AD7984 is an 18-bit ADC. Now I want to use my own controller to collect the output signal of CN0269. When ADC completes one acquisition, what bit is the output bit? How to determine the symbol bit of the input signal?
I'm not sure I fully understand your question.
There are no output bits or symbols to mark the end/beginning of a conversion. You start a conversion by using the convert pin of the AD7984 and then you follow the timing diagrams specified in the datasheet. The datasheet of the AD7984 will show you all the data transfer information that you will need to translate into your code for your controller. https://www.analog.com/media/en/technical-documentation/data-sheets/AD7984.pdf