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Momentary Supply Voltage Polarity Reversal on Op Amps

Other Parts Discussed in Thread: LM148, LF156

Thanks to Paul Grohe for his response regarding LM108 and LM148. I have a similar question about LF156 (National die) stemming from the same power supply reversal.

As with the above parts, the negative power supply was driven to about +0.3V for no longer than 35 milliseconds. During this time, the negative input voltage was at ground, resulting in violation of a note on the LF156 datasheet which states "the absolute maximum negative input voltage is equal to the negative power supply voltage". Th.

 Does the same general rule of 5ma on any pin apply to LF156? Is there a reason why parts with JFET inputs might be more prone to damage from this sort of event?

  • Hi Rob,

    Were the voltages measured with the parts in place? Or with the parts removed?

    1.1V sounds like diode clamping may have occured.  If this is the case, then it is hard to tell what peak currents may have been flowing during the clamp. Current is what causes damage...

    What are the other pins connected to? Are any of them to a low-impedance point or hard ground (i.e, a hard-grounded non-inverting pin?).

    Are the supplies current regulated? Are there large bypass caps? Are these TI or NSC parts?

    For NSC parts, generally it is allowable for a supply to go 0.3V "below" the spec. The usual "maximum current in/out of any pin" is about 5mA .

    If the part was truly floating, then you could go reverse by 0.6V across the part without much problem. But the problem is that there are other pins involved, and the supply is then referenced to the pin with the lowest potential. Current can also flow in/out of these other pins - so it is hard to say where the bulk of the current was floating.

    The most "delicate" pins would be the compensation pins, followed by the input pins - which could cause a shift in the offset if the current was flowing through the input stages.

    You would have to measure the peak currents in each pin to be absolutely sure.

    Fortunately, the parts of this era are pretty robust (large feature sizes) - so I doubt if a few mA going in/out of the pins would cause any issues.  They are pretty tough (as long as nothing exceedes the 40V process limit).

    The quick way to see if there was damage is to see if there is any change in the supply current, bias current, offset and pulse response/BW.

    Regards, Paul Grohe

    SVA Precision Applications (ex National Semi)

  • any bodies can tell alternate part no for lm108