ADS8548: I have a question about ADS8548.

I have two board,

Number 1, this board is "The refio pin is 4.966V", the power on sequence:

the green is AVDD, the blue is DVDD

the green is AVDD, the blue is HVDD

Number 2,the power on sequence

the fist on is HVDD, the second on is AVDD, the third no is DVDD.  this board is no problem. IF the number one''s ADS8548 move to 

Number 2,the REFIO is normal(2.5V).

Why is board 1 so different from board 2?  Can also try toggling the STBY pin low on board 1 and see if that recovers the reference to the proper 2.5V level?

The schematic and layout of board 1 is different from board2.  The board 2 is the avdd power on before the dvdd. The board 1 is the avdd and the dvdd power on simultaneously. So I think the avdd must power on before the dvdd. but  why?

Current limits on your power supply may be the issue.  Does toggling the STBY pin low revive board 1?

Does toggling the STBY pin low revive board 1。Yes。
Can I connect pin 9 /stby to pin 10 REset?

I am wrong. "connect pin 9 /stby to pin 10 REset"  can't work. 

Now I have many board 1.The refio pin of some boards is 4.966V.  what can I do?

Do you have email?  I can email you schematic.

I have sent you schematic though email.

Thank you - I will review the schematic later thia afternoon.  You would need to use an inverter on RESET to connect that into the STBY pin - they are opposite polarity.

now I have 100-200 board 1. I have three method:
one: The refio of ads8548 ouput 4.966 is few, so I can replace the ads8548.
two:  I product new small power system board, remove old power system, welding the new small board.  But this method is diffcult.
three: I add line from FPGA to STBY pin. But this method is too ugly.
I want to select the fist one.  what do you suggest？

If the 4.966 devices are working on the second board, you could certainly try to swap them out if that is the easiest thing for you to do.  Or try adding an inverter between RESET and STBY with something like the SN74AHC1G04.

If I use method 1, the normal ads8548 running long time Will become that the refio of ads8548 ouput 4.966 ?

Why the power system effect a few ads8548, but it  do not effect the most ads8548.

Long term I do not think you will have an issue.  As for why some parts are more sensitive, that may just be normal process variation along with your supply ramping up.  Can you try also to hold the EN pin of your 5.0V and 3.3V low (one at a time, try 3.3V first, and then 5.0V) to see if that has any influence?

“Can you try also to hold the EN pin of your 5.0V and 3.3V low (one at a time, try 3.3V first, and then 5.0V) to see if that has any influence?”
Wherther the 3.3V first or the 5.0V first, the refio of ads8548 is normal(2.5V).

Is there an easy means for you to delay one of those power rails?  Perhaps the RESET signal can be used?