Preamp

[Teleno]

If you think about it a bit further, the effects you mention, while completely valid, are of the 2nd order. Your useful signal is falling well under 100Hz. All the interferences above 100Hz will cancel out. Some even improve dithering.

The interference from the Tx pulse is repetitive with the same period as the signal. Not useful for dithering.

Anyway my design is open to everybody, modify according to personal preference and please, do share.

[Teleno]

This version includes a floating current sink (R16/U3) based on a high voltage depletion mosfet DN3545 or similar.

Source resistance R1 is fixed at 500 Ohm. Damping is active (see Moodz) by varying R16 depending on Tx current and coil.

Advantages:

- Less noise than the previous design (lower R1) and largely independent of the coil.
- Higher gain (ratio R2/R1 is larger).

Disadvantages:

- R16 depends on the L/C of coil and on the Tx current. Adjustment is delicate and sensitive to temperature.
- There's a small delay penalty for faster coils (around 1us)

[Sean_Goddard]

But WHAT IF you use a dual coil setup? Optimize for a PRF of say 1200Hz? Would that not be the basis of a fantastic nugget hunter?

[Teleno]

But WHAT IF you use a dual coil setup? Optimize for a PRF of say 1200Hz? Would that not be the basis of a fantastic nugget hunter?

Perhaps, but let's keep it simple by now and prove the concept on a monocoil first. If it works, then we can move further.

Coming back to the active damping ... the current sink circuit is very simple:



but it's very dependent on temperature, any ideas on how to compensate it? I was thinking an NTC in parallel with R16 but perhaps there are better ways.

Otherwise it will be left as another task for the MCU (one more DAC output).

[Qiaozhi]

But WHAT IF you use a dual coil setup? Optimize for a PRF of say 1200Hz? Would that not be the basis of a fantastic nugget hunter?

Or you could follow the example of the improvised coil thread and go for a triple coil setup.

[WM6]

Be careful, could be already patented by ML.

[Old cart]

Dual coil and temp compensation

Perhaps, but let's keep it simple by now and prove the concept on a monocoil first. If it works, then we can move further.

Coming back to the active damping ... the current sink circuit is very simple:



but it's very dependent on temperature, any ideas on how to compensate it? I was thinking an NTC in parallel with R16 but perhaps there are better ways.

Otherwise it will be left as another task for the MCU (one more DAC output).

I for one like the idea of a dual coil. That being said I have never used on a PI machine so don't know how they perform.

What changes with temperature? If you put an ntc in parallel with R16 won't it heat up and change value a lot? Maybe it could be part of voltage divider for the gate drive. The MCU drive is also a good idea , particularly it is can have an auto adjust , or maybe auto cal function.

[Teleno]

Be careful, could be already patented by ML.

Sure, as of now the circuit is available in the public domain and indexed by Google: http://elcid.demon.nl/metal_detector/

That should stop them.

[Teleno]

What changes with temperature? If you put an ntc in parallel with R16 won't it heat up and change value a lot? Maybe it could be part of voltage divider for the gate drive. The MCU drive is also a good idea , particularly it is can have an auto adjust , or maybe auto cal function.

The MOSFET current decreases with temperature (Rds_on increases), so an NTC would reduce the source resistance by the same ratio (hopefully), achieving temperature independency.

Auto cal. is another option, but not exclusive, since both methods can be implemented. Calibration would adjust to different coils and Tx currents.

[Achillion]

If used as a replacement for the two stage opamp MiniPulsePlus front end, do you reckon this front end could improve the sensitivity and noise rejection?

[Sean_Goddard]

OR you could sample every alternate pulse and use the delta as a reference (would that work I wonder)? I know that if you "ping" the coil with a low level pulse you can use that. It's been done but I can't remember where and it was used to take into account the sun heating up the windings, thermal heating due to current flow and a whole host of other things. Done in real time too!

[Old cart]

The MOSFET current decreases with temperature (Rds_on increases), so an NTC would reduce the source resistance by the same ratio (hopefully), achieving temperature independency.

Auto cal. is another option, but not exclusive, since both methods can be implemented. Calibration would adjust to different coils and Tx currents.

I suspect, but do not know for sure, that it is unlikely that an NTC would match the slope of the current change with temperature. Maybe it could be partially corrected and the rest done in software.
In a more advanced application software and a DAC could be used to correct for all errors, not just those created by temperature change.

[Sean_Goddard]

How low can YOU go? Check out the example circuit in this App note 0.95nV/Hz!!

http://cds.linear.com/docs/en/design-note/dn254f.pdf

Any use?

[Sean_Goddard]

Only one way to find that out I'd say ;-)

[6666]

I am curious, what function in the MCU will you use for outputing the target signal after it has gone through the ADC
are you going to use simple PWM audio ?