Vincenzo,

Why a multiplexed input ADC? Perhaps i missed your multi-channel requirement. If you need only 1 channel there are ADC's that perform conversions in 2us or less.

I wasn't aware Maxim made a version of the '0844 if it is similar to the original National part, check the National Datasheet the connection scheme for the 8039 will work just fine (although some 8052 specific alterations could enhance speed). Also note that the 8039 is very similar to the 8052 so the code can get you started upon your '52 solution.

regards,
p