An 8.2-$\mu$ W 0.14-mm2 16-Channel CDMA-Like Capacitance-to-Digital Converter

Abstract

This article presents a multi-channel capacitance-to-digital converter (CDC) circuits using the continuous-time Walsh coding multiplexing. The proposed circuits modulate the input capacitive sensors with orthogonal codes, enable simultaneous read-in of 16 capacitive sensors, and provide the combined 16 digitized outputs in a single conversion. By correlating 16 successive digitized outputs with the corresponding orthogonal codes in post-processing, the 16 capacitive channels can then be demodulated separately. The proposed continuous-time Walsh coding modulation preserves temporal information with minimum circuitry duplication and bandwidth requirement. This chip is fabricated in 130-nm CMOS technology and occupies an area of 0.14 mm2. With a reference clock of 4 MHz, the proposed multi-channel CDC achieves 11.4 ENOB with an average conversion time of 482 $\mu \text{s}$ . It consumes 0.51 $\mu \text{W}$ /channel and achieves an FoM of 87 fJ/conversion step.