Low power continuous-time delta-sigma ADC with current output DAC

Abstract

The paper presents a continuous-time (CT) Delta-Sigma (ΔΣ) analog-to-digital converter (ADC) using a current output digital-to-analog converter (DAC) for the feedback. From circuit analysis it is shown that using a current output DAC makes it possible to relax the noise requirements of the 1st integrator of the loopfilter, and thereby reduce the current consumption. Furthermore, the noise of the current output DAC being dependent on the ADC input signal level, enabling a dynamic range that is larger than the peak signal-to-noise ratio (SNR). The current output DAC is used in a 3rd order multibit CT ΔΣ ADC for audio applications, designed in a 0.18 μm CMOS process, with active-RC integrators, a 7-level Flash ADC quantizer and current output DAC for the feedback. From simulations the ADC achieves a dynamic range of 95.0 dB in the audio band, with a current consumption of 284 μA for a 1.7 V supply voltage; the resulting figure-of-merit is 262 fJ/conversion.