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ADS1262: Reference noise ENBW in ADS1262

zhongliang fu
Prodigy 10 points
Part Number: ADS1262

Dear Bryan Lizon，
    I have been studying the “Fundamentals of Precision ADC Noise Analysis“ you wrote before recently.There are a few questions about ENBW.

    First，ENBW is determined by the bandwidth of anti-aliasing filter and digital decimation filter. How to determine the bandwidth of the digital filter? Can it be understood as Fs/ (2*OSR).
Where Fs is the sampling clock rate and OSR is the oversampling rate.

    Second ，The contribution of reference noise to the overall noise of ADC.The document says that the contribution of reference noise only needs to be calculated within the ENBW bandwidth. But for Sigma delta ADC（discrete-time）, the reference voltage first acts on the modulator, and then is filtered by the digital decimation filter. In the modulator, the reference is actually sampled by the switched capacitor, so the aliasing of noise will certainly occur in this process, and the high-frequency noise of the reference module will be aliased into the low frequency. It is then filtered out by the digital decimation filter.

    Then it will be a problem to calculate only the reference noise within the ENBW bandwidth, because there is high-frequency noise aliasing into the band before the reference noise is filtered by the digital decimation filter.

    Looking forward to your reply. Thank you

Yours Zhongliang

  • 0
    TI__Guru 51906 points

    Hi Zhongliang

    Are you studying this material just for reference, or are you beginning a new design and hope that this information will aid you in that effort?

    Here are the answers to your questions:

    1. For most delta-sigma ADCs, the digital filter bandwidth is given in the datasheet, so you do not need to calculate it. But if you did need to determine the bandwidth of a filter, the best way would be to plot the transfer function and identify the -3dB point directly from this plot. Many of our delta-sigma ADCs have multiple filter types or orders, so there is no clear formula to determine the cutoff.
    2. I am not sure aliasing would occur in the modulator, but would occur during the digital filtering process. As a result, the reference usually has a lowpass filter between the reference output and the ADC input, which will limit the high frequency noise that can enter the system via the reference path. Moreover, the ADC inputs should have an anti-aliasing filter that similarly limits this high-frequency noise from aliasing back into the passband.

    -Bryan