Low noise Hydrophone Preamplifier - Page 1
Forum section: electronics / projects

author: madr
author_rank: Contributor
timestamp: March 05, 2025, 10:09:58 am
content: I am currently developing a low noise preamp for a new piezo sensor that should be used as a hydrophone. Since I am actually from the signal processing domain the topic was quite hard to get into for me. The sensor is relatively new and not well characterized. The only specifications I have are the following:- Capacitance of 7nF- Voltage of about 100 to 500 nV (no information of the type of excitation given so far)- D33=5.7 pm/VAs a ADC and DAC I am using consumer sound interfaces like the Steinberg UR44C, Focusrite Scarlett 4i4 or an RME Fireface UC. Here are the (from what I think) most relevant parameters:- Max. Input Lvl.: +10 to +22 dBu = about 2 to 10 V- Input Impedance: about 10 kOhm- Max. Output Lvl.: +15 to +20 dBu = about 4 to 8 V- Output Impedance: 75 to 430 Ohm- SNR: About 110 dB according to their datasheets - might be a little bit lower in practiceThe signals I desire to obtain can go up to 100 kHz.Coming from the ADC side I would by that think, that *theoretically* I could measure signals down to a couple micro volts - right?As a possbile solution many sources are recommending charge amplifiers for piezo sensors. My current design of the amplifier consists the following "idea": One charge amplifier directly and closely connected to the actual sensor amplifying the signal by 26 dB to then amplify this signal with a voltage amplifier with a gain of 44 dB - makes 70 dB total.schematic.pngWhen I simulate this I get the results desired: Flat frequency response from 10 Hz (cutoff set with the highpass) to 100 kHz.Spice prompt: .ac dec 100 0.1 100kacanal.pngSo I made the PCB and evaluated the amplifier isolated from the sensor element. I do this by connecting the sound interface in series with a capacitor of 7 nF to the input of the preamp and put the output of the preamp in the input of my sound interface. To characterize the system I use white noise, take the PSDs of in- and output signals and look if they are getting distorted by the amp. They don't look as in the simulation but they are pretty flat in my opinion.psd.pngThe purple psd is hereby obtained by picking up the signal after the charge amp and the magenta psd by pickung up the signal at the overall output of the preamp. The solid line is the signal (white noise) and the dashed line is the measured noise. The absolute y scaling is wrong but relatively it should at least be correct.When connecting this to the sensor I get a lot of noise and distortion. But since that would also be a lot to explain here I'd like to focus on my approach with this problem and my circuit for now.The overall question is roughly: How can I improve this circuit?Meaning:1. Do you see any errors regarding my basic approach to this circuit?2. Are there fundamental errors in the circuit design?3. What measures can I take to make the circuit more robust against interference/lower noise?4. Any other remarks/ tips? Anything I need to reconsider?Your help is highly appreciated.

author: moffy
author_rank: Super Contributor
timestamp: March 05, 2025, 11:31:28 am
content: This is very similar to something I worked on, a low noise current probe amplifier. The dominant noise was the 1/f noise of the opamp, both voltage and current, because of the low frequency response, 10Hz. All measurements had to be done in a shielded enclosure, and care had to be taken to ensure there are no drafts causing thermal noise. The input signal wires were particularly susceptible to noise. It didn't however use a piezo sensor input, so the use of a JFET low noise opamp makes sense, because of the high input impedance needed, so the LT1113 looks reasonable but there might be slightly better. It is also adviseable to get the max gain out of the first stage but then Gain Bandwidth Product(GBW) is a factor.Something like the JFE150:https://www.ti.com/lit/ds/symlink/jfe150.pdf?ts=1741110503866&ref_url=https%253A%252F%252Fwww.ti.com%252Fpassive-discrete%252Ftransistors%252Fproducts.htmlcould offer an improvement in noise performance.

