Pre amplifier for proton precession magnetometer

If the magnetometer sensor impedance is very low, then un ultralow noise bipolar amplifier like LT6018 would be ideal.  Definitely send details on the sensor, especially its impedance.  glen.brisebois@analog.com 

First thank you.

The magnetometer is actually a small NMR device.  The sensor is a coil, nominally 10 cm by 10 cm with 200 or so turns of wire.  Inside the coil is water or kerosene. .It is a supply of protons for the NMR The resistance is on the order of a few ohms and the inductance is 5 or more mH

The coil is activated by  8 - 12 volts dc for about 3 to 5  seconds.  This polarizes the protons.  Their spin is oriented with the applied magnetic field.

When the field is cut off the protons begin to precess about the Earths magnetic field.  The frequency is 1 - 4k. 

That is the signal that the preamp must process.  

There is of course quite a spike when the coil is turned off.  I am suppressing that with a diode and a resistor.  On the scope it looks good (modest spike of about 3 - 5 volts) but there could be spike before the diode "turns on".  A capacitor and resistor snubber board might get rid of the last bit.  I have been unsuccessful  with that but have not tried much.

It may be that spike that destroyed to old machine.

SO the LT6018 looks good.  It even has a low noise buffer buffer circuit on page 1!  The only thing that specifically attracted me to the ADA4625 wis the JFET input.  

Mouser has some Digi key does not have any a present.

Thanks

Tom Schmitt

Glen,

May I trouble you once again.

After the pre amp the signal is passed through a bandpass filter before amplification.

While the signal will be buffered at that point, it still would be small.

Would the LT608 be desirable for the op amps in the band pass filter and the initial amplification post filter.

That would make preamp,   filtering and the initial amplification the bulk of the cost of the unit.  Likely no better place to "spend money".

We will  make the boards 4 layer with bypass etc  Once we get a good signal there are many options for processing (that is redundant).  .

Thanks for the help. 

If we are successful this will be open source for other universities.

Tom Schmitt

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