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OPA2388: peak-hold detector circuit affected by opamp input leakage current

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brumbarchris:

--- Quote ---A non AZ OP may be better behaved in this respect
--- End quote ---

What does AZ stand for?

Cristian

Kleinstein:
The AZ stands for auto zero.
For an alternative OP one would look for a good JFET OP with good capacitive drive capability.
Another point would be a really good phase reserve, like > 60 degrees, as there are 2 OPs in the loop.  So 45 deg. Phase reserve would already be too little.

If it would not be for the case the OPA1642 could be a possible candidate.

brumbarchris:

--- Quote ---For an alternative OP one would look for a good JFET OP with good capacitive drive capability
--- End quote ---

As mentioned before, I had an earlier version of the circuit, using the ADA4891-2 opamp from Analog Devices; that one worked fine. I was unable, however, to find a JFET input opamp with a reasonable good offset voltage. That is why I eventualy selected the OP2388. However, it seems indeed that having this internal auto-calibration feature, while good for offset, is detrimental for a peak-detection circuit. I will have to select another one. I will check OP1642.

Regards,
Cristian

Poe:
1.  Linearly ramp the input signal down to 0V over 1mS.  If the output signal remains the same, the leakage isn't through the diode. 
2.  Add a DC offset to the input and see if the discharge rate changes.
3.  With the input grounded and the 'ground' pin of the capacitor disconnected from ground and instead attached to the positive rail, does the output trend up or stay low?  If it raises, it's the cap.

brumbarchris:
Hi,
I have clarified the leakage is not through the diode. As I reverted back to the original schematic (see my third post in this thread) I disconnected the feedback of the 1st opamp from the storage capacitor. Now it can hold the charge, but the "jumpiness" in following the peak is what is not good at the moment.

Regards,
Cristian

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