A CMOS OVERSAMPLING D/A CONVERTER WITH A CURRENT-MODE SEMIDIGITAL RECONSTRUCTION FILTER

Oversampling digital-to-analog (D/A) converters employing sigma-delta modulation noise shaping and single-bit quantization are attractive for use in digital audio applications because of their relaxed reconstruction filtering requirements and their tolerance of component mismatch. However, the use of a two-level D/A interface results in a large amount of out-of-band quantization noise that typically must be attenuated by a carefully designed analog reconstruction filter. This paper introduces a means of simplifying the reconstruction filter design through the use of a semidigital finite-impulse-response (FIR) filter. In particular, it describes an oversampling D/A converter wherein a current-mode semidigital reconstruction filter is used to implement a multilevel D/A interface that attenuates the out-of-band quantization noise without requiring precise component matching. An experimental implementation of the converter achieves a dynamic range of 94 dB and 72 dB attenuation of out-of-band quantization noise for a baseband of 20 kHz. The prototype converter, which consists of a linear interpolator, a second-order noise shaper, and a 128-tap semidigital FIR filter, dissipates 59 mW from a 5-V supply and occupies an active area of 3 mm2 when integrated in a 1.2-mum digital CMOS technology.