Nasty noise after the op amp

[stefanmarinov]

Leave 100uF there.
Do you have correct and stable -12v there or is it lower?
Interesting is how is done with C12, R8,D2,C3,D5 and C8!
I've seen something similar on one older design.

Ivconic, I am not the author of this scheme. I took it from a russian forum and changed it a bit. Here is the author's website: SurfMaster-Micro.
I just changed it a bit, but this noise is killing the whole idea. Now with the arduino's 10bit ADC, the sensitivity with 20cm coil is 17cm for 1€ coin. First I tough the reason is the small 10 bit resolution, but when connected it to the maple mini's 12 bit ADC the sensitivity is still the same. Some oversampling and digital filtering helps, but not much. The main problem remains the noise.

[green]

Ivconic, I am not the author of this scheme. I took it from a russian forum and changed it a bit. Here is the author's website: SurfMaster-Micro.
I just changed it a bit, but this noise is killing the whole idea. Now with the arduino's 10bit ADC, the sensitivity with 20cm coil is 17cm for 1€ coin. First I tough the reason is the small 10 bit resolution, but when connected it to the maple mini's 12 bit ADC the sensitivity is still the same. Some oversampling and digital filtering helps, but not much. The main problem remains the noise.

Have you determined if the noise is coming from coil pickup or the op amp? Maybe your scope, try connecting scope probe to common.

[ivconic]

Ivconic, I am not the author of this scheme. I took it from a russian forum and changed it a bit. Here is the author's website: SurfMaster-Micro.
I just changed it a bit, but this noise is killing the whole idea. Now with the arduino's 10bit ADC, the sensitivity with 20cm coil is 17cm for 1€ coin. First I tough the reason is the small 10 bit resolution, but when connected it to the maple mini's 12 bit ADC the sensitivity is still the same. Some oversampling and digital filtering helps, but not much. The main problem remains the noise.

Oh i knew i saw it somewhere!

[stefanmarinov]

Have you determined if the noise is coming from coil pickup or the op amp? Maybe your scope, try connecting scope probe to common.

green, at the moment I am only sure the reason is not in the scope. I replaced the coil with resistor and the noise is still there. Maybe really the mosfet driver is not good. I'm trying to check it now.

[ionut_mtb]

green, at the moment I am only sure the reason is not in the scope. I replaced the coil with resistor and the noise is still there. Maybe really the mosfet driver is not good. I'm trying to check it now.

i have same noise on my scope, but when i touch the gnd of the scope probe become smaller. after that i realized that my AC socket does not have grounding.

[green]

green, at the moment I am only sure the reason is not in the scope. I replaced the coil with resistor and the noise is still there. Maybe really the mosfet driver is not good. I'm trying to check it now.

I've had op amps oscillate with to much capacitance on the output in other applications. Don't see the capacitance in your schematic so probably not the problem. Driving a long coax can be enough. Could try a 100 to 500 ohm resistor in series with the scope probe. I'm always trying to reduce noise in my detector. Most of the time trying to learn and guessing what to try.

[stefanmarinov]

green, thank you for the suggestion, but at the moment I discovered another issue.

Seems, that something is really wrong here. I unsoldered the coil and the dumping resistor in order to observe around the mosfet. Instead of 12 volts there is only 5.6v on the gate and again 5.6v on the drain. On the source the voltage is stable 12.2V.
See the pictures:
At the IRF9640's gate
At the IRF9640's drain

[stefanmarinov]

The last post was a mistake. I just messed the x10 and x1 settings of the scope. Obviously it is time to go to sleep

[stefanmarinov]

I made a new PCB with new design and the noise is finally gone. In the last pcb I had some very long traces and some cable jumpers and I suspect they were picking some external noise. 600 meters away from my house is a "secret" military unit, so maybe they have some source of radio noise.