Purshottam:

<pre>
AD_MEAS: MOV A,CH_MEAS ;WHAT IS TO BE MEASURED 1
ANL A,#03h ;CLEAR MSNIBBLE REMAINDER HAS WHICH CH# TO MEASURE 1
ANL P3,#11111100b ;CLR BIT 0&1 OF P3 1
ORL P3,A ;SELECTS 4052 CHANNEL 1
<\pre>

You need to put this code at the tail end of your routine. You seem to face an issue of the setup time of the 4052. You are selecting a channel on the 4052 and immediately(before it settles) try to digitize it. This is the source of your problem. So, I suggest, you select the next channel to digitize after you have digitised the current channel. This gives the 4052 time to settle before you digitize the channel.

Hope this helps
Jerson

EDIT:
Having problems with the formatting. Please excuse me. I have used the pre tags with <> brackets but somehow the trailing \pre does not get interpreted as it is visible in the post.