VLF MD with digital signal processing : Bee-Buzz 1

[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

[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.

[pito]

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

mcp6022

[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.

[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.

[Aziz]

Hi Aziz,
To leave exotic ideas is good idea. Yes, you was obligate to try this solution! Using of NE5534 according me is possible, but this is not pointed to the future - now we have possibilities to use more good OpAmps with lower current consumption, lower noise and wider frequency band. All we, old (good) engineers have to switch for more new solutions.

NE5534A or LT1028A/AD797A? This is the art of ultra low noise design.
I say, you don't need the expensive ones. Take a cheap NE5534A and you can achieve much better results.
If the cheap op-amp NE5534 gets into trouble due to drive capability for instance (max 500-600 Ohm loads), then make a workaround (see below picture for the power driver).

For instance:
I can achieve at >10 kHz operating frequency with a RG1=1 Ohm, RG2=20 Ohm approx. (! super low drive resistance), RS=1 Ohm at Gain=20 (!) an input voltage noise density of 0.26 nV/rtHz. The 200 Hz bandwidth noise @10kHz is less than 0.1µV rms. This with a cheap NE5534 and a cheap medium power transistor BD179 to drive the feed-back-loop. But the draw back is, that the pre-amp draws almost 400 mW at 9 V.

Look at the below picture:


This is really impressive.

[JoyJo]

Common-base is generally the better approach to low noise. Here is a CB preamp I was messing with several years ago. It does not require a floating voltage source. I don't recall where I found this but probably an audio application. The values shown are what was used in my last sim.

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?

[Detectorist#1]

Hi Aziz,
To leave exotic ideas is good idea. Yes, you was obligate to try this solution! Using of NE5534 according me is possible, but this is not pointed to the future - now we have possibilities to use more good OpAmps with lower current consumption, lower noise and wider frequency band. All we, old (good) engineers have to switch for more new solutions.

[Aziz]

Hi all,

I know the reason for the bad performance with the old pre-amp. It's the fkn sound card, which had been operated at maximum power output (head phone output). No good.
It can get into oscillations, bad behaviour and very much noisy. And it will get hot and suck the battery of the Tablet PC rapidly empty.

Again, low power to the TX coil (even at line level output) and a good pre-amp with gain of max. 50 (this is absolutely max) is a good choice to start with. The sensitivity of the response is very good. And we are saving battery power. Everything is good.
So I'll leave the Richard's pre-amp and develop the previous pre-amp using ZTX-transistor pair with NE5534.

I have looked at three decoded bins around the resonant frequency using a narrow band chirping TX-signal. It is working but it isn't convincing much so far. I will look at the dual frequency response, which should give us the total benefit.
Cheers,
Aziz

[Aziz]

Hi all,

my USB Sound BlasterX G6 gets quite hot after 5-10 min operation at maximum TX-power. This is a good indication, that the power headphone amplifier is not efficient.
Anyway, I should wrap my choke L1 next time to get the dual frequency TX VLF/LF operation working.

Cheers,
Aziz

[Aziz]

It's not a rail to rail amp.

Yes, it's not a good idea.

I have plugged the 3V version and observed absolutely no benefit. It is even worse.
Using the other pre-amp gives absolutely no benefit too.

Back to the KISS principle. No pre-amp. No battery (power supply). Only passive circuit elements (both TX and RX side).
Cheers,
Aziz

[moodz]

It's not a rail to rail amp.

[moodz]

...read the spec sheet for the NE5534 too ....you are not supposed to operate the bipolar supplies below +/- 5 volts.

[Aziz]

Isn't that starting to get a bit messy. Why not stick with the original moving coil amplifier arrangement with a totally floating battery. Then if further op-amp buffering/gain is required, add a non-inverting gain stage but with it's own power supply which is not referenced to the 1st stage battery..

Yep, it isn't getting a KISS project anymore. I was quite surprised, how much the voltage rails can really vary. So much, that the op-amp can't regulate anymore and thus distorting the output.

That's funny. I can leave in the original 3 V design the gain setting resistor R5 and let the pre-amp gain define by the input impedance of the sound card.
I will do some measurements with the 3 V battery design before I finish the improved op-amp version.

[Olly]

Isn't that starting to get a bit messy. Why not stick with the original moving coil amplifier arrangement with a totally floating battery. Then if further op-amp buffering/gain is required, add a non-inverting gain stage but with it's own power supply which is not referenced to the 1st stage battery..