The SYNC command on the AD7768 can be thought of as a reset of the digital filter paths and logic within the AD7768. One of the major events that the sync triggers is the reloading of the digital filter coefficients based on the selected configuration.
A SYNC should be provided to the AD7768 after any changes to the AD7768 configuration, before valid samples are acquired.
The sync signal can be provided direct to the AD7768 SYNC_IN pin, to the START pin, or can be triggered over SPI, under software control.
To provide the SYNC over SPI or through the START pin, the SYNC_OUT and SYNC_IN pins must be tied together.
Once the SYNC is provided, it is no longer required to provide a SYNC again, until either the part is reset, or the configuration is changed.