Hi Rahul,
Negative AC resistances can occur in these measurements. It means that there is a negative voltage change for a positive current delta.
It will be very difficult to remove the signal you are talking about such that it will not be visible on a 16 bit converter. By my calculation, even a 1 Ohm output impedance would still yield 500 uV of output noise. You may need to add some additional filtering.
I hope this helps.
Mike