Linear processing of PWM signals using a RC time encoded integrator

Abstract

This paper describes a delay-and-addition cell that enables direct signal processing of pulse with modulation (PWM) encoded signals. The cell can be considered functionally equivalent to a switched capacitor integrator. However, both its input and output are synchronous PWM signals. As a difference to a switched capacitor integrator, the circuit does not require operational amplifiers and is composed of passive RC circuits, switches, comparators and digital logic. Circuit implementation non idealities such as offset and propagation delays have also been analyzed. The main advantage of this circuit is the possibility to operate at a low voltage. The paper shows measurements of a demonstration circuit implementing a first order filter. As an application example, the filter is used to attenuate the quantization noise of a sigma-delta signal, delivering a continuously varying PWM waveform from a synchronous bitstream.