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ADC32RF45: Aliased images due to interleaving?

Rosanah Murugesu
Prodigy 175 points

Part Number: ADC32RF45

I am noticing some strange noisy sections when measuring a channel response from DAC to amplifier to ADC. They occur intermittently and randomly during certain captures. These noisy section seem to fall into the frequency regions where the Fs/4 - Fsignal alias would fall. However, this is the only alias I see. Would you be able to explain what interleaving corrections are being performs and how they are being calibrated/updated?

  • 0
    TI__Mastermind 29220 points

    Hi,

    The interleaving correction is correcting for offset mismatch between the four sub_ADCs.  Figure 235 of the data sheet illustrates what the offset correction is doing.  The logic does not equalize the DC component amongst the four sub_ADCs but rather subtracts out the DC offset altogether for each of the four sub_ADCs.   For each sub_ADC there is a block of logic that generates an estimate of what the DC component is for that sub_ADC, and then this estimate of the DC offset is subtracted from the data stream.   There is a window of data over which this estimate of DC offset is generated and the window is 'large' so as to not have the estimate be misled by short term pathological cases of input data but the window is not so large as to prevent the estimate from following changes in environment over time such as temperature.  I do not know the time constant of this estimation logic.    The option exists to either freeze the value of the offset estimate and no longer update the estimate, or to not use the estimate at all and subtract zero from the data stream. 

    if you use the SPI register bits to disable (turn off) the offset correction, then you would see a significant spur right at Fs/4.   I do not tend to see spurs of the sort that would be Fs/4 - Fin.   The interleaving spurs due to sub_ADC offset are pretty much right at Fs/4 and multiples of that.

    Regards,

    Richard P.