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Piezoelectric hydrophone low noise amplifier design questions

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Sparker:

--- Quote from: David Hess on May 01, 2018, 09:41:26 am ---

--- Quote ---I'm quite constrained in space so I can't afford a nice & expensive toroid 40mm in diameter, so I stopped at this one: 10x6x4 mm toroid, T38 material. Probably I can fit about 60 turns total one layer. Is it a good choise?
--- End quote ---

Inductor for what?  For the transformer?  I would probably use a pot core for easier winding.  What transformer are you using for the transmit side?  I might use the same transformer or at least the same core for both.


--- Quote ---I am thinking if I could just move the protection to the "op-amp" side of the transformer, so its noise voltage is not amplified by the transformer, but probably it won't work well with the 10mm transformer mentioned above.
--- End quote ---

I was thinking the same thing but the transformer will increase the applied voltage from the transmit chirp making protection more difficult.  Applying the pulse to the transformer itself could be a problem if it saturates and the protection is on the secondary side so a larger than necessary transformer is needed for this.


--- Quote ---The problem with the transformer is getting sufficient self inductance, the primary impedance does need to be significantly higher than the piezo to not make a mess.
--- End quote ---

I was going to say getting a sufficiently high frequency response but obviously this is not a large problem given the existence of the transmit transformer which must be larger to prevent saturation.  I wonder if a tuned transformer would be better in this case.

--- End quote ---
I also thought of winding the RX winding at the TX transformer in the beginning, but I am planning to use one TX amplifier to drive two antenas through diodes like in the picture. So I can't use this approach.


--- Quote ---For a 100V RMS signal you're passing 100 mA and burning 10 Watt in the MOSFET (which it can't take for very long).

This needs to be considered but the chirps are pretty short.
--- End quote ---
Chirps will be transmitted about every 100 milliseconds, take chirp length 1 ms, and we get about 0.1 Watts average in this case.

David Hess:
Below are the examples I mentioned of diode bridges being used as series protection elements for low impedance inputs and outputs.  The current fed into the diode bridge limits the current between input to output.  The control inputs to the diode bridge could also be reversed to completely disconnect the input from the output.

Sparker:
Thank you! It's actually a very smart design. Do you think it will make a lot of noise?
It seems to me that it's not noisy at all, at least in my rough simulation a pair of 4148 diodes with 10mA running through them produce around 0.3 nV/sqrt(Hz). I'll try to simulate it in detail later.
Meanwhile I've assembled a rototype of our amplifier on a PCB, seems to be working fine. When I figure out how to communicate with the digital potentiometer, I'll try to analyze the self-noise of the circuit and share it here.  :)

David Hess:

--- Quote from: Sparker on May 12, 2018, 06:13:03 pm ---Thank you! It's actually a very smart design.
--- End quote ---

The Tektronix implementations were for protecting 50 ohm oscilloscope inputs with a 350MHz (485) and 1GHz (7A29) bandwidth.  I have never seen this used anywhere else.

Another interesting thing is how Tektronix used the 741 operational amplifiers in the circuit from the 7A29; they are configured as current feedback amplifiers with 2 high impedance differential inputs, 1 low impedance differential input (the output pin), and 2 current outputs from the supply pins.


--- Quote ---Do you think it will make a lot of noise?

It seems to me that it's not noisy at all, at least in my rough simulation a pair of 4148 diodes with 10mA running through them produce around 0.3 nV/sqrt(Hz). I'll try to simulate it in detail later.
--- End quote ---

Noise is where it gets interesting.  The noise is attenuated by the ratio between the impedance of the current source which is high and the impedance of the signal which is low.  Tektronix was protecting a 50 ohm signal, required the highest possible bandwidth, and had to operate down to DC so their current source had only a moderate impedance.  In your case active current sources could be used with cascodes to get incredibly high impedance at 240kHz and associated low noise.  The diodes and current sources have to be able to survive the maximum input voltage though.  I might also disable the current sources while the transmitter is actually active to limit power dissipation in the current sources and loading on the transmitter.

I just thought his was a neat idea which might apply to your application.  I have only used it for protecting low voltage signals like function generator outputs and trigger inputs and outputs.

Sparker:
So here are the results with the real circuit!
With the input of the first opamp completely shorted I am getting about 8 nV/sqrt(Hz) which totally satisfies me for now, until I shield the whole circuit completely.  :-+ I made the whole setup: I connected the MCU i want to use in this application, recorded 100 ms of data, calculated its RMS, divided it by gain and divided it by sqrt(noise bandwidth of the filter). I think the circuit is mostly a success.

But then I have a problem: I use the LM2594 DC-DC converter running at 140 kHz to generate the negative -2.5V rail for the op amps. When I connect the transformer to the input of the opamp (and remove the short between opamp's input and GND of course), somehow the 140kHz and 280kHz from the converter get coupled in so much that the output of the circuit gets saturated. :palm: I've provided much filtering for the -2.5V rail with an additional LC filter, which should result to about 2uV of ripple at the -2.5V rail. From all this I conclude that most likely the transformer is responsible for this. What do you think?
 I should have probably used the pot core for the transformer because it's better shielded, but honestly I thought that toroids don't leak electromagnetic field and thus don't pick it up.
I've tried putting a tiny tin box over the toroid but it didn't change anything much.

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