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I'm designing schematics with the audio codec CS42448. The datasheet suggests output filters and I chose to follow the passive filter recommendation. I'm not sure though how to follow the calculation for the capacitor (C). Can I choose any value for the resistor (R ext)? And the Fs capacitor, does it refer to the 3.3uF (0.0000033)?

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No, the Fs is the cutoff frequency you want for the filter. Rext is the load resistance of the device where the audio is being output. C is the capacitor for the lowpass filter.

See the evaluation board schematics for example values for the passive filter.

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  • \$F_S\$ is the sampling frequency. You want to remove it.
  • \$R_{ext}\$ represents the impedance of the load you expect.

\$R_{ext}\$ and the 560 ohm resistor are effectively in parallel to each other. If \$R_{ext}\$ is much larger than 560 ohms, then it matters very little. If the output of the filter goes to an op-amp input, \$R_{ext}\$ will be very large - the equation reduces to \$C = \frac{1}{4 \times \pi \times F_s \times 560}\$

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