I am testing the AD7998 on several identical boards. I get 50 samples each second and provide the average of these conversion data.
VREF is provided by REF195. The input data are buffered and filtered and perfectly stable (measured with a precision voltmeter)
On some boards the ADC data is perfectly stable and repeatable at each powerup.
On other boards the conversion results float slowly every 3-4 sec in a range of 10 ADU
On other boards after power-up with stable input the conversion results starts from different values (also 100 ADU far from the stable value) and reach a stable value after some seconds (also a minute).
I am reading the data after 5us the falling edge of CONVST.