I want to collect data from a hydrophone over a period of days or weeks. A desired typical collection period would be 30 days.

Hydrophones are a piezo-electric transducer that output millivolts (thousandths of a volt). A response from a hydrophone might vary from 0.001 Volt to 0.099 Volt, for example. The needed sampling rate is about 10 kHz. I want to avoid doing anything that would distort the signal. Signal purity is very important. I want to attach the recording mechanism right to the hydrophone in a water-tight box. I can make the housing, but I do not know how to record the data, or what could be used to record and digitize such a low voltage signal.

In general, the goal is to to collect and digitize the low voltage signal as purely as possible, minimizing any noise or distortion. Non-linearity in the hydrophone output is not an issue because I have calibration curves for the hydrophone that describe its frequency response in great detail. So, the data can be adjusted after the fact to equalize for the variations in sensor sensitivity.

The signals of interest are primarily between 30 Hz and 500 Hz. Signals between 500 Hz and 6000 Hz are interesting, but are of secondary importance, so if the system design would benefit significantly by ignoring them, then that is ok.

Cost for components I am hoping for would be less than $1500 (not including the hydrophone, battery and other stuff). In other words, the budget just for signal processing components and data storage would be $1500.

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    \$\begingroup\$ Do you have some specific hydrophone? \$\endgroup\$
    – Eugene Sh.
    Commented Jun 30, 2015 at 15:23
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    \$\begingroup\$ Well, no distortion at all isn't possible since your ADC will introduce some too. But why do you need to be so extreme? Isn't something like 0.0003% at 1kHz low enough distortion to work with? \$\endgroup\$
    – PlasmaHH
    Commented Jun 30, 2015 at 15:31
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    \$\begingroup\$ Let's turn this around. What is the largest amount of distortion you CAN accept? And please, "no distortion is acceptable" is not a valid answer. SOME distortion is always present, from lossy transmission to differential non-linearities in the ADC, to ADC quantization errors. So, just exactly how much distortion is acceptable, and why? \$\endgroup\$ Commented Jun 30, 2015 at 17:01
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    \$\begingroup\$ I have to ask you why, if you're so concerned with distortion, are you only sampling at 10KHz? Considering Nyquist, that means you can only get a potentially correct copy of sounds up to maybe 5KHz, but even doing that would require some pretty heavy filtering to keep out audible distortion from the higher human-audible frequencies going all the way up to 20KHz. It seems to me like you'd get far less distortion sampling at 160KHz through an el-cheapo amp, than sampling pure levels from a chip sitting inside your mics at only 10KHz. (unless you're looking for sub-audible frequemcies). \$\endgroup\$ Commented Feb 8, 2016 at 4:41
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    \$\begingroup\$ The "do not want to amplify" requirement is nonsense. \$\endgroup\$
    – Curd
    Commented Feb 9, 2016 at 21:56

7 Answers 7


Basic parameters

To summarize, you want to log a analog signal of up to 100 mV amplitude at 10 kHz sample rate. That's quite doable.

You want to collect data for up to "weeks". Let's see how much data that is. You didn't specify resolution or signal to noise ratio, but let's say 12 bits/sample, or 1.5 bytes/sample. That's 15 kBytes/second = 900 kBytes/minute = 54 MBytes/day = 380 MBytes/week = 1.5 GBytes/month. That's also doable.


I do not want to amplify the signal because that would introduce distortion.

That's just silly nonsense. Amplifying this signal can be done relatively easily without adding distortion at the level of 12 bits/sample. How do you think they take even smaller signals from air microphones and ultimately get to the volts or 10s of volts signals to drive HiFi audio speakers? This has been a well solved problems for many decades, even before there were transistors and opamps.

Basic design approach

I want to attach the recording mechanism right to the hydrophone in a water-tight box. I can make the housing, but I do not know how to record the data, or what could be used to record and digitize such a low voltage signal.

