On 3 out of 4 boards I have built that use an AD7689 SDO is stuck low from power up even if CNV is high. I used this chip successfully on multiple boards therefore I am pretty confident the circuit is good.
I noticed that if VDD ramp is slower than VIO, SDO will go high for a brief period of time (probably until VDD reaches its minimum level). If I increase VDD slew rate then SDO stays low.
SCK and DIN are driven high by the SPI master.
The chip does not heat and I am pretty sure we did not apply voltage outside the maximum ratings.
Are there any known causes to this issue?