Electronics > Projects, Designs, and Technical Stuff

Why is this circuit's transient response settling twice

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nikola22:
Sorry, I was not trying to imply they were exactly the same, just that they had similar output performance. I have taken photos of both an inductive (150mH/85ohm load), and a purely 85ohm load. Inductive is the first image, and purely resistive is the 2nd image.




SiliconWizard:
What is "Vout" exactly? The voltage across the load, or the potential at the voltage buffer? With your initial "yellow is VL" statement, it was a bit unclear. But it does look like it's the second option, as otherwise obviously the current and voltage would be exactly proportional for a pure resistive load.

nikola22:
Its the voltage at the load. The current is set by the voltage across R5, so Vout - Vbuffer its a howland current source topology. Iout =( Vout-Vbuffer) / R5.

duak:
When I see a 10 to 20 ms settling time i think either C1 is the wrong value or maybe there's some thermal issue.  Could there be a problem with the VCC and VEE connections to the TCA0372?  Maybe it's not powered and the OPA4172 is actually sourcing/sinking all the load current through the ESD diodes of the TCA0372.  The OPA4172 then heats up and limits current.  It's sort of in line with the short circuit current graph on p.18. Is the OPA4172 getting hot?

BTW, does an input pulse of the opposite polarity act differently?

SiliconWizard:
Yep.
Also, the scope capture with a resistive load looks suspect. If the actual circuit is exactly like on the schematic, any current flowing through R5 should also be flowing through the resistive load, with only negligible current flowing through R2 and R4 (where else could it also be flowing?). But as we can see on the capture, the voltage across the resistive load looks nothing like the voltage across R5 (proportionally) after ~20ms, which is highly suspect. Some incorrect wiring and/or part overheating is indeed possible. Did any of the opamps feel hot to touch? I'm also trying to figure out if an excessive unbalance of the 10k resistors could explain this.


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