Preamp

[ivconic]

For a moment we could stop focusing only on mono coil design in this setup.
I don't see what's the problem to consider use of double D or coplanar coaxial coil in this design?

[Davor]

That's spot on - you can't get input noise below 3nV/sqrt(Hz) with damping resistor connected in series - regardless of the amplifying device's capability to go much lower. 1nV/sqrt(Hz) corresponds to ~60 ohm, and that's too low for a meaningful damping resistor.
Going down with noise will require a non-inverting op amp with some sort of blanking/clamping device. Alternatively it may be a FET or a transistor, but with no capacitors in biasing circuitry to avoid long recovery. It is much easier and cheaper achieving below 1nV/sqrt(Hz) noise with discrete devices, but I have no idea what role may non-linearities have in PI. It is quite simple to demonstrate importance of front-end linearity in VLF, but things are not as straightforward with PI. Intuitively I'd say linearity must play a role in early sampling, but I may also be wrong.

[Teleno]

[Teleno]

Common base preamp:







PI_common_base.zip

[green]

http://www.geotech1.com/forums/showthread.php?22661-coil-to-plus-input&p=208180#post208180

I tried what Davor said with a Bss127 and a FDV302. Couldn't get it fast enough, maybe someone that knows what they are doing could help.

[Teleno]

Common base amplifier, gain is Rc/Re, where Re is valued fo critical damping

[Davor]

"Can you also look as active blanking or can we get away with not needing that?"
You can't get 1nV/sqrt(Hz) with traditional series resistor approach, so blanking is absolutely required. It may be active but also passive. Moodz' approach is active and it also incorporates flyback acceleration, but fun part is that the same functionality is available with passive approach.

If n-channel mosfet is placed in series with preamp input (drain connected to coil) and gate biassed at positive rail, the input is protected from positive flyback voltage. Adding a p-channel mosfet (if needed) biassed at negative rail in between the n-channel one and a preamp input protects from charging voltage. At this moment I'm not up to seeking small mosfets for voltage, charge, and availability, so feel free to step in.

As of recently I'm mostly engaged in some other endeavours that prevent me from participating more in forum activities, so my visits are far less frequent.

[ivconic]

Sean:
"...Davor and Ivconic, can you take on the front end ampl please between you as a team as you BOTH have excellent ideas and I think that 1nv/Hz would be just about dandy ;-))..."

Sean, it is fair from your side, i appreciate.
But i am afraid it's not possible to do things like this, in "groups", especially online.
Different people, different habits, different conditions. Different opinions and different intentions.
I suggest you to be the one who decide and the rest of us can only throw ideas and suggestions.
That's the only way to keep the continuity.
Just an example:
if i chose front end like i posted on other topic; it will need specific power supply. So i will have to design quite different power supply, than; most of the guys here will not be happy to accept it, in place of the one already proposed.
One thing usually leads to another. At the end everybody will force something quite different on their own. And we will end up with dozen different and ALL unfinished designs; all those most probably will have nothing in common with Surfmaster at the end!
That's where "project manager" term is coming on scene!
What we need here is exactly the "PROJECT MANAGER" who will decide and manage.
Most logical choice is you Sean, of course.
You can collect all the ideas and than propose two compatible approaches; one modern (4-layer pcb, smt etc...) and one "traditional" ( single or double layer pcb, no smt, conventional analogue components)...
So, people will pick the option which they find most suitable for them.
...
Personally, i can not participate in "full time" and all the time here, i can not maintain continuity in work here. Because of my personal responsibilities and fact that i am also working on 4 other projects at the same time.
But i will be here from time to time and i can help by drawing schematics and pcb's in case you need.
Also later i can build "traditional" version very fast and test it for you here.
That you may count on, from my side.
Cheers!

[Qiaozhi]

Discussions related to the design of the preamp.