Reza -
Please review the datasheet and the VREF spec. The device only supports a minimum of 0.5V for VREF.
For lower reference voltages, instead of the standard buffer, you would need to provide attenuation. This circuit would need to use a low-noise, high PSRR op-amp and precision resistors to maintain a good reference across designs.
Is there a reason the reference needs to be so low? Using even a 2.5V reference, gets the LSB = < 0.3uV.
There are a few references that go down to 1.2V and 1.024V (see links below). Not sure if they would meet your requirements.
http://www.ti.com/lsds/ti/power-management/shunt-voltage-reference-products.page#p232=1.2;1.2
http://www.ti.com/lsds/ti/power-management/series-voltage-reference-products.page#p232=1.024;1.024