A 9.6 nW, 8-Bit, 100 S/s Envelope-to-Digital Converter for Respiratory Monitoring

The signal's envelope contains important features for a host of applications empowered by machine learning algorithms. Conventionally, the raw analog signal is digitized first, followed by feature extraction in the digital domain. This brief presents an ultra-low-power envelope-to-digital converter (EDC), with respiratory monitoring as one of its applications. The EDC consists of a passive switched-capacitor envelope detector and an inseparable successive-approximation-register analog-to-digital converter (ADC). The two blocks at different sampling rates directly integrate without a buffer between them thanks to a ping-pong operation of their sampling capacitors. An EDC prototype was fabricated in a 180-nm CMOS. It provides 7.1 effective bits of ADC resolution and supports input signal bandwidth up to 5 kHz and an envelope bandwidth up to 50 Hz while consuming 9.6 nW.