Have you got a logic diagram to indicate how you generate the START and OE strobes? What is the clock source? How do you stretch the nRD and nWR signals to meet the requirements on page 4 of the datasheet? What's the MCU clock frequency?

You've said,

     ORG 2003H        ; A15-Enable the decoder and A0=1,A1=A2=A3=0;
     MOV DPTR,#0001H  ; conversion data is received from this 
                      ; address
     MOVX A,@DPTR 

I'm guessing that you've said that you use A15 (P2.7?) to generate a negative level for what the diagram shows as negative "Address Decode", though that's just a guess.

You should probably have a look at the pulse width of the START/ALE pulse, to ensure it's at least the required length, based on the clock frequency you're feeding it.

RE