When using an ADC with the ADS7863A, the differential AMP is used in the previous stage when the ADC input signal is small.
The analog side of the ADS7863A is 5 V and the digital side is 3.3 V.
The AMP of the THS4531A has a power supply of 5V and a midpoint of 2.5V.

Question 1 
 The differential AMP of the THS4531A is used, but the COM terminal of the THS4531A is the reference for the input.
 Should the COM pin of the THS4531A differential AMP be generally connected to the REFIN pin of the ADS7863?

Question 2
Also, can the COM pin of the differential AMP of the THS4531A be used without problems
 even if it is not connected to the REFIN pin of the ADS7863?

Question 3
If the reference voltage of the THS4531A drops, if it is not connected, 
you can change the register settings of the DAC of the ADC.
There is a discrepancy between the reference voltage on the input side 
and the REFIN voltage on the ADC side, but it can be set close to the reference 
voltage of THS4531A by register setting.
Is it correct to interpret that there is no performance problem?

Question 4
Is it correct to assume that it is a good idea to connect the COM pin of 
the THS4531A differential AMP while connecting the REFOUT and REFIN pins of the ADC?

Question 5
Even when the COM pin of the differential AMP of the THS4531A
is not connected to the REFIN of the ADS7863 (not connected)
Will it be canceled by changing VREF in the register setting of the DAC?