Improved low-distortion sigma-delta ADC with DWA for WLAN standards

Abstract

An improved low distortion sigma–delta ADC (analog-to-digital converter) for wireless local area network standards is presented. A feed-forward MASH 2–2 multi-bit cascaded sigma–delta ADC is adopted; however, this work shows a much better performance than the ADCs which have been presented to date by adding a feedback factor in the second stage to improve the performance of the in-band SNDR (signal to noise and distortion ratio), using 4-bit ADCs in both stages to minimize the quantization noise. Data weighted averaging technology is therefore used to decrease the mismatch noise induced by the 4-bit DACs, which improves the SFDR (spurious free dynamic range) of the ADC. The modulator has been implemented by a 0.18 μm CMOS process and operates at a single 1.8 V supply voltage. Experimental results show that for a 1.25 MHz @ −6 dBFS input signal at 160 MHz sampling frequency, the improved ADC with all non-idealities considered achieves a peak SNDR of 80.9 dB and an SFDR of 87 dB, and the effective number of bits is 13.15 bits.