AD7329 reference voltage

Hi all!

I'd like to use the AD7329 in my design.
  From what I understand, a 2.5V reference can be used to measure up to ±10V and a 3V reference can be used to measure ±12V at the analog input pins.
  Reading through the datasheet, it's not clear to me what reference voltage is needed when I want to be able to measure ±16.5V at the analog input pins.

Thanks in advance!