ADC/DAC interface resistors

For the Myriad RF schematic, on the ADC/DAC digital interface (RXD[11:0], TXD[11:0]) there are 22 ohm resistors and for the SPI, there are 47 ohm resistors. What are these for and are they required?


as far as I understand to reduce the possible ringing on the SPI or DATA lines. Especially if lines are very long.