John,

Please pay attention to the fact the both Vref+ and Vref- must be in the rage AVDD+0.2V and AVSS-0.2V.
So the Vref- is not allowed to be –2.5V.
If you want to have full dynamics put Vref- at ground and Vref+ to VDD supposing VDD=5V as required for C552.
If you have to measure negative voltages use an OPAMP to translate it to positive range and scale from 0 to 5V.

Regards,
Cristian Ceausescu