Q. Should I solder the centre pad of the CMX990 to the PCB or leave it floating? If soldered, what should it be soldered to?
A. The centre pad of the CMX990 should be soldered to Analogue Ground observing the usual precaution that no digital or RF or high-current return paths cross this region. This pad is isolated in the QFN package and bonding to analogue ground helps reduce signal coupling into the die substrate.