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INA231: INA231 conversion time

Fred Cheng
Expert 4670 points
Part Number: INA231

Hi team,

The datasheet said the noise impact may reduce by increasing conversion time. My question is as we known in delta-sigma ADC the noise shaping is strongly related to the sampling frequency.

But since the frequency is almost fixed around 500kHz(+-30%) , why would noise performance benefit from increasing conversion time? 

Regards,

Fred

  • +1
    TI__Mastermind 18780 points

    Fred,

    While you are correct that for an SD converter sample frequency is important, remember that in a SD converter, there are not one but two frequencies inside the degrees of freedom that form the output data rate, fs and fd. The conversion time here is the time domain representation of fd, as there is an output made at the end of every conversion for the INA231. The relationship of these variables in an SD is that the decimation ratio is the ratio of fs/fd, and as you say, fs here is fixed at 500kHz, leaving the only variable as fd. Therefore, as the conversion time becomes larger, fd becomes smaller, and the decimation rate grows. Larger values of fd will envelope more of the shaped noise floor, and therefore have more noise. 

    This app note covers the subject in depth. 

  • +1 in reply to Carolus Andrews
    TI__Expert 4670 points

    Thanks understand.

    It helps a lot.

  • 0 in reply to Carolus Andrews
    TI__Expert 4670 points

    Btw is the averaging mode of INA231 here  is not referring to the "averaging" digital filter in the delta-sigma right?

  • +1 in reply to Fred Cheng
    TI__Mastermind 18780 points

    Fred,

    No, the averaging mode of the INA231 takes multiple samples (the number is determined via the averaging register discussed in section 8.6.1.1) and averages these measurements together to further reduce noise, as the average of the peak to peak distribution of the noise should limit to zero as additional samples are averaged together.