How to calculate the noise using a different MCLK?

  • Using the same ADC power mode, filter setting, and decimation rate, the ADAQ7768-1 has a slightly different amount of noise with a different MCLK. This is due to the core ADC having a higher noise spectral density (NSD) with lower MCLKs, while the analog front-end (AFE) NSD remains constant.
  • In the ADAQ7768-1 datasheet, the “Noise Performance and Resolution” section lists down the noise performance of the device across varying MCLK frequencies, filter settings, and decimation rates.
  • If the MCLK used is not in the table, the noise can be calculated as below:
    1. Calculate the core ADC’s NSD (NSDADC) at the operating MCLK frequency (MCLKOP) using the formula:
    2. Root-sum-square the NSD of the core ADC and the AFE to get the total NSD. Use the NSDAFE from the table.

       

      RTO NSDAFE (nV/rtHz)

      Gain0

      23.1

      Gain1

      25.7

      Gain2

      31.2

      Gain3

      37.7

      Gain4

      55.1

      Gain5

      91.4

      Gain6

      172.5
    3. The total RTO noise is:

      where VN_RTO = referred to output RMS noise in Vrms
      BW = effective noise bandwidth in Hz
    4. For example, using MCLK = 12.288MHz, MCLK_DIV = 2, Filter = Wideband Low Ripple, DEC_RATE = 128, leads to a:
      • BW = 20.8kHz
      • RTO Total NSD = 49 nV/rtHz
      • RTO Total Noise = 7.07 uVrms
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