Hi Tak-san,
The analog inputs of the AD9257 are not internally biased, so you need to apply the input common mode voltage from an external source. This "external" source can be from the VCM pin of the AD9257, which is nominally 0.9V. An example of this is shown in Figure 46 and Figure 47 of the datasheet.
You would like to apply an input common mode voltage equal to 1.1V. If this is the case, then you need to use a different external source for the input common mode voltage.
Before we discuss how to apply the 1.1V common mode voltage, please let me confirm about your input signal. When your Differential input voltage = +/- 1.0V, does this mean that your signal will go 1V above the input common mode voltage, and 1V below the common mode voltage? Please describe your input signal a little bit more.
The full scale input voltage of the AD9257 is 2V peak-to-peak-differential. This means that each single-ended signal swings 1Vpp, which is also saying that, for full scale, each single-ended signal swings 0.5V above the input common mode voltage, and 0.5V below the input common mode voltage. Does your signal fall within these boundaries?
Thank you very much.
Doug