Input cap values on ADAU1701

Hello,

22 uF capacitors should work fine. Input coupling capacitors like these allow your AC audio signal through while blocking DC. This blocking action is needed because your input signal fluctuates around ground potential, while the ADC input pins operate at 1.6 V above ground. The caps allow only your input audio without this 1.6 V difference to appear across the 18 K resistors. In turn the resistors admit a fluctuating current into the ADC pins proportional to your audio signal, by Ohm's Law. The ADC converts this current to the numerical sample values which in turn drive your DSP signal flow.

The coupling capacitors must be sufficiently large in value to allow your low-frequency (bass) through. If the caps are too small, you'll lose bass response -- with a "tinny" sounding result. How much capacitor is enough? You can calculate the "3 dB point" (the frequency where about 70 % of the original signal gets through) with the formula: F = 1 / (6.28 x R x C). For 18 K and 22 uF, F = 1 / (6.28 x 18000 x 0.000022) = 0.4 Hz. This is much lower than 20 Hz, the lowest audible bass frequency. Thus 22 uF or even 10 uF is sufficient for most applications.

Best regards,

Bob