Hi Seung,
From the Oscilloscope capture, it almost seems like the device is in Auto mode. Can you verify the mode by reading the AMC Configuration Register 0, 0x4C.
Also, you mention that there are no internal or external trigger signals. What is the state of the device in the above picture? Meaning, can you let me know what registers were set prior to this, or if using the EVM software, what settings were selected.
Thanks,
Matt