Hi Shubham,
DSYNC resets the digital filter, so if you reset after each reading you will lose the advantage of having the SINC3 digital filter. It may be a good idea to DSYNC after a channel change, but unless there is a large difference in the voltage levels you will not notice much difference.
The timing delays required between commands and data is given as time t6 in the timing specification table on page 9 of the datasheet. In most cases this will be 50 tosc periods. If fosc is 1MHz, tosc is 1us and the delay required will be 50us.
The ADC itself will sample at the modulator rate unless in power down or sleep condition. The DSYNC command resets the digital filter and restarts the conversion from that point.
Best regards,
Bob B