Hello Peter ,Ijaz,Charles & Steve
Sorry for providing incomplete information
& thanks for the concerned you showed.

ADC used is Cirrus make CS5526 & multiplexer
is Maxim make DG409.
Steve , as per your suggestion ,I grounded the ADC i/p momentarily the ADC gives proper
count but when another channel is selected
after momentarily grounding ,ADC again goes into oscillation .
If one of the ADC i/p is grounded ,then there is absolutely no problem.
But I am using differential (more technically
speaking fully differential signal or floating signal ) so with grounding, it simply becomes single ended signal ,the concept of differential mode is completely lost.


Thanks