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Half of AD7793 always read 0xFFFFFF

Category: Hardware

Currently have 4 AD7793 parts on a PCB set up using the ratiometric 3-wire RTD example from the datasheet. All 4 have the exact same schematic and are setup and read with the same code (only the CS line changes). However, when reading the RTDs, 2 out of the 4 always read full scale (0xFFFFFF) no matter what (always the same 2). Reading back the registers from the chip seems to show that they are being set correctly. The status register does show that the output is usually being clamped to 0xFFFFFF, but measuring Ain+ and Ain- with a scope and multimeter both show that the actual voltages are what would be expected. What could I be missing?

After more troubleshooting, It seems that the external reference is the issue. The reading works with the internal reference selected. As far as I can tell, all of the external references are very close (using 0.1% resistors), so I still don't understand half of them having different behavior.

Edit to add more information discovered
[edited by: sgcc23 at 9:04 PM (GMT -4) on 13 Mar 2023]