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Anti-Aliasing Filter for PGA900

Hunter Long61
Prodigy 225 points
Other Parts Discussed in Thread: PGA900

Hello,

I am trying to design an anti-aliasing filter for the PGA900. I know the sigma delta modulator frequency is 1MHz, but I am not sure if this means the Nyqyuist frequency is 500kHz. The other possibility is that since the max output rate is 15.625kHz so I also thought the Nyquist frequency might be 1/2 this or 7.812kHz.

Below is my filter as currently designed. I was thinking about reducing the capacitors by a factor of 10 to 0.1uF and 0.01uF. Before doing this I wanted to make sure that the Nyquist rate is 500kHz and not 7.812kHz.

Thanks,

-Hunter

  • 0
    TI__Intellectual 2845 points

    Hunter, 

    I don't believe your image you shared uploaded correctly. but the correct frequency for your anti-aliasing filter would based off of your output rate rather than the modulator frequency. 

  • 0 in reply to Matthew Sullivan
    Prodigy 225 points

    Ok, thanks for your reply!

    This post: says that

    "

    Instead of needing to filter everything beyond the Nyquist frequency as in SAR ADCs, the anti-aliasing filter for delta-sigma ADCs only needs filter out unwanted frequency content around multiples of fMOD.  This allows for an even more gradual filter roll-off that is much easier to design with a few discrete, passive components. Typically, a single-pole RC filter is all that is required, but you may need a second-order filter for lower OSRs.

    "

    So this made me think the anti-aliasing filter would be based on the modulator frequency instead of the output rate.

    Here is the filter as I currently have it designed (hopefully the image works this time).