On those boards, the XMOS chip acts as an I2S master controller. It has no knowledge of what you connect it to. It will output MCLK/BCLK/LRCK (that you'll need to connect to your ADC), as well as output one or several I2S data lines (for DACs), and input one or several I2S data lines (for ADCs). Your ADC will have to be set up as an I2S slave. As long as it supports the sample rate that the XMOS will use, there's no need to know anything more about it. I2S is standard, and in most cases, audio data is received/sent as two 32-bit words regardless of the sample width, in an MSB-first way (the remaining bits being usually zero), so basically, BCLK (bit clock) will be the sample rate times 64 (two channels, L/R, per I2S frame).