The ID register is made up two fields: DEVICE_ID and SILICON_REVISION. The DEVICE_ID field (4 bits) designates the generic, being:
0001b = AD7606B
0010b = AD7606C-16
0011b = AD7606C-18
For the SILICON_REVISON, AD7606B is at 0100b and AD7606C-16/-18 at 0010b.