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OPA2192 voltage offset problem

Tom Saluzzo
Expert 1280 points
Other Parts Discussed in Thread: OPA2192

Customer is using OPA2192 and has discovered a problem.

 

We noticed that occasionally the offsets were bad after running our offset correction routine.  After investigating the problem, we realized that when we shorted the input to the channel, the offset of the OPA2192 shifted by about 400uV (not always, just sometimes).  Our offset correction routine would zero the output of the channel with this input offset. When we cycled power, the offset of the OPA2192 would become very small, and the channel output offset would then change by the 400uV multiplied by the channel gain, causing failures.

 

We eventually replaced the OPA2192 at the input of the amplifier with a competitor's opamp (ADA4610), and the problem then moved to the next OPA2192 in the amplifier.  We now knew that the OPA2192 had a problem.  We know that the opamp has an electronic trim which is applied at the factory.  We suspect that the trim gets reset by some mechanism involving changing the input suddenly, and that the proper trim is re-applied on the next power cycle.  I do not think that the inputs to the OPA2192 ever go above the positive supply or below the negative supply.  FET switches in front of the OPA2192 would clamp the input excursions at the supply voltage.

 

We then replaced the OPA2192's on the board with new parts that had a different date code.  The module now works correctly and we can't make it fail.

 

The parts that failed had this date code:

53H

Z4W4

 

The parts that work have this date code:

57H

ZG91

 

The bad parts were sent to the customer as samples.  The good parts were obtained through distribution. Has TI seen anything like this?  If so, has it been fixed?  If you need more information please let me know.

  • 0
    TI__Guru 60570 points

    Tom,

    It is very unlikely, if not impossible, that electronic Vos trim bits get improperly reset by sudden change in the input voltage – we have done plenty of characterization of OPA2192 and have never seen such problem.  However, from your description of the problem, it is most likely that the problem is caused by sequencing of the power supply.

    If the positive power supply is turned on first and the negative supply is turned on second (thus it is temporarily floating), for short period of time there is no proper current path from the positive rail to negative rail and thus the current finds parasitic path inside the op amp that may improperly load OPA2192 Vos bit or even cause permanent damage to IC– see diagram below.     

    By connecting Transient Voltage Suppressors (TVS) across both power supplies as shown below, the TVS diodes are reverse-biased in normal operation BUT will become forward biased and provide a direct path for current from positive to negative rail in case the negative rail is floating. This prevents the device from going into an abnormal state which may cause improper loading of the voltage offset bits in case of OPA2192.

    Therefore, please recommend to your customer to add TVS protection diodes to their application circuit to eliminate the input offset issue.  This also protects the circuit from potential damage to the IC caused by transient over-voltages often occurring during turning-on of the supplies.

    For more information on the topic of electrical overstress (EOS), please watch the series of TI Precision Labs videos by following the link below:

  • 0 in reply to Marek Lis
    Expert 1280 points
    We sequence our positive supply on first (otherwise analog switches in the circuit latch up). But we have never seen this problem occur when the supplies turn on. The only time we have ever seen the OPA2192 offsets change from very low (microvolts) to very high (hundreds of microvolts) is when we switch the input to ground. We did put a diode from the negative supply to ground as was suggested in the diagram you sent us. This did not fix the problem. Where do we go from here?
  • 0 in reply to Tom Saluzzo
    TI__Guru 60570 points

    Tom,

    You must use TVS with fast turn-on time and not a random diode which may be too slow to provide the current path before problem occurs.  If despite that you continue having problem with OPA2192, please provide us with detailed schematic showing all the voltages and component values used.  You say the problem occurs when you switch the input to ground: what supplies (single, dual) and circuit configuration do you use?  

    You previously said you see the problem after auto-calibration and cycling of the supplies:  

    ‘Our offset correction routine would zero the output of the channel with this input offset. When we cycled power, the offset of the OPA2192 would become very small, and the channel output offset would then change by the 400uV multiplied by the channel gain, causing failures.’

    That is different than taking the input to the ground as you claim now:

    ‘The only time we have ever seen the OPA2192 offsets change from very low (microvolts) to very high (hundreds of microvolts) is when we switch the input to ground.’

    In order to be able to assist you, we must have full understanding of the conditions under which the problem occurs.