VLF MD with digital signal processing : Bee-Buzz 1

[Aziz]

Is it possible to use this scheme with unipolar power supply 3,3v
as an input stage of a metal detector located in the coil?

You need special low voltage op-amps to operate it at low voltage rails. I don't know, whether there are available.

[Carl-NC]

Is it possible to use this scheme with unipolar power supply 3,3v
as an input stage of a metal detector located in the coil?

That might be pushing the limit but I don't know without trying it. It will require a rail splitter to drive the ground node.

[pito]

You need special low voltage op-amps to operate it at low voltage rails. I don't know, whether there are available.

mcp6022

[JoyJo]

That might be pushing the limit but I don't know without trying it. It will require a rail splitter to drive the ground node.

Let me also ask, what do you mean by a splitter? In theoty, there are quite affordable chips for obtaining negative voltage in the sot23-5 case by TI company. Yes, with bipolar power supply, the dynamic range will be wider, but the input stage circuit will become more complicated.

[JoyJo]

mcp6022

Not only this chip))) Although it is the most common. There are quite a lot of such chips: AD8606, AD8656, OP262, OP1662, LT6237.
I used these chips as input amplifiers with a 3.3V unipolar power supply

[Aziz]

Again, you dont need special chips. Use discrete transistor preamp just designed to operate at 3 V.
I should look at them too. Could be very useful.
Cheers

[Altra]

Here are three pre-amps that I made and tested. The first (equinox) had some problems with oscillations and low bandwidth. It used some flea sized components, so I blamed it on pcb design and poor soldering. The second (vanq) worked well but the bandwidth rolled off in the 30khz range. I modified the third to to work up into the 40khz range. These operate at 3.3v and have a low impedance differential output which allows using simple twisted pair cable. Credit to Orbit on helping reverse the vanquish pcb. I am interested in how it would be received and fed to the ADC.

[JoyJo]

I guess that's what I was looking for! Thank you!

[Aziz]

Hi all,

well, ok. I have got rid of the op-amp and made the AC pre-amp fully discrete now (the good old days style ).
It operates now at 3 V battery voltage and allows max. 15 mV input voltage at gain of 47. Beyond the limit, it begins to disturb the output as there isn't enough voltage range due to the transistor voltage drops.
Power consumption: 25 mA at 3 V.
Noise-Specs? It's Ultra
See below:

Cheers,
Aziz

[JoyJo]

Here are three pre-amps that I made and tested. The first (equinox) had some problems with oscillations and low bandwidth. It used some flea sized components, so I blamed it on pcb design and poor soldering. The second (vanq) worked well but the bandwidth rolled off in the 30khz range. I modified the third to to work up into the 40khz range. These operate at 3.3v and have a low impedance differential output which allows using simple twisted pair cable. Credit to Orbit on helping reverse the vanquish pcb. I am interested in how it would be received and fed to the ADC.

Equinox has such an input part in the block. The second differential input is used to monitor the current in the coil Tx.

[Altra]

Thanks! That completes the basic hardware picture. The rest is code.

[pito]

Hi all,

max. 15 mV input voltage

Aziz

and minimum ? = sensitivity

[Aziz]

and minimum ? = sensitivity

Hi pito,

you have misunderstood me. Due to the low battery voltage of 3 V only, the input voltage is limitted at a specified gain of approx 50.
Input 0 - 15 mV -> no distortion expected. Output voltage peak-to-peak would be approx. 1.5 V.
Input above (> 15 mV) -> distortion to output happens.

[pito]

so you can say sensitivity is 1 microvolt and output will be 47 microvolt noise free ?
15 mV -> no distortion expected. Output voltage peak-to-peak would be approx. 1.5 V.
15x47 = 0.705 V not 1.5 V

[Detectorist#1]

Hi Aziz,
The high results in the MD area depends more from side factors than from our knowledge in hardware design of the very fast or very low noise solutions. But this is not capable to stop us!