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.