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Need help understanding this OpAmp behavior.
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Mechatrommer:

--- Quote from: David Hess on July 10, 2019, 03:10:22 am ---The 324/358 is subject to phase reversal if the negative common mode limit is exceeded:

If either input or both inputs are lower than –0.3 V with respect to the negative supply, excessive input current can flow and the output may display phase reversal, also called inversion.

http://www.ti.com/lit/an/sloa277/sloa277.pdf


--- End quote ---
this... please do take this advice seriously. some jellybean opamp doesnt have to go -ve on its input to phase reverse, i also suggest you characterize your opamp, to see when phase reversal happened. attached are example of knock off brand of NE5532 setup as buffer or voltage follower its +ve input feed from function generator (blue and pink traces = voltage rails, yellow = input, cyan = output). the 1st is really bad china knock off where reversal happened near zero (but still +ve) input, the second and third pictures are mexico knock off batch of NE5532 which can take some -ve input abuse before reversal occured. also note the clipping on the output on both +ve and -ve rail due to non rail to rail behaviour, so it cant follow the input on those extremities fwiw...
Wigo:

--- Quote from: Mechatrommer on July 11, 2019, 06:16:00 am ---
--- Quote from: David Hess on July 10, 2019, 03:10:22 am ---The 324/358 is subject to phase reversal if the negative common mode limit is exceeded:

If either input or both inputs are lower than –0.3 V with respect to the negative supply, excessive input current can flow and the output may display phase reversal, also called inversion.

http://www.ti.com/lit/an/sloa277/sloa277.pdf


--- End quote ---
this... please do take this advice seriously. some jellybean opamp doesnt have to go -ve on its input to phase reverse, i also suggest you characterize your opamp, to see when phase reversal happened. attached are example of knock off brand of NE5532 setup as buffer or voltage follower its +ve input feed from function generator (blue and pink traces = voltage rails, yellow = input, cyan = output). the 1st is really bad china knock off where reversal happened near zero (but still +ve) input, the second and third pictures are mexico knock off batch of NE5532 which can take some -ve input abuse before reversal occured. also note the clipping on the output on both +ve and -ve rail due to non rail to rail behaviour, so it cant follow the input on those extremities fwiw...

--- End quote ---

@Mechatrommer
Wow thanks for that explanation and meassurments. That clarifies a lot.

I had not have time to do some more testing, but i will perform the recomended tests on my OpAmps as i have the time to do so.

@All
Thanks a lot for your help and the indepth explanations. I appreciate it and it helped a lot to gain more understanding on the hidden traps that lie within those ICs.
StillTrying:

--- Quote from: Wigo on July 12, 2019, 08:36:40 am ---I had not have time to do some more testing,
--- End quote ---

Make some. :)
Have you checked what actually comes out of the piezo yet. I've since done some tapping on a bare 25mm piezo disk. Finger taps can easily give 10Vp-p and tapping it with a plastic pen can easily be 50Vp-p or more, even when driving a 10k load as well as the scope probe. I'll add a scope shot from a few days ago, when/if I find the memory stick...

...Finger tap in Blue, plastic pen tap in Yellow.
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