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Why is this circuit's transient response settling twice

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nikola22:
This is a more complete view of the schematic, ive tried removing R9, and the snubber component, and the buffer op-amp, none of these seem to change the improper transient response. BTW the resistors are 0.1% tolerance, I don't think the issue lies there, unfortunately :(

nikola22:
I don't believe there is a thermal issue, as I can run this circuit all day, and it will give me the correct output current of ~50mA, but only on transient do I see unexpected behavior. There is a proper connection to the rails of the op-amp, and I have even removed the buffer at one point to measure the transient response. The problem is still the same... A pulse of opposite polarity, acts exactly the same EXCEPT for a very small pulse of approximately +/- 10mV, then it seems to have a proper transient response (critically damped ...)

SiliconWizard:
Are you sure the 47pF capacitor is not actually 47nF?

I still don't get the pure resistive load case. How can the current and the voltage across the load not be proportional (which they don't seem to be during the first 20ms)? Where does the current flow?

This 20ms time constant also seems suspect. It's very long (that's why I asked about the cap.) With an inductive load, you can expect some ringing, but not long steps like this.

Are you also 100% sure about your input voltage? That may sound obvious, but I don't think we have actually seen it on your scope captures. It could have a nasty transient maybe?

Anyway, I'll attach an LTSpice sim file if you or anyone's interested in simulating this. Not with the same opamps, but I've failed to get anything close to what you are observing here. There must be something we don't know or are overlooking here...

nikola22:
My simulation showed exactly the same! I am utterly confused... Now I am wondering if its because of the input diodes on the OPA4172, wondering if that its in the simulation model.

The 12V rails look clean, unfortunately problem does not seem to be there... :(

T3sl4co1l:
Pic of the breadboard?

Tim

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