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AD7788: Using it with low Vref

His question is on the specs in the datasheet:
There is a Reference Voltage Range specified, from 0.1V to Vdd.
1) Do you have any datas or an idea on the performance loss (missing codes)
when he decreases the REFIN from the specified 2.5V to low ranges of e.g
0.1 to 0.2 V ?
2) Does it influence the measurement if he sets his reference on REFIN(+) = 4.2
and REFIN(-) = 4 V or on 1.2V and 1.0V respectively?


With a 0.1V or 0.2V reference, the rms noise from the part  remains unchanged. 
However, the LSB size is now smaller so the p-p noise will degrade. 
For example, when Vref = 2.5V, the rms noise is 1.5 uV and the p-p resolution
is 19 bits. 
So, with Vref = 0.2V, the p-p resolution will be 15.5 bits. 
With a 0.1V reference, the p-p resolution will be 14.5 bits.

Having the reference biased about 4V is fine. 
The difference between REFIN(+) and REFIN(-) will be 0.1V or 0.2V. 
The reference pins have good common mode rejection.