1. How does the serial interface operate?
The serial interface on these families of converters is implemented as a state machine. The interface works by counting clocks on each data transfer. Therefore…
Yes, tying DIN high between write operations prevents invalid data being written to the ADC. It might be a different case when AD7190 is in continuous conversion read mode. Because in this read mode, the part also monitors the DIN and 40 consecutive 1s will reset the chip. Unlike in the standard read mode the ADC only monitors the DOUT line and don't care the DIN line during this read operation.