Singh,
Now that OSR = 128, you get one number at the ADC output when you apply an input that is 128×Fs.
If the clock frequency is Fclk, and Fclk = OSR × Fs. Just apply a sine wave at the input. The input signal frequency is
Fsig = Fclk × n / 2
m.
n is a odd number so that n and 2
m are coprime numbers. Then do 2
m-point FFT with the ADC output data.
Choose a big enough number for 2
m so that the frequency bin is small enough to identify signal, distortion, and noise.
You need to read a book named
Understanding Delta-Sigma Data Converters.
http://www.designers-guide.org/Books/#Schreier-2004Yawei