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ADS1261: Will the AC Excitation method remove sensor offset

Dale Yones
Prodigy 30 points

Part Number: ADS1261

Hello,

I am looking at applications note SBAA290 for the ADS1261.

Will the AC Excitation mode remove any voltage offset due to just the sensor - including the offset voltage created by bridge tolerance differences in the four resistors of the bridge, or due to imparted strain on the bridge during assembly (i.e. when glued down)?

Thanks,

Dale

  • 0
    TI__Mastermind 44760 points

    Hi Dale,

    Unfortunately, chopping is unable to account for the sensor's offset. If you take a look at this related reference design: http://www.ti.com/lit/pdf/tiduac1, you'll see an explanation of how chopping removes offsets that occur after the chopping.

    However, you can still perform calibration while using AC excitation to account for the sensor's offset voltage. By measuring the offset with no load applied, you can subtract this offset from all future measurements. Typically, this kind of calibration is required anyways, as weigh pans placed on top of a load cell to help hold additional weight would need to have this weight accounted for, just like any other offset.

    I hope that helps!

  • 0 in reply to Christopher Hall
    Prodigy 30 points

    Hi Chris,

    Thanks for your feedback. I have to sit down and go thru the app note above in more detail.

    So the statement in section 2.1.1. does not apply to the sensor offset?

    "The effect of ac bridge excitation is much like continuously measuring and applying a system offset calibration (removing the overall offset of the measurement system, as opposed to input chopping, which only removes the ADC’s contribution to the system offset)."

    i.e. it only applies to offsets up the sensor connections?

    Thanks again,

    Dale

  • 0 in reply to Dale Yones
    TI__Mastermind 44760 points

    Hi Dale,

    That is correct, AC excitation only removes the offsets up to the sensor. AC excitation will help with removing offsets on the sensor wiring (since the wiring is after the chopping), but if the actual Wheatstone bridge is producing this offset then this cannot be removed except by performing additional calibration with the sensor.

    The statement in section 2.1.1 probably could have been worded better...AC excitation produces a similar effect as continuously calibrating and accounting for the system's offsets, but with the sensor removed from this calibration process.

  • 0 in reply to Christopher Hall
    Prodigy 30 points

    Thanks Chris,

    OK, well the hope was to look for a way to cancel the offset other than in code or a laser:)

    I'll have to do the math and see what the resolution  left after we account for the offset.

    Quesions, do the demo boards for the ADs1261 allow testing AC excitaion as well as DC?

    Thanks again,

    Dale 

  • 0 in reply to Dale Yones
    TI__Mastermind 44760 points

    Hi Dale,

    Yes, the ADS1261EVM has an on-board MOSFET driver which uses only two GPIO signals from the ADS1261 to switch the bridge's excitation voltage polarity. By default, the AVDD/AVSS supplies will be used for the excitation voltage, but you can remove R6 and R25 to connect a different excitation source, if needed: