Hi,
You are correct that you can provide an external VREF when SENSE is tied to AVDD. The AD9235 is characterized for operation between 1 to 2Vpp differential. Greater voltages like 3Vpp differential can be accommodated, but distortion performance will degrade.
The internal reference generator has not been characterized to output 1.5 V, so I would not depend on that to generate the VREF in that case.
You may gain some headroom with higher analog supply voltage, which in-turn will increase the Vcm that the AD9250 outputs and give the input sampling circuitry a little more margin.
Anthony
