Talking to ADS8698 (18-bit ADC) over SPI
Per the Datasheet PG47

During power-up or reset, the default content of the command register is all 0's and the device waits for a command to
be written before being placed into any mode of operation.

If your still working with this I'm going to be tackling this same thing very soon, hopefully I can get it to work with DMA.
https://forum.pjrc.com/threads/34863-Teensy-CC-Dummy-Load-300W/page4