The basic strategy is to amplify the signal to maximize it within the voltage range of a A/D, then grab samples from this A/D and store them in a flash memory.

Distortion requirements

In general, the goal is to to collect and digitize the low voltage signal as purely as possible, minimizing any noise or distortion.

Without numbers, that's no spec at all. Surely there are other tradeoffs, like cost, space, and power, just to name a few obvious ones. Don't say "the best you can do" since that's meaningless. Best in what context? $1,000,000 budget? Surely you know something about the signal. Since you haven't given us any guidance, I'm going to proceed assuming 12 bits/sample is good enough, because probably it is.

Use a microcontroller

The process of grabbing samples from the A/D and stashing them in a flash memory would certainly benefit from using a programmable part like a microcontroller. Plenty of microcontrollers have 12 bit A/Ds built in, which makes things easy.

Amplifier parameters

It's not clear whether your "0.099 volts" maximum amplitude is RMS, peak, or peak to peak. For sake of example, I'll round off the "0.099 volts" to 100 mV and assume that's peak. That means the raw hydrophone signal goes from -100 mV to +100 mV, with a range of 200 mV.

The microcontroller will run from 3.3 V, and let's say you run the A/D from a separate 3.0 V reference to ensure it's nice and clean. You therefore want to expand a 200 mV range to a 3.0 V range, which means you need a gain of 15. Again, that is really no problem, and won't add any meaningful distortion or noise.

The amplifier will also contain a high pass filter, or "DC blocking" filter. You need to specify what your lowest frequency of interest is. For example, HiFi audio considers everything below 20 Hz unhearable. You are therefore free to throw it away if the end result is for human hearing. The DC blocking filter will also allow you to offset the midpoint of the amplified signal to the middle of the A/D voltage range, or 1.5 V. This is again easy to do electrically.

Sampling and aliasing

Since you are sampling at 10 kHz rate, the maximum theoretical frequency you can capture is 5 kHz. Any input frequencies over that will fold back to lower frequencies as "aliases". This is something you want to avoid. To do this, you need to eliminate frequencies above half the sample rate from the signal before you sample it.

Making analog filters sharp and accurate is difficult, but at your sample rates is not so hard digitally. Assuming you want flat frequency response close to the 5 kHz limit within reason, I would actually sample significantly faster, then filter and reduce the sample rate digitally. This is done often enough it has a special name, which is decimation. For example, you might sample at 100 kHz (still easy for 12 bit A/Ds in microcontrollers to do), low pass filter digitally so that 5 kHz and above is attenuated to oblivion, then pick 10 kHz samples out of the resulting stream.

This allows for a sloppy but effective analog filter before the 100 kHz sampling. For example, three poles of simple R-C filtering with rolloffs at 10 kHz attenuates by 125 (-42 dB) by the 50 kHz aliasing limit of the 100 kHz sample stream. Most likely your microphone and other physical things in your system will attenuate significantly at that frequency too. I'm assuming you don't have to worry about a full amplitude 50 kHz signal being received, so -42 dB at 50 kHz is good enough.


Once you have the 10 kHz sample stream, you have to store it somewhere. With today's technology, that is best done in a flash memory. You need less than 2 GBytes, so there are many options available. At that relatively small size, you can use flash chips directly, managed by the microcontroller. Some commercial memories, like SD cards, can be interfaced with over something like a SPI bus, which is something the microcontroller will have a built-in peripheral to handle. If you are willing to make your own device to read the data off this flash card, the you don't have to use a file system. Otherwise, you probably need to implement something like FAT32 with just a single file at a known location.

Power: The real problem

You need to consider the power requirements. Let's say you need 100 mA at 3.3 V to run all this, and that the power system from battery to ultimate consumption is 80% efficient. 100 mA at 3.3 V means you are consuming 330 mW. Taking the 80% efficiency into account means you need a battery that can supply 413 mW. That's 413 mJ/second, 25 J/minute, 1.5 kJ/hour, 36 kJ/day, and 1 MJ/month. This is your real problem, not amplifying or sampling or storing the signal.

