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I am looking at part of the datasheet for a hydrophone I am using in a project. It includes this figure, which plots resistance, impedance, and reactance vs. frequency:enter image description here

Part of my project involves designing a preamplifier for the hydrophone. Because the output of the hydrophone will be in the range of 20kHz to 40kHz, I must make the input impedance of the preamplifier much greater than that of the internal resistance of the hydrophone to maximize voltage transfer between the two. When looking at this chart, I am unsure of which value I need to use for the equivalent series impedance. The parallel capacitance of the hydrophone is 3.4nF, and its datasheet can be found here.

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  • \$\begingroup\$ It looks like the curves match up with your statement about the parallel capacitance. No surprises there. Do you know how to calculate this? (\$\mid\, X_C \mid =\frac1{2\pi\,f\,C}\$) If you use that equation I think you'll find that the red line matches the magnitude (the sign of capacitance reactance is taken as negative.) The resistance looks very low at your frequencies of interest, but the reactance will vary by a factor of two over the range. \$\endgroup\$
    – jonk
    Jan 27 '20 at 0:41
  • \$\begingroup\$ I'm good with impedance of a capacitor, but is the capacitor is in parallel with the hydrophone then I'm confused with how to factor this into my circuit diagram. Considering that the resistance is small, could I model the hydrophone as a voltage source in shunt with a capacitor to ground, neglecting series resistance (because its small). \$\endgroup\$
    – Saunders
    Jan 27 '20 at 0:51
  • \$\begingroup\$ I think you should consider looking for hydrophone circuits developed by Winfield Hill, as an example. This is smack in the middle of what he's done in his life, so I'm sure there must be something out there with his name on it. However, my instincts say you need a JFET for the 1st stage/pre-amp (lower noise at the required impedance.) In fact, I'm almost sure of it. That can be followed up with a shielded twisted pair wire to whatever you need (if you need circuitry remote from the hydrophone.) \$\endgroup\$
    – jonk
    Jan 27 '20 at 0:52
  • \$\begingroup\$ I will look into his work. I have a textbook of his as well which I can look through. \$\endgroup\$
    – Saunders
    Jan 27 '20 at 0:58
  • \$\begingroup\$ I found one possibly useful reference: Miguel Alvarado, "Construction and Testing of Low Noise Hydrophones," Naval Postgraduate School, Master’s Thesis, 2003, p. 8. But I have a question for you. Why haven't you contacted the manufacturer? They will know and will likely have a white paper on the topic, if not both that and also a reference design for you. \$\endgroup\$
    – jonk
    Jan 27 '20 at 0:58
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It looks like the impedance is about 1K-2K through your band of interest. I don't know what you mean by "much more," but 20K input impedance for your preamp should probably be easily achievable, and 200K wouldn't be much of a challenge. Note that your internal resistance is significantly lower--almost negligible--but the impedance will still affect both the output level and (perhaps more significantly) the phase shift. If phase shift is critical (which I doubt, but you're the expert on your requirements) it could be compensated for in the preamp.

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