Systems, methods and apparatus for wireless charging are disclosed. A charging device has a resonant circuit comprising one or more transmitting coils, a driver circuit configured to provide a charging current to the resonant circuit, a zero-crossing detector configured to provide a zero-crossing signal that includes edges corresponding to transitions of a voltage measured across the resonant circuit through a zero volt level or corresponding to transitions of a current in the resonant circuit through a zero ampere level and an Amplitude Shift Keying demodulator. The demodulator may be configured to receive a plurality of samples of voltage or current in the resonant circuit captured at times determined by the edges included in the zero-crossing signal, and demodulate a modulated signal obtained from the charging current using the plurality of samples of voltage or current in the resonant circuit.