Let's put this in perspective. First let's look at how big a lithium battery this implies. Assuming average 3.5 V out of the battery, you need (1 MJ)/(3.5 V) = 286 kAs = 80 Ah. That could be, for example, twenty 4 Ah cells bundled together. That's quite a bundle, and will be the bulk of the size and weight of your overall device, but it is at least within the realm of reasonably doable.

Again, you really should be focusing on the power requirements. That's your biggest problem, and where good engineering can make the most difference.

  • \$\begingroup\$ Thanks, very valuable analysis. I updated my answer with budget and signal frequency information. I was thinking that by using a direct-to-memory solution and avoiding a computer I could sidestep the power problem. Your suggestion is to use a microcontroller which would have a lower requirement than a computer or miniboard. I wonder if there is a configuration using discrete components that can get the data from hydrophone output to memory chip without using a full blown microcontroller (which has a power-sucking CPU on it). \$\endgroup\$ Commented Feb 11, 2016 at 19:09
  • \$\begingroup\$ microcontrollers can be very low power. \$\endgroup\$ Commented Feb 11, 2016 at 19:15
  • \$\begingroup\$ Here's an incredibly low-power MCU that can do everything suggested in this answer & had a 12-bit ADC capable of sampling @ up to 1MHz for anti-aliasing even up to 500KHz noise signals (which your piezos are highly unlikely to be able to pick up that high, anywise): digikey.com/product-detail/en/EFM32LG840F64G-E-QFN64/… \$\endgroup\$ Commented Feb 11, 2016 at 20:01
  • \$\begingroup\$ @Tyler: Without a micro you won't be easily able to oversample, low pass filter, and decimate. You still need some logic to manage the A/D and the writing to flash memory, which might be over SPI. All the little control logic and sequencing will add up. A micro will definitely be physically smaller, and probably only a little more power, but allow a lot more useful things to be done. \$\endgroup\$ Commented Feb 11, 2016 at 21:12
  • \$\begingroup\$ @Tyler: I just read your updated question. The frequency range is roughly what I was assuming, but you still haven't said anything about signal to noise ratio. The electronic parts shouldn't cost anywhere near $1500. They shouldn't even cost $50. \$\endgroup\$ Commented Feb 11, 2016 at 21:16

You need three things: an amplifier (sorry, unavoidable), an ADC, and some digital storage. With the quality of modern op-amps, you totally do not need to be concerned about noise and distortion; you can easily achieve >100dB SNR. Nonlinearities in the hydrophone itself will be a far worse source of distortion.

If you don't amplify the signal, you will be using only 100mV (of 2500mV total) dynamic range, which means you get about 11 bits of precision from a 16-bit ADC. Also, the ADC does NOT have a very high input impedance and it may load down the piezo output; you might get absolutely nothing. The amplifier stage is therefore critical to match the (high impedance, low level) source signal to the (somewhat low impedance, higher-level) input of the ADC.

Audio ADCs are commonly available and support 16-24 bits of precision. 8 bits is available too but I suspect would be insufficient dynamic range. Your 5kHz bandwidth tells me these are long-range signals and you need enough dynamic range to ensure that nearby noises do not saturate your input stage.

An audio ADC will expect an input of a couple of volts, hence the need for the amplifier. It sounds like you need probably about 20x gain, which is very achievable with a high impedance op-amp, with low noise and distortion.

You could consider an automatic gain control, but that will introduce distortions and make the output much harder to process.

You take the streaming ADC output (I2S, usually) from your ADC and stream it through a microcontroller into some flash storage, probably an SD card. 16 bits at 22kHz is 3.6GB/day; a 64GB card is pretty affordable and will give you 17 days of recording.

