A CMOS oversampling bandpass cascaded D/A converter with digital FIR and current-mode semi-digital filtering

An oversampling bandpass digital-to-analog converter has been designed so as to eliminate the carrier leak and in-band SNR degradation that accompany I and Q channel mismatch in wireless transmitters. The converter combines a cascaded noise-shaping sigma-delta (ΣΔ) modulator with digital finite impulse response (FIR) and mixed-signal semi-digital filters that attenuate out-of-band quantization noise. The performance of the converter in the presence of current source mismatch has been improved through the use of bandpass data weighted averaging. An experimental prototype of the converter, integrated in a 0.25-μm CMOS technology, provides 83 dB of dynamic range for a 6.25-MHz signal band centered at 50 MHz, and suppresses out-of-band quantization noise by 38 dB.