author: mtwieg
author_rank: Frequent Contributor
timestamp: March 05, 2025, 01:20:21 pm
content: I'm guessing the "sound interface" you're referring to is functionally the same as a VNA. It's important to characterize the contributions of your test equipment to the noise (both at the input and output). What do you see if you disconnect the source (i.e. short out V1 in your schematic) while just measuring the output noise? Is that what you're showing with the dashed lines in the plot?The y axis in the psd plots doesn't make sense. If it's in dB/rt(Hz) then that implies it's noise spectral density, not power spectral density (which would be something like dBm/Hz). But "dB" doesn't give any sense of absolute scale, should be something like "dBV" or "nV", etc.Not sure why you say you're getting "a lot of noise and distortion", it's hard to tell what's going on without knowing more about the test equipment itself (except that the second stage has around 44dB as expected).Also you may want to check whether your power supply noise is leaking into the signal path. I would try powering it directly from batteries to see it that has any effect.I'm curious what U2 is doing in your schematic. I've seen transimpedance amps with amplifiers in the feedback network before, but not quite like that.

author: David Hess
author_rank: Super Contributor
timestamp: March 05, 2025, 04:13:36 pm
content: Why did you select the piezo accelerometer amplifier to work from rather than the low noise hydrophone amplifier on the previous page?

author: richnormand
author_rank: Unknown
timestamp: March 05, 2025, 11:11:05 pm
content: Built this one several years ago.Works well but I did not check the ultimate frequency response.In addition to the cylindrical pzt I also changed the geometry of the sensor head with a planar disk piezo. Works even better at high frequencies.I used a small digital recorder that has a variable playback speed to down shift the high frequency hiss from a leak with great success.https://www.instructables.com/Lets-Build-Some-World-Class-Hydrophones/

author: moffy
author_rank: Super Contributor
timestamp: March 06, 2025, 03:16:51 am
content: OP posted same question here:https://forum.allaboutcircuits.com/threads/low-noise-hydrophone-preamplifier.205669/

author: madr
author_rank: Contributor
timestamp: March 06, 2025, 08:04:50 am
content: @moffy Is that a bad thing? If yes, please tell me why (honestly curious). I started even here:https://electronics.stackexchange.com/But there weren't really answers coming in so I started seeking help somewhere else. If this is bad practice let me know.Anyway thanks for your answer. Please excuse my longer time to answer yours but since I am not really experienced in electronics I need to at first comprehend your replies.Quote from: moffy on March 05, 2025, 11:31:28 amWe do have electromagnetically and acoustically shielded enclosures for characterization purposes but we won't have them in the real measurement use case.Quote from: moffy on March 05, 2025, 11:31:28 amThe sensor element is placed on a small rectangular frame on the pcb and connected to the preamp with little wires bonded to the cathode/ anode and solder pads on the pcb. I'd assume this is probably not the best setup in terms of noise but the material scientists told me that there is no really good other way to connect the sensor to the pcb.Quote from: moffy on March 05, 2025, 11:31:28 amIn terms of gain I could theoretically go up to approx 35 dB since I need a bandwidth of 100 kHz and LT1113 has a GBW of 5.6 MHz. I only used 26 dB beacause I was told that it's better to have "not so much gain" on the first stage and then boost the signal in the second one. Was that a bad tip?Quote from: moffy on March 05, 2025, 11:31:28 amThanks for the suggestion. Tbh I don't know how to properly use JFETs especially with piezos yet so I have to look that up first before I can really respond to that.

