I sincerely appreciate community suggestion in subjected matter.
The target is to ADC DC voltage that can be in range from -0.15V to +0.15V in harsh environment (work close to internal combustion engine, alternator, igniting units and VHF transmitter) .
The chosen setup is AD8138 as front end and ADC AD7450A (12bit SAR) where AD8138 transforms bipolar signal to differential with gain equ 1.
Point where help is needed - is decision on whether to chose variant A. where ADC input buffer stays with gain 1 and ADC works with rather small signals in noisy environment with the smallest Vref to match input voltage or variant B where ADC input buffer has a gain greater than 1 to scale the input voltage up to the max of Vref but also to amplify all the noise components. Currently input stage of the ADC buffer has the first order low pass filter with small filter elements in both legs of the ADC buffer.