# Calculating Passive Analog Output Filter for CS42448

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)?

• $$\F_S\$$ is the sampling frequency. You want to remove it.
• $$\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}\$$