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ISO124: No input rails causing output to drive to rails

David Kraemer
Prodigy 40 points
Part Number: ISO124


Hello,

I have a design where the input rails are not always provided as they are derived from an input side that can be off. With the output rails enabled the output drifts toward either the VS2+ or VS2- node. Is there a way to force the output to lock at the GND2 node in this situation?

VS2 rails are +-15V

Once the input side rails are provide the output is as expected except for a 25mV to 45mV negative offset.

Thanks,

David Kraemer

  • 0
    TI__Guru**** 182021 points

    Hi David,

    Unfortunately there is no easy way to get the ISO124 outputs to track to ground with the input side un-powered. Is this a new project? We may have some newer isolation amplifiers that would be of interest to you.

  • 0 in reply to Tom Hendrick
    Prodigy 40 points
    Tom,

    I assume you meant to say there is no easy way to track to ground. It is a new design but we already have boards in hand so the footprint is stuck in a sense. The device is used to monitor scaled down high voltage lines so precision is key. I would love a device with a better input bias current but I don't think there are a lot of good options. My worry is that a downstream ADC may over range if the ISO124 output were to drift low.

    Thanks for any thoughts.

    David K.
  • 0 in reply to David Kraemer
    TI__Guru**** 182021 points
    Hi David,

    Yea - somehow I dropped the 'no', sorry about that. We have a couple new parts in the pipeline, but the package would be different. If you can entertain a smaller package with better isolation capabilities, let us know and I'll have someone provide you with more detail.
  • 0 in reply to Tom Hendrick
    Prodigy 40 points

    Tom,

    Thanks for the info. I will certainly entertain some other parts. Please let me know what you have in mind that might help solve the floating node problem.

    David K.