author: madr
author_rank: Contributor
timestamp: March 06, 2025, 08:21:52 am
content: @mtwiegQuote from: mtwieg on March 05, 2025, 01:20:21 pmThe sound interface(s) I'm using are normal sound interfaces like Steinberg UR44C, Focusrite Scarlett 4i4 or an RME Fireface UC. The setup is always the Line Output of these soundcards connected an equivalent capacitor of 7nF in series and then connected to the input of the preamp. The output of the preamp is then routed to the input of my soundcard. I measure the input signal with no output signal (which is in my opinion the noise of the DAC Noise+Sensor Noise+Preamp Noise) - in my plots the dashed line - and the input signal when I output white noise - solid line.Quote from: mtwieg on March 05, 2025, 01:20:21 pmYeah I'm sorry. I estimate the psds with the following command in MATLAB:Code:pwelch(ppfHydIn , flattopwin(iNfft), [], iNfft, fSamplerate, 'psd')With no overlap, iNfft = 8192 and fSamplerate = 192 kHz.Quote from: mtwieg on March 05, 2025, 01:20:21 pmYou're right. I'll do some more measurements and get back with some more meaningful results on that phrase...Quote from: mtwieg on March 05, 2025, 01:20:21 pmI already checked that but it seemed like there was no big difference. Of course the 50 Hz are more dominant but distortions below 1 kHz are fine for me since I am rather using everything above that. We don't have transducers to produce such low acoustic frequencies underwater.Quote from: mtwieg on March 05, 2025, 01:20:21 pmI got that from the LT1113 datasheet. LT uses these type of circuit on almost every of their example circuits. It's a DC Servo which is supposed to suppress the DC offset voltage.

author: moffy
author_rank: Super Contributor
timestamp: March 06, 2025, 08:25:00 am
content: Quote from: madr on March 06, 2025, 08:04:50 amPersonally, I am less likely to invest time in a problem, if the poster is unresponsive, happens a lot where posts are not followed up by the original poster. Or if the OP tries a scatter gun approach posting in multiple forums, which leaves the people trying to help with half the information. But that's just me personally, I mention your other post so that others are aware and can make of it what they will.

author: madr
author_rank: Contributor
timestamp: March 06, 2025, 08:30:24 am
content: Quote from: moffy on March 06, 2025, 08:25:00 amOkay I get that. But like I said - in the other forum there was nobody really replying so I thought I'd give it a shot at other places (eevblog and all about circuits). I never really used forums much so I didn't know where my question is placed the best. But as you see - I answer ASAP and appreciate every help.

author: madr
author_rank: Contributor
timestamp: March 06, 2025, 08:31:27 am
content: Quote from: David Hess on March 05, 2025, 04:13:36 pmI was told that charge amplifiers are better for piezo sensors like the one I want to connect. So I sticked with this one. Just looking at their specification of output noise it also looks like the Accelerometer Amp is better (6nV/rt(Hz) vs 128nV/rt(Hz))?

author: madr
author_rank: Contributor
timestamp: March 06, 2025, 08:36:30 am
content: Quote from: richnormand on March 05, 2025, 11:11:05 pmI actually baught one of those but it was pretty expensive to ship etc. and I don't know why and how but it seemed damaged since no matter what I used as input and which signal I used to excite it was just outputting highly distorted signals. So I didn't order (or rebuilt a new one) yet.

author: David Hess
author_rank: Super Contributor
timestamp: March 06, 2025, 09:41:48 am
content: Quote from: madr on March 06, 2025, 08:31:27 amThe noise of the two designs is the same once gain is taken into account. The hydrophone amplifier has a voltage gain of about 26dB which when multiplied by its input noise of 6nV/SqrtHz is 128 nV/SqrtHz.

author: SteveThackery
author_rank: Super Contributor
timestamp: March 06, 2025, 10:25:28 am
content: Quote from: madr on March 06, 2025, 08:31:27 amI, too, am interested in building a hydrophone amp.@madr: could you tell me what document you are referring to? Thanks.

author: moffy
author_rank: Super Contributor
timestamp: March 06, 2025, 10:34:27 am
content: Found a nice article describing a low noise amp using the JFE150:https://www.ti.com/lit/an/slpa018/slpa018.pdf?ts=1741056995232Since the accelerometer signal preamp is integrating the charge, its output is quite different to the output of a hydrophone preamp which is non integrating, unless I am missing something.

