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LAUNCHXL-F28379D: below zero differential signal measuring

Ethan1
Prodigy 90 points
Part Number: LAUNCHXL-F28379D

Hi all,

I'm setting up my experiment setup with Launchpad 28379D. One of the signal I need to measure is a differential signal. The problem is that this signal is sometimes below zero, and that part is needed too. I've known that it's possible to add a offset externally to make all signal level above zero, but is there a way around it using the functionality of the Launchpad? Any thought is appreciated.

Best Regards 

  • 0
    TI__Guru 62975 points

    Ethan,

    The Launchpad is only powered by positive supplies so there is no built-in method to handle negative voltages.

    Are you using the differential buffering circuit accessed through J19+J20?

    If so, you might be able to wire in a high-impedance bias voltage (at R56/R57 and R63/R64) to inject a little offset into the input signals through voltage-divider effects.  It would resemble this example from the F2805x TRM:

    -Tommy

  • 0 in reply to tlee
    Prodigy 90 points

    Hi Tommy,

    Thank you! That's really informative. But maybe changing/wiring to the launchpad is not my best choice.

    I'm actually using the differential input pair in J21 and I use bread board to build a little offset circuit today, which solved my problem.

    And I'm wondering what's the SMA input, llike why are there three pins at J19. I googled and did not find anything similar.

    Best Regards

    Ethan

  • 0 in reply to Ethan1
    TI__Guru 62975 points

    Ethan,

    You can think of the SMA connector as a miniature BNC or coax connector.  There's one signal terminal in the middle and the rest are intended for ground shielding.

    -Tommy