The PCM1808 Audio ADC has an internal oversampling digital filter operation. The operation of this filter is dependent on the system clock (SCKI) that is connected externally to the IC.

For a 48 kHz sampling rate, you need to set the system clock frequency appropriately. According to the PCM1808's data sheet, the system clock (SCKI) must be 64/128/192/256/384/512/768 times the sampling frequency (fs). For your case, let's select 256 times as it's common for audio applications.

So, the system clock (SCKI) frequency should be:

[ f_{SCKI} = f_{s} \times 256 ]

Substituting ( f_{s} = 48) kHz into the equation gives:

[ f_{SCKI} = 48 \mathrm{kHz} \times 256 = 12.288 \mathrm{MHz} ]

To set up the system clock for the SCKI pin (P.SCKI) of the PCM1808, you would need a clock source, like a crystal oscillator or a clock module, that operates at 12.288 MHz. Incorporate this oscillator into your schematic and connect its output to the SCKI pin.

Keep in mind that this is the clock frequency required for a 48 kHz sampling rate specifically. If you want different sampling rates, you may need different clock frequencies. Always check the datasheet to make sure you have the right system clock frequency.

And remember that, if employment of multiple PCM1808's is desired, they all have to share the same system clock to ensure synchronized operation.