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Weird signal instablity of an opamp amplifier
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SiliconWizard:

--- Quote from: VanitarNordic on November 19, 2018, 08:25:55 pm ---
--- Quote from: SiliconWizard on November 19, 2018, 05:09:44 pm ---It doesn't seem synchronized to mains.

Why don't you select a longer timebase so you can actually figure out the exact period of the apparent modulation? That will help to diagnose the source of the noise.

--- End quote ---

Actually, this instability intensifies when I increase the signal frequency, which I think should not if the noise is the low-frequency 50Hz hum. It does not get fixed when I use a battery.

so far I could reduce this instability by choosing a different value for the input capacitor (opamp AC coupling). reducing from 10uF to 2.2uF reduced the instability significantly.

--- End quote ---

Does the modulation change in frequency too?
Is the opamp powered by a single- or dual-supply? Can you post a schematic maybe?
StillTrying:
There's more than enough evidence to show it's mains hum + SMPS noise, and not the op amp for me, I've seen it too often!
KE5FX:

--- Quote from: StillTrying on November 20, 2018, 12:43:37 pm ---There's more than enough evidence to show it's mains hum + SMPS noise, and not the op amp for me, I've seen it too often!

--- End quote ---

I have seen some misbehavior from otherwise-trustworthy opamps when using high source impedances.  That waveform looks a bit like what I observed a while back on the LT1677.  (Link goes to a Usenet thread where I asked for some peer review.)

The takeaway from that experience, at least from my point of view, was "don't use opamps with R-R input stages if you don't actually need that particular feature, and don't use high source impedances when you do."  I don't know if the TL071 would be vulnerable to anything like that or not.  The datasheet does say that the common-mode range includes the positive rail.  Try (much) lower resistances and see if that changes the behavior?
VanitarNordic:

--- Quote from: StillTrying on November 20, 2018, 12:43:37 pm ---There's more than enough evidence to show it's mains hum + SMPS noise, and not the op amp for me, I've seen it too often!

--- End quote ---

You mean the small noises in valleys? it happens because of the connected voltmeter. otherwise, if I disconnect the voltmeter, the peak and valleys are clean, but this something like signal dancing effect (I don't know what this specific signal behavior called) still exists. Anyway, for a project, I work with small signals amplification and if I could not remove this annoying phenomenon, I can not go to the next steps and examine other parameters in practice.
VanitarNordic:

--- Quote ---Try (much) lower resistances and see if that changes the behavior?
--- End quote ---

I have tested a few more part numbers such as LM318, TL072, TL081/2 and all show this behavior. I think it might be the hum noise, but I should remove it before I go to the next steps
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