Different specification on AD7691

I want to try an 18 bit ADC with an evaluation board and have thought of AD7691. However, when looking at it I find different specifications:

Here it says 16 bits on the evaluation board:

PulSAR ADC PMODs [Analog Devices Wiki]

And then about the reference voltage here it says from 2.3V to Vdd

http://www.analog.com/en/products/analog-to-digital-converters/ad-converters/ad7691.html#product-overview

And here from 0.5V to Vdd.

http://www.analog.com/media/en/technical-documentation/data-sheets/AD7691.pdf

I am actually interested in using a low Vdd ( 0.6V aprox)and 18 bits so could you please tell me which of all these is correct.

Also in the datasheet I see that SNR decreases with decreasing Vref, however it is only shown until 2.3V. How low does it go to at 0.5V.

thank you