I'm working with an AD9154 board, and it looks like the blanking state machine is blocking the DAC outputs: I'm reading back 0x00 from register 0x147, which according to Table 73 in the datasheet indicates that the blanking state machine (BSM) is holding the output data at midscale.
From what I can see in the datasheet (in Figure 73 and text on page 68), this should only happen if the TXEN0/1 signals are active or if the LMFC sync machine is in the middle of a rotation. As far as I can tell, neither of these things are true here: the sync machine is locked (register 0x03B = 0x0D), and the TXEN0/1 inputs are both being driven high after the DAC is configured. The sync machine is configured for one-shot operation (bits 3:0 of register 0x03A = 0x1) with a continuous SYSREF input.
Are there any other conditions that could cause the BSM to hold the output data at midscale?