Can anyone tell me what is the major difference between these two chips?
BTW: Is the SMA (SMB?) adapter in the attached picture included in the EVAL-AD7986 package? If not, where can I get it?
As we can see, you are right that both AD7986 and AD7641 are very much alike. But the AD7986 is in PulSAR technology while the is a SAR. Although these are almost the same and PulSAR was created base from SAR technology. Please refer to this thread for details of PulSAR. ( What is the Technique of PulSAR?)
In terms power consumption the AD7986 has the advantage (PulSAR) and Reference volatge range, AD7986 has wider with internal of 4.096V and external can be 2.4V to 5.1V against the AD7641 with 2.048V. This mean that AD7986 is more variable in terms of Voltage Reference settings.
The AD7641 is able to communicate in both parallel and serial interface while the AD7986 communicates in serial interface.
The SMA male to SMA male barrel adapter is not included in the EVAL-AD7986 package. You may check this website for details on the adapter. SMA Male to SMA Male Barrel Adapter [0752-0424] - $7.49 : Hobby Wireless - FPV RC shop, the best in wireless systems