ADS122C04: Measuring voltage reference to ground

Part Number:ADS122C04

Hi,

I am using the ADS122C04 to measure voltage between A and B (across R2) and between B and GND (see schematic below which is a simplification of the actual circuit). 

 As far as I understand, I need to power the ADS122C04 with AVSS below 0 to properly read negative voltages.

To measure VAB I have configured the MUX with value 0 (AINP = AIN0 and AINN = AIN1). Problem is with VB. Apparently, I cannot set the MUX to AINP=AIN1 and AINN = GND (see Table 19 in the datasheet) but only AINP=AIN1 and AINN = AVSS. Using this configuration I get an overflow even if the voltage is below:

Questions:

1) How can I measure VB respect to ground with AIN1? Which is the correct configuration? (Please, note that AIN2 and AIN3 are used for other purposes).

2) Why I get an overflow measuring VB respect to VSS?

Thanks.