Input voltage on differential ADC (MCP33131D-05-E/MS)
The question is: can I apply a negative (-VREF) signal to the Ain-


No. The Ain signals are DIFFERENTIAL, which means that Ain+ can be 0, and Ain- can be VREF, which is -VREF in internal ADC representation. If Ain+=VREF and Ain-=0, it is +VREF. But both inputs must be positive in relation to ground.