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Original question:

[FAQ] FDC1004 Capacitive Sensing FAQs

FDC1004: How do I specify the offset decimal part ?

Daisuke Okahara
Prodigy 20 points
Part Number: FDC1004

I have a question about the offset specification of the FDC1004. (It's not a CAPDAC)
In 8.6.4 of the manual, there is a description of the structure of the offset register.
How do I specify the decimal point?
For example...
-If I want to write -2.99876, how do I specify the decimal part 0.99876?

  • 0
    TI__Genius 14726 points

    Hello Daisuke,

    Welcome to the E2E forum, thank you for asking your question.

     

    The offset registers use 16bits for storage with the first 5bits configured as a two’s compliment representing the integer, and the remaining 11 used for the decimal.

    This means the resolution for the decimal portion of the register can be calculated to be 1/ (2^11) = 1/2048 (pF). To calculate the bit representation of a decimal, you take the number you wish to represent ex .99876 and multiply by the resolution (2048): (.99876)(2048) = 2045.46048. The resultant number will be rounded to the nearest integer (2045) and converted back to binary to represent the 11 decimal bits: (111 1111 1101). This would equate to the decimal representation 2045/2048 = .99854

     

    Please let me know if you have any questions.

     

    Thank you,

    Jacob