An ASK demodulator for use in an RFID transponder having a limiter circuit associated with the antenna circuit and converting the ASK antenna fieldstrength modulation into an ASK limiter current modulation by limiting the antenna voltage to a fixed value and thereby causing the limiter current to be substantially proportional to the ASK antenna field strength, and a current discriminator circuit that discriminates the ASK limiter current modulation. By converting the fieldstrength modulation into a proportional limiter current and discriminating that limiter current, a linear relationship and a stable demodulator sensitivity are achieved. The current discrimination can be made accurately under low-voltage conditions.