To synchronize multiple AD777x, an external pulse in the SYNC_IN pin is required. This pulse does not affect the data programmed in the internal registers and must be synchronous with MCLK.
There are two different ways to achieve a synchronous pulse:
Applying an asynchronous pulse on the START pin, which is then internally synchronized with the external MCLK clock, and the resulting synchronous signal is output on the SYNC_OUT pin.
Triggering the SYNC_OUT internally. When the AD777x is configured in SPI control mode, toggling Bit 0 in the GENERAL_USER_CONFIG_2 register generates a synchronous pulse that is output on the SYNC_OUT pin.
The SYNC_IN and SYNC_OUT pins must be externally connected if internal synchronization is used.
In synchronizing multiple AD777x devices, the SYNC_OUT pin of one device can be connected to multiple devices. This synchronization method requires the use of a common MCLK signal for all the AD777x devices connected, as shown in the figure below.
If the START pin is not used, tie it to IOVDD
AD777x also features SRC (sample rate converter) allowing the synchronization of multiple ADCs without interrupting the data flow. This is discussed in detail in the analog dialogue Newest Sigma-Delta ADC Architecture Averts Disrupted Data Flow When Synchronizing Critical Distributed Systems