ADC nyquist filtering

https://www.analog.com/en/analog-dialogue/articles/front-end-amp-and-rc-filter-design.html

Could I replace the RC filter with a 2nd order sallen-key LP filter? and use it without the RC, connect the output of the filter directly to the ADC input?

I see the RC address the settling issue, but the BW is way much bigger than my Nyquist frequency and allows too much noise in. What do you recommend to limit the input BW and also good settling time?

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  • 0
    •  Analog Employees 
    on Jan 18, 2019 9:47 PM over 2 years ago in reply to ahgu

    Any lower frequency filtering needs to be done on the input side of the ADC Driver (Amplifier).  So the signal chain should be:

    Nyquist Filter      ->      ADC Driver      ->      RC filter      ->      SAR ADC

      (500kHz)                    (Amplifier)                (high BW)              (AD7667)

  • Can I combine the filter and ADC driver together? For example, use Sallen-Key LP filter and then drive the RC network?

    Sallen-KEY--------RC----------ADC

  • 0
    •  Analog Employees 
    on Jan 21, 2019 7:29 PM over 2 years ago in reply to ahgu

    You *can* combine the ADC driver and Sallen Key filter.  It will complicate the circuit (and the analysis), and comes with tradeoffs.  A few things to consider:

    • You will still need the RC filter after the driver and before the ADC.
    • There will be noise from the Sallen Key filter, which is dependent on the component values in the filter.  You can use Filter Wizard to quickly simulate the noise - you can optimize the design for low noise, and observe how the noise changes as you vary the component sizes.  You can also export your design to a set of LTspice simulation schematics, which includes a schematic for noise simulation.
    • The Sallen Key filter will have an impact on the on the ringing/stability of the driver circuit, and the lower bandwidth will result in any ringing taking that much longer to settle out.  I haven't done much to examine this, but looking at this just a bit, it looks like it would be pretty easy to design a Sallen Key filter that wouldn't settle sufficiently due to ringing if it were connected directly to the RC Filter + ADC input.  

    My understanding is: this can be done, and has been done.  But it complicates the design challenge.