Hi Kato-san,
The timing diagram below compares CPOL=1&CPHA=1 against CPOL=1&CPHA=0 to illustrate the details that allow 18*N. Note that to give SCLK margin, we recommend SCLK ≤ 36MHz for CPOL=1&CPHA=1.
ADS8885 and ADS8881 propagation delay is the same, but the ADS8885 can only reliably tolerate up to 16-MHz.
Regards,
Rafael