Hi :

  I have simulated a third order one bit delta sigma modulator , the input signal is 20KHz,but after simulation i found  the fft result contain a dc component. I don't know what cause this dc component.

 and I think delta sigma modulator do good to the SNR, but due to nonlinear (like switch capacitor circuit nonideal )cause THD worst.  Is that right?

 and I also want to know doing a 16 bit delta sigma modulator, usually how much THD equal ?  

                                                                                                        Thanks!