If not, why is this one register addressable on 9 bits, and all the others on 8
The registers 0x36 and 0x107 allow the user to control the buffer currents.
These buffer currents let you control the SFDR in certain input frequency
ranges. The initial design did was not intended to provide these registers to
our customers. But in characterizations, we found out that these registers
offer a great deal of flexibility in providing a high dynamic range for the
part at different Nyquist zones. Shown attached is the relationship of the SFDR
with the Analog input frequency at different buffer current settings (i.e.
Figure 49 in the datasheet).
So this is not an error, but by design. Unfortunately there is an extra bit for
the register address, but if the IF is below 150MHz, you really do not need to
program these registers. They can be set as default.