1. Field of the Invention
This invention relates to a pulse width modulation digital-to-analog converter for converting input digital data into a pulse width modulated waveform for conversion into corresponding analog signals.
2. Prior Art
Various types of digital-to-analog converters are known in the art for converting input digital signals into corresponding analog signals, such as a pulse width modulation or PWM system for converting input digital data into a pulse width modulated waveform, or a pulse amplitude modulation or PAM system for converting input digital data into a pulse amplitude modulated waveform, for conversion into corresponding analog signals.
On the other hand, in digital audio signal processing systems, such as CD or DAT, several techniques have been proposed for reducing quantization noise at the time of decoding analog audio signals from digital data. These include an oversampling technique for converting digital data with a sampling frequency fs according to Nyquist's theorem into data having a sampling frequency which is a multiple (n.multidot.fs) thereof, and a noise shaping technique which changes the frequency distribution characteristics of the noise for improving the S/N ratio. For example, as disclosed in the Japanese Patent Publication 61-177819 (1986), digital data which have been subjected to oversampling or noise-shaping, as mentioned above, are converted into analog form by a digital-to-analog converter producing analog audio signals having a reduced quantization noise level.
Meanwhile, although it is possible in principle with the above mentioned PAM digital to analog converter to produce analog output signals with excellent linearity and reduced distortion, it has the drawback that high accuracy resistance summation or current summation circuits, accurately registered with the weights of the input digital data bits, are required. If it is desired to improve resolution, the circuit scale must be increased and the circuit as a whole must be designed to provide high accuracy.
On the other hand, the above mentioned PWM digital-to-analog converters have a less complex circuit organization. In a conventional PWM digital-to-analog converter, as shown in FIG. 1, pulse width modulated waveforms having pulse widths W.sub.7, W.sub.6, W.sub.5, W.sub.4, W.sub.3, W.sub.2 and W.sub.1, corresponding with 3-bit 7-value input digital data D.sub.7, D.sub.6, D.sub.5, D.sub.4, D.sub.3, D.sub.2 and D.sub.1 are generated and freed of high frequency components in a low-pass filter to produce analog output signals. However, as shown in the spectral distribution diagram of FIG. 2, which illustrates the results of an FFT analysis of a pulse width modulated waveform generated with the use of the conventional circuit from a 10kHz sinusoidal signal, the even-number order distortion components, and above all, the second order distortion components HD, are large.