Design issues for a 20-bit D/A converter based on pulse width modulation and noise shaping

A 16-bit quality digital to analogue converter (DAC) using pulse width modulation (PWM) can be realised with near zero harmonic distortion. Noise shaping is employed to reduce the data word length but this interacts (mildly) with the PWM. The system can be extended to handle 20-bit signals, however the interaction becomes so significant that it should be taken into account during design. The two effects of concern are the modulation of high-frequency noise into sidebands at audio frequencies, and the intermodulation of this noise with the input signal. These effects are demonstrated for different types of noise shaping filters and a systematic solution to the design problem is presented. Computer simulations are used to verify system performance and demonstrate that the choice of input signals is crucial for distinguishing between these effects