A 30-μW 94.7-dB SNDR Noise-Shaping Current-Mode Direct-to-Digital Converter Using Triple-Slope Quantizer for PPG/NIRS Readout

This article presents a low-power, high-resolution, high dynamic range (DR) current-mode direct-to-digital converter (CM-DDC) using a noise-shaping triple-slope quantizer. Both thermal and quantization noises can be first-order shaped out of band, increasing the resolution effectively. High resolution can be realized without using a high sampling rate or high-order shaping architecture. A coarse-fine zoom style quantization is exploited, which reduces the conversion time and, thus, the power consumption of active circuit blocks with dynamic biasing. On the circuit level, a chopped current digital-to-analog converter (DAC) is proposed to reduce 1/ <tex-math notation="LaTeX">f</tex-math> noise of the current DAC, enhancing the DR. The CM-DDC with light-emitting diode (LED) driver and digital control circuitry is implemented in a standard 55-nm CMOS process and characterized experimentally. The converter achieves a signal-to-noise and distortion-ratio (SNDR) of 94.7 dB and a cross-scale DR of 136.5 dB in 20-Hz bandwidth with 30- <tex-math notation="LaTeX">μ</tex-math> W power for photoplethysmography (PPG)/near-infrared spectroscopy (NIRS) applications. Thanks to the shortened conversion time, the converter’s sampling frequency and signal bandwidth can be adjusted to 200 Hz and 2 kHz, achieving 90.5-and 79.2-dB SNDR, respectively, which allows various current sensing applications. The proposed CM-DDC is also validated with on-body chest PPG measurement.