Q) Since the AD9680 has multiple supply domains (1.25V, 2.5V and 3.3V), is there a need to sequence these supplies in any order?
A) The AD9680 has an internal Power On Reset (POR) “Nanny” circuit that manages all the power rails. Until this POR circuit is satisfied with the power rail levels, the device will be in reset mode. In reset mode, if SPIVDD is where it is supposed to be (1.8/2.5/3.3V), you will read 0xFF through the SPI ports.
As a troubleshooting technique, if you find out that the device is not getting configured and you read 0xFF for any SPI read cycle, the first place to check would be the supply voltages at the various pins of the AD9680. Chances are, one of them is out of range as far as the POR circuit is concerned.
The SPI port also sends out 0xFF for a short while after a SPI soft reset (0x000 = 0x81). This is temporary though, and lasts for about 5ms while the bootloader loads all the SPI defaults.