Hi Marjorie,
The clock is not applied until I manually enable it, well after power up. To double check, I removed the AC coupling caps on the clock inputs and powered the card again. Good thought though. Same behaviour with or without AC coupling.
Measured voltage on rails is 1.88-1.91V. Our schematic is labelled 1V8 but it is actually 1V9 (or very close), sorry about the confusion.
It does look like the POR is happening properly. Following your steps: Step 1, I read 0xFFFF, Step 2, cycle ECEb HI then LO, I read 0x0000. This is very repeatable. Out of twenty restarts, it worked every time.
I did notice a potential clue... the power on calibration didn't run for two of the restarts. I have a LED attached to the CalRun pin and can see the LED light up briefly on startup, indicating calibration had started/finished. The step 1, 2 process still worked in both of these instances.
Do you have any other ideas about what could be happening?
This is our first design with this ADC. It is the third build of boards, and as mentioned, all previous builds have behaved as expected. They've been running for over a year now with no issues.
Thanks for your continued support.
Rob