  • \$\begingroup\$ Add a low-pass filter (can be combined into the amp stage) to this, for eliminating some of the near-range high-frequency/volume interfering noises from your recording & I think this answer becomes you 'best case' solution, @TylerDurden. \$\endgroup\$ Commented Feb 11, 2016 at 19:41
  • \$\begingroup\$ I agree, a low-pass filter is necessary. If nothing else, you must exclude everything above the Nyquist limit. \$\endgroup\$ Commented Feb 12, 2016 at 2:59

You need to amplify the Piezo signals with a fairly high impedance amplifier, or a charge amplifier. I recommend using a good FET-input op-amp in a non-inverting configuration.


First of all, you have to specify the effective number of bits you want to capture. If you only really need 1 bit of resolution, then some comparator might do the job. But really, you must be interested in at least 8 bits.

You're saying that you don't want to amplify the signal to leave out any distortions. You might get away without it, but please realize that the ADCs themselves introduce distortions.

Now don't quote me on this, but I don't think ADCs in general like to operate very close to their rails. Here you're talking about a signal 0-10mV. I figure you'll have a hard time finding a suitable chip.

Another thing to consider is the impedance of your hydrophone. If it has impedance comparable to that of your ADC, you have a bad day, it will screw with your results.

So after all that, you really need to "isolate the impedance" of your hydrophone from the ADC. While at it, amplifying the signal is just a little step.


You are going to need a low noise amplifier. If you have money get one of http://emelectronics.co.uk/ some of the amps have 280dB of gain. And noise levels that approach low ohmic resistors (10 ohms). They also have low 1/f noise which would be useful to you if your signal of interest is below 10Hz


Most of the answers so far suggest parallel DACs (digital-analog-converters, eg. 12-bit@10khz), but I think that a 1-bit delta-sigma DAC might be a better fit for your project as the analog filtering circuit (nyquist theorem) is simpler, which means fewer components in the analog path and therefore probably lower distortion.

Pay attention to noise. Piezo-electric usually means "high impedance". If the impedance of the hydro phone is 1MOhm and your desired bandwidth is 5kHz, thermal noise voltage will be already about 10uV. This gives about 7-bit resolution for a 1mV signal (13-bit at 100mV).

Oscillator drift could be an issue, depending on your application. Typical oscillators are only 50ppm (worst case 130s drift per month).

[1] https://en.wikipedia.org/wiki/Delta-Sigma-Modulation


While expensive the AD7177-2BRUZ-RL7 should offer you the resolution you need to sample without using an amplifier.

In the datasheet in the absolute maximums area it doesn't list anything for the minimum reference voltage so if you make some assumptions of what your maximum voltage you are going to need to read is you can have a reference voltage of that to increase your usable resolution.

The datasheet indicates you get 19.3 noise free bits at 10ksps, so you'll need to discard the 13 least significant bits.

  • \$\begingroup\$ I am willing to pay 10x what this chips costs to get good performance. \$\endgroup\$ Commented Feb 8, 2016 at 12:55
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    \$\begingroup\$ You're looking in the wrong place for the minimum ref voltage. That isn't an 'Absolute Maximum' spec. You'll find it on pg5 of the datasheet clearly specified as 1V minimum. \$\endgroup\$
    – brhans
    Commented Feb 9, 2016 at 1:26
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    \$\begingroup\$ 10x ? Then you're not doing engineering, you're doing... something else. Your insistence on not having an amplifier is born of ignorance. Nor have you replied to others seeking specifics about your distortion requirements. @William Brodie-Tyrrell answer is the correct one & should be marked as such, otherwise you're just wasting everyone's time. \$\endgroup\$ Commented Feb 9, 2016 at 5:23
  • \$\begingroup\$ 1 / 2**19 = 0.00000190734 Still plenty of range. Still reading uV when he's talking about measuring in the mV range. \$\endgroup\$
    – HilarieAK
    Commented Feb 9, 2016 at 5:44

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