author: mtwieg
author_rank: Frequent Contributor
timestamp: March 06, 2025, 12:21:29 pm
content: Quote from: madr on March 06, 2025, 08:21:52 amOk, I'm not very familiar with "sound interfaces" but it sounds like a VNA.QuoteDon't forget the sound card (i.e. the receiver) will also contribute some noise as well. To properly characterize the noise performance of your DUT, need to characterize this.QuoteOk, so solid line is approximately gain (assuming the injected noise is far greater than the noise floor of the DUT and receiver noise).QuoteShould have noticed you mentioned the incorrect y-axis in your OP, no big deal.QuoteSo what specifically are you looking to measure here? Are you trying to derive a noise figure/factor for your preamp, or some other sort of metric? In the case of noise factor, if the source impedance is purely imaginary (because the source is a perfect capacitor) then noise figure will basically be infinity. Need a more realistic model of the transducer in order to get a sensible result.QuoteNeat, I'll check that out.

author: madr
author_rank: Contributor
timestamp: March 06, 2025, 01:41:56 pm
content: Quote from: SteveThackery on March 06, 2025, 10:25:28 amThe datasheets of most LTs ICs have example circuits for low noise hydrophone preamps in them:https://www.analog.com/media/en/technical-documentation/data-sheets/1113fc.pdforhttps://www.analog.com/media/en/technical-documentation/data-sheets/1792f.pdfI've tested them on other conventional pzt hydrophones like the Reson TC4033 and there they worked pretty good.

author: madr
author_rank: Contributor
timestamp: March 06, 2025, 01:45:52 pm
content: Quote from: moffy on March 06, 2025, 10:34:27 amI'll have a look! Thank you. Might take a day or two till I'll be able to really respond to this.

author: madr
author_rank: Contributor
timestamp: March 06, 2025, 01:55:38 pm
content: @mtwiegQuote from: mtwieg on March 06, 2025, 12:21:29 pmThey are nothing more than AD/DA converters with different inputs and outputs. However, they are mainly designed for the audio range, which is why they are usually not the best choice for unit-accurate measurements in voltage or current. VNAs would probably be better here for analysis. However, for the application we need arbitrary signals.Quote from: mtwieg on March 06, 2025, 12:21:29 pmI also did some "loopback" measurements with the sound cards but the measured noise was always below the noise of the preamp. In my opinion thats what I want right? Otherwise I would give away SNR, right?Quote from: mtwieg on March 06, 2025, 12:21:29 pmMy overall goal is to maximize SNR of the sensor when it is connected to the preamp and submerged in water. I already did measurements that led me to the conclusion that the preamp noise is still too high (or the sensor signal is just too small since the piezo is really not that sensitive...). But those measurements arent something I could really show off here till now, since they'd probably confuse everyone (because I have to make them more understandable).

author: mtwieg
author_rank: Frequent Contributor
timestamp: March 06, 2025, 02:13:02 pm
content: Quote from: madr on March 06, 2025, 01:55:38 pmRight, I'm assuming "the application" needs specific audio equipment. For characterizing noise, the sound interface might be sufficient, so long as it's used properly.I would try just measuring the preamp output noise spectrum when its input is connected to the transducer (or an equivalent impedance), with no external signals being injected (i.e. from your DAC). Measure that noise, convert it to a PSD/NSD spectrum, and compare it against a .noise simulation in LTspice.Also measure the noise spectrum with your receiver/ADC disconnected (or terminated to a similar impedance to your preamp's output impedance). Subtract that noise power from the first measurement to correct for the receiver noise (hopefully it isn't significant and this step isn't necessary, but it's always smart to check).

author: David Hess
author_rank: Super Contributor
timestamp: March 06, 2025, 03:25:38 pm
content: Quote from: SteveThackery on March 06, 2025, 10:25:28 amThey are found on pages 69 and 70 of Linear Technology application note 67, as shown below. I recognized the design.

author: madr
author_rank: Contributor
timestamp: March 07, 2025, 07:22:47 am
content: Quote from: mtwieg on March 06, 2025, 02:13:02 pmI already did similar measurements but it was my "first" run doing these kind of measurements so I'll do them again with some lessons learned. After that I'll post the results with correct y-axis... But this may take a week. I will be back.