A system and method is disclosed for demodulating RF amplitude modulated signals in a demodulator circuit of an EPCO compliant RFID tag. One advantageous embodiment of the invention comprises first and second input ports, a +ve envelope detector circuit for each of the first and second input ports, a −ve envelope detector circuit for each of the first and second input ports, a +ve envelope differentiator circuit, a +ve low pass filter, a −ve envelope differentiator circuit, a −ve low pass filter, and a zero crossing detector. The zero crossing detector detects a transition in the RF input signal using a voltage difference between a +ve filtered differentiated envelope signal and a −ve filtered differentiated envelope signal.