Pulse-width modulation in sigma-delta modulators

Time-encoding machines map analog or multi-bit digital signals to a binary stream. The information contained in the input signal is preserved by the high resolution achieved in the time domain. Pulse-Width Modulators (PWMs) rely on a similar operation principle. This paper explores two applications of PWMs to the design of data converters based on Sigma-Delta Modulators (SDMs). In the first case, the multibit output sequence of a digital SDM is PW modulated and, then, converted to analog by a low resolution Digital-to-Analog Converter (DAC). This paper shows that, with a proper selection of the PWM, the output signal is free from the errors produced by the sampled nature of the signal to be converted. What´s more, it does not exhibit even-order harmonic distortion, either. In the second case, the output of the quantiser in a multibit continuous-time SDM is converted to a digital PW modulated signal and then fed back to the input of the loop filter. In both cases a multibit DAC can be replaced by a single-bit one and the dynamic-element matching circuitry, which increases the modulator complexity and power consumption, can be removed.