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As a follow up to my question (see circuit diagram there), I am thinking further if I may be a cause of the problem. My idea that the second part of op-amp and another op-amp chip may have been damaged because, at some point in time, I did not connect their positive inputs, and was "hot-" connecting these inputs to the signal source on the same board.

I think it is very low probability, as initially, when things did not move properly, everything was connected, but anyway worth checking.

I saw this and this. My chips did not heat (I was checking it almost every time when operating them), no smoke and no craters up to physical damage. All op-amps were configured as voltage followers all the time, thus, as I understood, must drive output to some specific level, not saturate or get to "latch-up" situation.

Am I correct?

Update: op-amp is AD8512A (this info is available in my original question).

@PlasmaHH: looking to datasheet I see

The AD8510/AD8512/AD8513 have internal protective circuitry that allows voltages as high as 0.7 V beyond the supplies to be applied at the input of either terminal without causing damage

No word about unconnected input pins, or floating input pins.

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  • \$\begingroup\$ It will surely depend on the opamp and what protection it has built in (you do not say which one you use) but a floating pin has an unknown "random" voltage. Check the datasheet if that voltage falls into the range of allowed voltages at that pin. \$\endgroup\$ – PlasmaHH Jun 14 '17 at 7:51
  • \$\begingroup\$ I am really interested in your solution if you ever figure this out. The high input impedance on the op amp means it doesn't take much to get it higher than the rail. In similar designs to yours with big caps on the inputs, I have blown inputs at power down. If you shut down with a voltage on the DAC Vout pins, the op amp input will be driven negative, possibly below the negative rail (now unpowered). You might want to put a diode in parallel with C58 and C67. \$\endgroup\$ – John Birckhead Jun 14 '17 at 13:00
  • \$\begingroup\$ @JohnBirckhead not sure it can be the case - DAC is powered from +3.3 V, its output at no circumstance should be less than -12 V and more than +12 V (op-amp is powered from 2*12 V). I will edit original circuit diagram to include op-amp powering circuit. Current is limited by 100 Ohm resistor, thus input should be safe. Putting diode may distort the DAC output. Do I miss anything? Please explain. \$\endgroup\$ – Anonymous Jun 14 '17 at 14:35
  • \$\begingroup\$ When power goes down if all supplies go to zero, the C57 and C66 could have a 3-volt charge. Assuming the DAC output looks like a diode to the power rail, when the power goes to zero, the rail will pull down the DAC side and the Op amp side could go negative with the cap discharging through the op amp. Easy to check with your scope; shut down and watch the op amp input. It all depends on how the power supply goes down. I am suggesting the diode would be back biased across C58 and C67 so if this occurs the diode will prevent the cap from going negative more than 0.7V. No distortion \$\endgroup\$ – John Birckhead Jun 14 '17 at 20:00
  • \$\begingroup\$ @JohnBirckhead you are right, op-amp input (DAC circuit output) as well as its output go down to approximately -1.2V relative to ground, and then exponentially to zero. Which diode you recommend, and are you sure it is a problem for op-amp because its input can go down (relative to ground) to -13 V per datasheet? \$\endgroup\$ – Anonymous Jun 14 '17 at 21:10

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