We have AD7173-8 running off +/-2V5 rails but everything decoupled to GND.
Is it best to decouple everything to AVSS (-2.5V) as in effect that is the GND for the device??
You can refer to the AD7173-8 Evaluation Board. For analog circuitry it is recommended to decouple to AVSS but for digital it must still be DGND.