A 0.9 V Adaptive Sampling Rate Differential Level Crossing-SAR ADC for Biomedical Signal Acquisition System

In this work, a successive approximation register(SAR) analog-to-digital converter(ADC) is proposed based on the level-crossing(LC) method, which is suitable for biomedical applications. The proposed architecture employs a differential topology at the input to remove even harmonics resulting in a higher effective number of bits(ENOB). The LC method and delta sampling approaches are used to eliminate the disadvantages of requiring two comparators in conventional LC-based ADCs. In other words, by adding a judgmental switching input before the comparator and employing a SAR quantizer, the required oversampling ratio (OSR) is decreased, causing a higher signal-to-noise-and-distortion ratio (SNDR), along with a reduction in the silicon area of the capacitive array. In addition, an array of the metal-oxide-semiconductor capacitor (MOSC) is added to the comparator stage to adjust the comparator’s offset and the threshold voltage of the LC topology. The proposed ADC was fabricated using TSMC 180nm complementary metal-oxide-semiconductor (CMOS) technology. The simulation and measurement results show that for a signal bandwidth of 4 KHz and an oversampling rate of 16, the output sampling frequency, power consumption, and SNDR are automatically adapted to different input signal frequencies. The measurements report a power consumption of $580 \sim 763 \mathbf{~ n W}$ and an SNDR of $49.38 \sim 60.23 \mathrm{~dB}$, equivalent to an ENOB of 7.9 9.7 bits for input frequencies of $1 \mathrm{~Hz} \sim 4 \mathrm{kHz}$, respectively. The core of the proposed ADC occupied a silicon area of 0.1476 $\mathrm{mm}^{2}$.