Reading 125KHz AC signal using Arduino

hi,

how can Arduino due read AC signal with 125KHz, what about the negative part

With a 16 MHz. Atmel 328, your 125 KHz. data rate translates to 64 clock cycles per read. I have seen somewhere that an analogRead requires about 144 clocks from the 328. I think you need to find a much faster chip. And don't forget that to faithfully capture an analog signal digitally, your sample rate (Nyquist) must be twice the highest frequency component of your signal.