I need to measure an magnitude of an AC signal with a constant frequency. The problem is offset of the AC signal varies. What I am thinking is filtering the signal and than using Delta Sigma ADC to measure it. However, I haven't experienced any Those ADC's before. Is that a good approach or should I think different solutions.
You are getting ahead of yourself. You can't pick a technology before knowing what you want to do.
Delta-sigma is one of several easily available A/D technologies. They can be very high resolution at the expense of speed. However, there is no way to tell whether that is a good tradeoff in your case from the information given.
As for the DC offset, you basically need a high pass filter. Those also can be realized in various ways, including a simple C,R up front or software later. The advantage of putting the high pass filter at the input before the A/D is that it reduces the range the A/D has to have to only the peak to peak AC level.