AD9266 digital output level

Evaluation board: AD9266-80EBZ.

Changes: I have changed the power so that DRVDD is 3.3V.

Problem: The signal level on the digital output is not going to GND. Here is a scope trace. CH1 (yellow) is the input signal. CH2 (blue) is D15/D14 (for this input signal the MSB is 1 on the peak and 0 in the trough) measured before RN808 and the buffer.

Is it normal for the output to sit at an indeterminate level like this? The datasheet doesn't mention anything about using a pull-down on the digital line, and I don't see one included in the evaluation board (apart from the PULLDOWN specified in the ucf file).

The output from Visual Analog (HSC-ADC-EVALCZ evaluation board) is attached below.