Nasty noise after the op amp

[stefanmarinov]

After building several analog metal detectors, now I want to play with direct sampling on the op-amp's output, but there is some nasty noise, or even combination from two or more noises. I can not find the source of it. What do you think?

Scheme
On the op-amp's input
At the op-amp's output ZOOMED
After D6

The ADC was not connected during the capture.
Tried with Maple Mini, Arduino, Atmega8 and still the same noise, so the noise is not from the MC. The optimal damping resistor is 320ohms.
The power is from 12V lead acid battery.

[ivconic]

R9 is too high.
I don't understand the role of D3 and D6?
I would add 100uF capacitor in parallel with C5.
Noise can be partially coming from measuring instrument too.

[kosacid]

D3 i don't ether D6 is a pull down to 0v and clip the -v replace D3 with a cap R10 adjust to the max voltage you need, is D6 a 5v zenner ? just that R10 is a bit low
i would swap D3 with a cap and D3 with D6 and use pull down resistors to set my max voltage

[Xtrem]

After building several analog metal detectors, now I want to play with direct sampling on the op-amp's output, but there is some nasty noise, or even combination from two or more noises. I can not find the source of it. What do you think?

Scheme
On the op-amp's input
At the op-amp's output ZOOMED
After D6

The ADC was not connected during the capture.
Tried with Maple Mini, Arduino, Atmega8 and still the same noise, so the noise is not from the MC. The optimal damping resistor is 320ohms.
The power is from 12V lead acid battery.

I dont think your mosfet driver are correct, remove Q4, R1, R2(short),Q3 BD139, R5 50 Ohm direct to collector of Q3.
Supply with regulated power the 5534 (+5, -5 ), short remove R9, replace BAV99 with 4148 ???, decrease (trimm) R11, so that you have the best signal.
Maybe your problem are BAV99 ???

[stefanmarinov]

D3 in combination with R10 rectifies the signal. It removes the negative part of the signal. D6 is a 3.3v zener and protects the ADC from voltages higher than 3.3V.
Ivconic you are right about the 100uf capacitor in parallel with C5. I will try this today. Hope it will help. What do you mean with R9 being too high? R9 and R11 make the proper ampliffication.

[Xtrem]

Q3, Q4, contuct at the most time of switching TOGETHER.....think about....

[stefanmarinov]

Xtrem, I made an analog detector with the same driver and there was no such noise. This is why I think the problem is not in the driver.

[kosacid]

no need for D3 D6 is doing the job of cutting the -v R10 is so low that's why your getting some -v take it R10 is to adjust the level of the signal with that resistance to ground its a wonder your getting a signal out it must be right low

[stefanmarinov]

D6 alone can not remove all the negative voltage. There is still -0.6V left and the maple mini can't handle voltages below -0.3V. The ADC will die.

[kosacid]

ahh got ya

[green]

The 300kHz+ noise looks continuous. Try removing Q2 and then a jumper across D1.

[Carl-NC]

Gain looks OK; not sure about the MOSFET drive, I've never done it that way but I guess it's working. On D3/D6, you might consider a precision rectifier but that adds an opamp. On the noise, it might be environmental; try rotating the coil to different angles, see if it gets worse/better. Or, replace the coil with a resistor and see if the noise goes away.

[stefanmarinov]

A lot of headache with this experiment
Tried the advice of ivconic and put 100uF in parallel with C5, but no luck. The noise is still there. Now I will try the Carl's and green's suggestions. Hate whet it happens like this!

[ivconic]

A lot of headache with this experiment
Tried the advice of ivconic and put 100uF in parallel with C5, but no luck. The noise is still there. Now I will try the Carl's and green's suggestions. Hate whet it happens like this!

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]

This attracted all my attention, i will try it with Arduino some day...