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LMP91000 reference-worker clamp fet (internal or external?)

David Strawn
Prodigy 40 points
Other Parts Discussed in Thread: LMP91000

The eval kit for the LMP91000 features a fet to clamp reference and worker when power to circuit is OFF.  Why is this implemented external to the LMP91000 when the chip has an integral fet clamp?  Should I employ an external fet clamp in my design or use the integral clamp?  I'm using the LMP91000 to amplify current from Alphasense H2SD4 toxic gas sensor.

Application is toxic gas sensing instrument used in Oil & Gas.

Thanks,

Dave Strawn

  • 0
    TI__Guru 54076 points
    Hello David,

    The internal FET clamp only functions when power is applied (in normal or deep-sleep modes). By default, it is off and must be programmed to turn on.

    Space is provided for an external JFET clamp for protection when the board is unpowered or used in it's default power-up configuration state.

    Since this is an evaluation board, the external JFET could interfere with characterization, so it was left off. The assumption being that the sensor would be removed and placed back into the shorting ring for storage.

    In a "real world" application, it is recommended to have the JFET clamp to protect the sensor when the power is removed.

    Some of those sensors take hours to recover after allowing the terminals to float, and each event also shortens their lifetime.

    So unless you can GUARANTEE that the power will never be removed or that the internal FET will always stay on if programmed...I would also include the external JFET.

    Regards,