Choice between Pseudo-Differential Unipolar and Pseudo-Differential Bipolar ADC Input
Pseudo differential takes a full positive swing on the +IN input whether it is unipolar or bipolar. If it is bipolar the -IN needs to be fed the nominal dc offset of the signal fed to +IN.

This is an advantage on analogue systems using a mid-rail generator (such as a voltage reference and op-amp buffer). If the voltage reference moves up or down a little, it doesn't affect the ADC code value produced because that offset movement will affect both input signals together.

For a unipolar input where -IN is 0V and a mid-rail generator is used, if the mid-rail generator shifts a little, then that is reflected as a dc error in the converted result.

I tend to use unipolar configurations (even when a mid-rail generator is used) because there appears to me more choice in ADCs AND I always choose really good references PLUS, in a lot of signal processing I do, I don't care about (say) a 10mV offset end-to-end.


  For example, the input signal is a single ended sin function with
  range from 0 to 3V and a DC bias of 1.5V. Should we connect the IN– to
  1.5V or to GND?



If the dc bias of 1.5 volts and very stable it doesn't matter which you use
If you don't care too much about small dc offset variations then use either
If you do care about offset variations then use bipolar pseudo differential