Hi Terumasa,
The scan rate is the max rate at which you can multiplex between channels. If the output data rate is 250 ksps and the sinc5+sinc1 filter is used, the settling time is 20 us. So, you can switch between channels at 20 us max.
With the sinc3 filter, the settling time is 3/output data rate. So, at 250 ksps, the settling time is 12 us. At 50ksps, the settling time is 60 us. This is the time for which RDY is high after initiating the single conversion.
Regards,
Jarwin