Hello,
I really need help to solve this problem as it is my teacher's question on final exam.

In application note AN-1192, the Bridged-Paralel circuit of four LM3886 circuits. And there, in the schematics, are four servo circuits, which are integrators of DC voltage. It is DC offset protection. Those circuits needs LOW-OFFSET OpAmps like LF412, AD712, OPA2134 etc.

But, when I made a ciruit od PCB, many times happended, that one of the OpAmp got destroyed, probably because of voltage-peak. Mostly it happened in inverting type of servo.

When I changed LF412 or AD712 for cheap TL082 with input offset 3 mV, no problem occured and they (TL082) were worked properly. But while I put back AD712 after some test, it was destroyed.

So why are those low-offset amps so sensitive? When I looked to datasheets, they look almost same inside.


Thank you for suggestions

 

Hard to say. It's probably related to different input amplifier structures between the two devices. I doubt that it's related to whether they are low-offset types or not. You need to clamp whatever over-voltage is zapping the op amps.