Demodulating AM Square Signals via a Digital Timer for Sensor Applications

This article evaluates theoretically and experimentally the performance of a timer-based demodulator applied to low-frequency amplitude-modulated (AM) square signals coming from sensor circuits. The demodulator extracts the amplitude of the AM square signal by measuring the period of a reference triangular signal that is altered by the AM signal itself, as already suggested in a previous article but for AM sinusoidal signals. The demodulation and digitization are carried out simultaneously via a digital timer and without requiring a rectifier, a mixer, a low-pass filter, or an analog-to-digital converter, thus resulting in a simple and low-cost design solution. Unlike the sinusoidal case, this article proposes to infer the amplitude of the AM square signal by measuring the bias of the period measurement with respect to the ideal (known) value. Experimental results show a nonlinearity error lower than 0.03% full-scale span and a resolution of 13.3 bits.