The AD7683 is pseudo differential input and AD7864 is true differential input.
It is known that the pseudo differential input structure features less CMRR
than true differential input structure. However, in the datasheet, the CMRR
spec of the two parts is the same (65dB @ fin=100KHz). And the ADC simplified
schematic shown in figure 20 is the same. Does that mean the common mode
rejection capability of the two parts are the same?
The CMRR is the same since the parts are identical. The difference being at
final test, the AD7684 is programmed for a full differential input span.