with the faster sample rate. This effect outweighs the gain from the oversampling.

Vladimir Vassilevsky DSP and Mixed Signal Design Consultant

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Reply to
Vladimir Vassilevsky

Thanks, Vladimir.

Could you clarify what is meant by "gear shifts"?

Reply to
Nicholas Kinar

Caution: d-s ADCs often have analog bandwidths well below what you'd expect from the rate at which they deliver data. They do a lot on internal digital filtering.

John

Reply to
John Larkin

Thanks, John. I took a look at a few 24-bit ADC datasheets and found that the digital filter can greatly restrict the passband. This information has made me take a more critical look at these ADCs.

Reply to
Nicholas Kinar

Maybe; or maybe the sensor will deliver good signal into a short circuit load.

Never. Your acoustic signal is AC, converting to current it's still AC, but the log amp will never take the logarithm of a negative number correctly. Good only for positive definite input.

What you want, is a variable gain amplifier that takes a gain control voltage so that voltage (or power) gain is proportioinal to exp(Vprogram), and feed AC into the amplifier and take AC at the correct range for digitization from the output.

Like a floating point number, the gain control voltage is the exponent part, and the ADC reading is the mantissa part.

Reply to
whit3rd

Nicholas Kinar a écrit :

What accuracy do you aim at on the 0.5uV signal?

One thing is good to have in mind: WRT your 30kHz BW, if one guess a

30kHz brick wall filter, which I read you don't want because you want to do some time of arrival measurement, 0.5uV rms noise is less than 3nV/rtHz input referred noise. Doable but you begin to need to be careful. Now if you want some meaningful measurement and you want 0.15uV rms noise, so that your 0.5uV signal is detectable, that is less than 0.9nV/rtHz. Still doable but you have to be increasingly careful.

If now you throw in your increased BW requirement, or you want more accuracy, then, well, hem... Unless your signal is repetitive and you have a trigger to sync on, in which case averaging is possible.

WRT your dynamic range, can't you have some PGA front end? That'll ease your unrealistic ADC requirement.

One possibility, if you can't switch gains, is to have several gain stages working in parallel, digitize all of them at the same time and select the right one in digital domain, with "a bit" of calibration and post processing.

I've seen much more acrobatic processing done with good results.

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Thanks,
Fred.
Reply to
Fred Bartoli

Thanks, wit3rd! That's a great idea, and would work quite well to be able to vary the gain based on the amplitude of the signals which are measured.

Perhaps some sort of DSP could be done to predict when the time-varying signal is increasing in amplitude, and the gain could be dynamically adjusted, thereby eliminating clipping.

Reply to
Nicholas Kinar

Thanks Fred!

Measuring the presence of a 0.5uV signal. Any better accuracy would be great.

Sure, perhaps what could be done is to increase or decrease the gain as the signal changes. But I suspect that there will also be components in the signal which will be measurable at much larger voltages, and I would also like to measure the smaller voltages at the same time.

Very neat idea, Fred. I would wonder if it would be possible to then combine all of these gain stages into one signal, rather than selecting the right one.

Reply to
Nicholas Kinar

Would you know of an example schematic or circuit that I could look at?

Reply to
Nicholas Kinar

The signal consists of a number of frequency components. Some frequency components have larger voltage amplitudes (5V p-p), whereas other frequency components have smaller voltage amplitudes in the microvolt range.

Reply to
Nicholas Kinar

But I would also suspect that the frequency components can take on larger and smaller voltage amplitudes, so using separate analog band-pass filters for specific frequency components may not work.

Reply to
Nicholas Kinar

Here's another post placed elsewhere which deals with saturation at higher amplitude levels:

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Here's an excerpt from the post located at the above address:

"Two reasons come to mind. You will see more quakes with a more sensitive instrument. You can not really compensate with more amplification because the dynamic range is smaller with the low bit count devices. Yes, you can raise the amplification level so that an 8 bit device will respond to the same voltage signal that the 24 bit device will see, but the 8 bit device will be saturated after only 256 counts while the 24 bit device would have only recorded 256 counts out of 16.7 million possible counts."

I think that this might explain that I need to use a greater number of bits to be able to detect small and large signals.

What is the best way to do this, given the bandwidth and sampling rate?

Reply to
Nicholas Kinar

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.

Reply to
Nicholas Kinar

There's an ADC converter board offered by this company:

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Here's what the system can do:

"The new PS2400 analogue-to-digital converter (ADC) board from Earth Data Ltd, part of the Southampton-based Kenda Group, provides three independent channels of 24-bit conversion and offers true 24-bit dynamic range. It can resolve signals from

1µV to ±8V in its normal gain mode or 50nV to ±400mV in its high gain mode."

But how to attain true 24-bit accuracy? Are there some ADCs capable of attaining true 24-bit accuracy?

Reply to
Nicholas Kinar

A DLVA (detector/log video amplifier) might work, if your accuracy requirements are modest (say 0.5 dB).

Cheers

Phil Hobbs

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Phil Hobbs

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