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TL084 - damaged op-amps

Eric Johansson
Prodigy 120 points
Other Parts Discussed in Thread: TL084

We have a situation where we are seeing damage to one of the 4 op-amps in a TL084. The package is running off +/- 15 volt supplies. All four op-amps in the package are configured the same, as voltage followers. The inputs to all four channels are the same (RC network, R=100K, C=1uF), and the outputs go directly to an open frame header connector. The symptoms are either railed output (~13.5V) irrespective of input voltage, or excessive offest (up to 3.5 - 4V) - that is, with the input grounded, the output gives -3.7V, but with a 3.3V input, the output is -0.7V. We have seen this failure on multiple systems - generally, the failure is on channel 1 (i.e., op-amp with output on pin 1), but we have also seen a case where the failure was on channel 4 (i.e., output on pin 14). For this latter case, the offset voltage for channel 1 was about 40 mV. In all cases, the other amplifiers in the package appear to be working correctly.

What are possible explanations for this? Could ESD damage be the culprit?

  • 0
    Genius 5430 points

    Eric;

    This is just a guess but it sounds like the 1uF capacitor may be dumping its charge back into the op amp input when the power supplies are shut off. You might put a 1k resistor between the capacitor and the input pin to limit that current.

  • 0 in reply to Neil Albaugh
    Prodigy 120 points

    I took a look at the fall time for the +/- 15V supplies when the supplies are shut off. The fall times are actually quite slow, and the worst-case capacitor current works out to be about 35 uA, which I'm assuming is too low to create damage.

  • 0 in reply to Eric Johansson
    Genius 5430 points

    Eric;

    Unless the output or the supplies are shorted it seems unlikely as you have shown. Could the connector be mismated accidentally? It might take only a momentary mis-connection. Depending on what the op amp is driving, maybe a resistor in series with the output could provide a measure of protection.

    I assume there are no big voltage transients in the system....

  • 0 in reply to Neil Albaugh
    Prodigy 120 points

    Just getting back to this issue. In answer to the questions, there are no voltage transients in the system, and it is very unlikely that connectors are mis-mated. All four op-amp circuits are identical, the input devices to these op-amps are similar to each other, and connectors are keyed and different from other connectors used in the system. The outputs feed open headers, but it is very unlikely that these are getting mis-mated, given the way this PCBA mates to the other PCBA in the system; also, this device appears capable of tolerating unlimited duration short circuits to both ground and either supply, so even if the connector could get mis-mated, it seems like the part should not suffer permanent damage. The design uses other types of op-amps, and these do not get damaged - just the TL084's.