Concerto ADC VREFHIGH

Hello,

I want to adjust VREFHIGH for a design which uses F28M35H52C. 

In "Hardware Design Guidelines for TMS320F28xx and TMS320F28xxx DSCs " it is mentioned that  "The F280x ADC accepts 2.048 V, 1.5 V, or 1.024 V on the ADCREFIN pin."  However, in conceto ControlCard design  3.3 V is used as VREF. I will appreciate if some one helps me to find out the correct value for VREFHIGH. 

Thanks in advance.