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OPA2388: peak-hold detector circuit affected by opamp input leakage current
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Kleinstein:
The circuit with the 2nd OP inside the loop of the first OP ideally wants the 2 nd OP to by considerably faster than the 1 st OP.  One also has to be careful with the size of the capacitor, as this acts as capacitive load. The resistor as drawn does not help here as it is inside the loop.
To some degree it can help to have a transistor as emitter follower instead of the diode.
It helps if the 1 st OP is chosen for good capacitive drive capability.  The 2nd OP does not need a low offset. The 1 st OP would correct the offset of the 2nd OP, even if large.

The easy point to try would be reducing the capacitor to something like 100 pF. One may be lucky and get away without the oscillations.
With some OPs is may help to give the 1 st OP some additional current loading to stay on one side of the output stage.
brumbarchris:
Thank you for your replies and simulations!

I'll try to play a bit with the 20k feedback resistor value, to see if I get some different results.


--- Quote ---To some degree it can help to have a transistor as emitter follower instead of the diode.
--- End quote ---
I am aware of that scheme, but I need to try and make something out of the current PCB layout. Equally, this means I am stuck with using a dual-in-package opamp, so they will both have the same speed. Although, I would suppose the capacitive load of the 1st opamp artificially makes it slower anyway, right?


--- Quote ---One also has to be careful with the size of the capacitor, as this acts as capacitive load. The resistor as drawn does not help here as it is inside the loop.
The easy point to try would be reducing the capacitor to something like 100 pF. One may be lucky and get away without the oscillations.
--- End quote ---

The oscillations are not there because of the capacitor. They are there while uC_PEAK_RST is HIGH, which means the reset MOSFET is active and shorts out the capacitor. I am pretty sure they appear because the limited output current capability of the 1st opamp, which must constantly source current through the 10 Ohm resistor and through the MOSFET.

Regards,
Cristian
exe:

--- Quote from: brumbarchris on March 03, 2020, 08:53:26 pm --- soon Mouser will not be distributing TI parts anymore

--- End quote ---

Can you please share a link describing the situation?
Kleinstein:
The reset phase should not be much longer than needed. Especially there should be a little time between the reset phase and the peak to come. The OPA388 needs some time to recover from effective short at the output and thus recovery for the AZ part of the OP. There is a chance that the oscillations are not classical loop instability, but possibly some residual from the internal chopping. A non AZ OP may be better behaved in this respect.

A capacitor at the OPs output would not help - it would make things even worse, as there would be additional phase shift / delay. If at all it would be an extra capacitor in feedback.
fcb:

--- Quote from: brumbarchris on March 03, 2020, 08:53:26 pm ---I got parts from Mouser. Although soon Mouser will not be distributing TI parts anymore, I do not believe that what I have is fake, at the moment.

--- End quote ---
Are you sure? AVNET/Farnell have been running stocks down for a year+, and I received this from TI earlier this week:

"As you may know, we (TI) are in the process of winding down our business relationships with certain distributors. We are writing today to help ensure you continue receiving the products you need, when you need them. Please see below, and forward this email to your procurement and purchasing colleagues as appropriate.

Actions our TI customers should take to ensure business continuity of TI products
If you currently purchase TI products through Avnet, Compel, Eastronics, MT, Telsys, WPI or WT, we encourage you and your supply-chain partners to immediately start transitioning your new and existing TI business to our long-term supply-chain options:
   •    Arrow Electronics, our authorized broadline distributor in all regions except Japan.
   •    TI.com.
   •    DigiKey or Mouser, our authorized catalog distributors.
   •    TED or Macnica, our authorized broadline distributors in Japan.
"
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