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Original question:

CCS/MSP430FR2633: Conversion Count - Offset Subtraction

MSP430FR2532: CapTIvate Conversion Gain below 100

Kenton McFaul
Prodigy 155 points
Part Number: MSP430FR2532

Good afternoon!

In the CapTIvate documentation, the minimum Conversion Gain value is specified as 100.  However, in a recent test, we set the Conversion Gain lower (e.g., 50, 25).  The CapTIvate software did not prevent this, and there appeared to be an expected increase in sensor sensitivity.   As a result, I was wondering...

  • What potential impacts on sensor operation can occur if Conversion Gain is set < 100?
  • What causes the Conversion Gain to have a theoretically minimum of 100?

Thanks!

  • 0
    TI__Mastermind 28070 points

    Hi Kenton,

    In order to reliably detect touch, the conversion gain value must be sized accordingly. Too small of a conversion gain results in delta values that are just too small to correctly detect with reasonable capacitance values. As illustrated here, conversion gain needs to be high enough for you to have a detectable difference between touch and notouch results. 

    This page provides more information on the conversion gain and conversion count parameters.

    If you want to get higher sensitivity, I would recommend increasing conversion count rather than decreasing conversion gain. 

    Best Regards,
    Brandon Fisher

  • 0 in reply to Brandon Fisher94
    Prodigy 155 points

    Thank you for the response, Brandon!

    The following phrase is somewhat confusing to me: "conversion gain needs to be high enough for you to have a detectable difference between touch and no touch results."

    Based on experience and the documentation, the opposite seems to be true (i.e., sensitivity to capacitance-change increases when conversion gain is decreased).  Is this a result of the relationship between gain and offset subtraction?

    Thanks!

  • +1 in reply to Kenton McFaul
    TI__Mastermind 28070 points

    Hi Kenton,

    I've checked with our hardware expert here, and there is actually an additional factor at play in setting that 100 minimum value. The MSP430 Captivate technology performs best with a Conversion gain value above 100 because internally the device is most linear in that range. 

    He also recommended setting higher conversion count rather than going below 100 on conversion gain if you need more sensitivity. 

    Best Regards,
    Brandon Fisher

  • 0 in reply to Brandon Fisher94
    Prodigy 155 points

    Thanks again, Brandon!

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