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ADS1231: Noise Performance

Neel Madhav
Prodigy 90 points
Part Number: ADS1231


Hello,

My concern is Noise performance of ADS1231. The following image shows the Noise performance in tabular form. This table is table 1 from datasheet of ADS1231. In this table NOISE-FREE BITS is 6th column. This value of noise free bits includes the sign bit?

Regards,

Neel Madhav

  • +1
    TI__Mastermind 48020 points

    Hello Neel,

    Yes, you are correct.  The ENOB (we call this Effective Resolution on newer devices) and Noise-Free Bits includes the sign bit.  Also, these noise numbers assume Vref=5V.  If operating at 3.3V, these number will be reduced by 0.6b.

    Regards,
    Keith Nicholas
    Precision ADC Applications

  • 0 in reply to Keith Nicholas
    Prodigy 90 points

    Hello Keith,

    I have just one concern. We are operating at 5V supply. For ADC raw count whether, we should consider ENOB bits or Noise free bits. We are using this for load cell application, and we want to increase resolution by adding more bits into ADC count. 

    Thank You.

    Regards,

    Neel Madhav

  • 0 in reply to Neel Madhav
    TI__Mastermind 48020 points

    Hello Neel,

    Noise free bits simply means that the code is not changing with a steady load.  At 10sps, if you truncate the lower 7 bits (24-17), these bits will be constant and produce the same 17b code with a fixed DC input.  The ENOB (or effective resolution), the first 17b will  be constant and the next 3 bits will change from reading to reading.

    You can improve the Noise free bits (and the ENOB) by further filtering in your processor.  An averaging filter is a common approach.  Averaging will decrease the noise by 1/√n, where n is the number of samples averaged.

    PGA=128, Vref=5V

    Noise free bits n=4: 18.4b

    Noise free bits n=16: 19.4b

    Noise free bits n=64: 20.4b

    Regards,
    Keith