Negative Voltage Reference for Bipolar ADC
You are over-complicating this. The power rails for the circuit in your question are +2.5 volts and -2.5 volts and it uses a REF3125 to produce a REFP voltage that is precisely 2.5 volts above REFN. This makes REFP 0 volts but if REFN drifts to -2.6 volts, REFP must also track to -0.1 volts.

The internal reference voltage needed by the ADC is REFP-REFN hence, providing REFN is within the constraints dictated by the chip, it doesn't need to be tied to -2.5 volts. For instance if your power rails are 0 volts and +5 volts then you tie REFN to 0 volts and use a regular reference chip to produce (say) +2.5 volts.

In other words REFN doesn't need to be precisely set or even fairly constant but REFP needs to be a fixed value above REFN.


  What is the standard way to get good quality bipolar reference voltage
  in a circuit?


You don't need one - a common/semi_rough -2.5 volt rail (that also feeds the op-amps) is fine for REFN. You need REFP to be precisely set above REFN - that's all.