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ADS8363: Minimum Configuration example

Mihir Gupta65
Intellectual 2155 points

Part Number: ADS8363

If I follow the Minimum Configuration Example (chapter 9.1.2 on page 44), there are 2 formulas for Ffilter and R

With a tacq of 100 ns and a capacitor of 1nF as mentioned, I end up with R = 4.243 ohm and Ffilter = 221 MHz. This value looks strange to me…

Am I right saying that

  • R should be less than 12 ohm to respect the input impedance (page 20)
  • Ffilter should be 500 kHz to respect the anti-alias filter of the Nyquist theory when the sample rate is 1 MSPS
  • C should be higher than 1nF (recommendation on page 44)

All those constraints don’t seem to fit with these 2 formulas.

Where is my misunderstanding ?

Regards

Mihir

  • 0
    TI__Guru 67806 points

    Hello Mihir,

    Thanks for your post.

    The input RC filter servers primarily to provide charge to the internal sampling capacitors. It's not always possible to use this stage of the signal chain as both a "charge bucket" circuit as well as an antialiasing filter. You're correct about the Nyquist frequency at 500 kHz, but additional filter may be needed prior to this final stage before the ADC.

    A larger capacitor may overdampen the input transient response and not allow enough time to reach the desired fully-settled voltage level. We have an in-depth training series and simulation tools that can help fine tune the exact R and C values that will give optimized settling performance. Check out Section 5 in TI Precision Labs - ADCs!

    Best regards,