If my signal path is as the following: enter image description here

My input DR is 20mVpp and goes through the above path, and the ADC used is 16 bits. Would the LSB be 5V/(2^16)=76uV or the gain will be added to the LSB as (5V/(2^16))/90=0.84uV. If the gain is added I am having a hard time understanding why it improves the bit resolution.