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2-Wire Hydrophone Amplifier

Joe Palka
Prodigy 40 points
Other Parts Discussed in Thread: LMV1012

Hi,

 

I am looking to make a 2-wire hydrophone amplifier that can drive a 15km cable.  I am looking for something that can operate similar to the LMV1012 where the signal and power share the same wire.  I am not sure if this is even possible, or the best rout to tackle it.  Based on my hydrophone, I need a gain of at least 60dB.  I am only just beginning, so I would appreciate any advice before I begin.

 

Thanks

  • 0
    TI__Guru* 93105 points

    Hi Joe,

    There are probably a number of solutions for your hydrophone amplifier application. My thought is to start with a phantom microphone circuit and adapt it to a hydrophone. One of my colleagues wrote an EDN article some years ago about a phantom amplifier circuit. You can find the article here:

    http://www.edn.com/design/analog/4340681/Microphone-uses-phantom-power-

    Some work would likely have to be done to the line driver portion of the circuit. I suspect the line appears mostly capactive at hydrophone frequencies and that can bring stability concerns.

    Regards, Thomas

    PA - Linear Applications Engineering

  • 0 in reply to Thomas Kuehl
    Prodigy 40 points

    Thanks Thomas,

    I think this is a start for me.  I see that there is still two conductors and a shield for this circuit.  I think I can only use one conductor and a shield, I will have to confirm this with the customer.

    I am wondering if something like this can be pulled off using some FETS?  Basically, using the signal to vary the resistance across the FET?

    Do you think one could use the LMV1012's in parallel?

    Also, are there some simple courses on amplifiers offered anywhere?  I have little experience.

  • 0 in reply to Joe Palka
    Prodigy 40 points

    So I tested the idea of putting two LMV1012 in parallel, an amazingly, it works.  I can get 31 dB with two of them (25dB with one).  I would have a long ways to go to get 60dB.  I would assume I will run into supply issues when I get to 40dB.

  • 0 in reply to Joe Palka
    Guru 21975 points

    Joe,

    The LMV1012 is intended to be used with electret microphone capsules. Is this the type of transducer you are using? (Most of the hydrophones I have heard about have been low impedance moving-coil devices.) What is the signal bandwidth required?

    The LMV1012 is not suited to drive such a long cable. A 15km cable is going to require some special drive circuitry. Do you have information on the cable type or impedance?

    Regards, Bruce.

  • 0 in reply to Bruce Trump
    Prodigy 40 points

    Hi Bruce,

    We are still figuring out the hydrophone, I think we are going to go with a spherical shape ceramic, which I do not know the impedance properties yet.

    The signal bandwidth is 100Hz to 90kHz.

    The customer's operating voltage is from 22-30Vdc as well.

    Cable properties (15km cable):

        • ZC= 30 Ohms
        • R= 5.9 Ω/km
        • L=160 µH/km
        • C=177 nF/km

    Based on the estimated sensistivity of the sphere, we will need a 60dB gain.  I would prefer to go with something fiberoptic, but that isn't an option.

     

    Thanks,

     

    Joe

     

  • 0 in reply to Joe Palka
    Guru 21975 points

    Joe,

    You have a complicated problem. The cable length and frequency range is such that you will need to carefully consider transmission line effects. One or both ends must be terminated with 30 ohms. You can read this thread describing a similar situation:

    http://e2e.ti.com/support/amplifiers/etc_amplifiers__other_linear/f/18/p/215943/770937.aspx#770937

    Another possibility is to FM modulate a much higher frequency signal and demodulate at the receiving side. This could greatly improve noise rejection and signal fidelity.

    Regards, Bruce.