Kuldeep,

Unfortunately I do not know is causing your problem. I can only guess. If noise is being coupled into your adc circuit who knows what effects that might cause. It could be a combination of effects. It is up to you to perform tests to isolate the problem. I cannot do this as I am not in front of your equipment.
I gave you a suggestion - have you performed the test I proposed? Don't get to caught up on the effect on the EOC signal - try and find the cause. Once you have found the cause then you may be able to explain the effect.

With any problem solving exercise - stop -think carefully - devise some tests (the best tests are usually very simple) - check the 'obvious'. From the test results you will find some things don't make a difference and some do. This can help lead you to a solution.

Some more tests:
disconnect the signal wire from the hall effect sensor (I gather it has three wires gnd,signal and power). Try the other wires, see if there is any change. We do know that disconnecting the three wires fixes the problem. This test alone should take you a couple of minutes and may give enough information as to what the cause might be. Some measurments with an oscilloscope would also give more evidence.