Hi, Youhei,
The VCMO input sets the output common-mode level for the baseband outputs, as described in Table 3 on p. 6 of the AD8347 datasheet. The input range when operating under 5V supply is from 0.5V to 2.5V (Table 1 on p. 3), but the output swing will be reduced when VCMO is below 1V, due to the output signals clipping near Ground level. In normal operation, the VCMO pin is connected to the VREF pin, which provides a 1 V reference to set the output common-mode level (See Fig. 46, Basic Connections, on p. 18).
Benjamin