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Linda,

Can you share some more information about your hardware? Your schematic and/or layout would be a great start.

Linda,

I do not have the appropriate software to view your layout, but I've got a few follow up questions along with some suggestions...

1. Where is the controller in this set-up? Is it on another board or is it part of this board? If it is on another board, is there a quality ground connection?
2. On this schematic I do not see where AGND and DGND are tied together - this could be related to BUSY not toggling and related to the REF5025 output. I might have missed this on your schematic or it could be on another page...
3. C149 seems pretty large, perhaps consider reducing this. As it is right now it's a bit of time until the cap charges up and takes the ADS8556 out of reset.
4. Perhaps increase the capacitance at C40
5. Right now the reference buffer is disabled but you are applying the reference voltage at the REFIO pin of the ADS8556. Observe the block diagram of the ADS8556 on page 1 of the datasheet. Note that the REFIO input goes through a buffer before feeding the converters. Right now you have REFBUF_EN tied high so the reference buffers are disabled. This may be related to the output you see on the REF5025 and it is certainly going to be an issue for actually using the devices.

Play around with these points and if your issues persist please provide us a few oscilloscope captures - ideally I'd like to see the RESET pin on power up, BUSY with respect to CONVST (I'd like to see the BUSY signal at the pin of the device, not after the logic circuit), and a scope capture of the REF5025 output.