Continuous-Time Acquisition of Biosignals Using a Charge-Based ADC Topology
This paper investigates continuous-time (CT) signal acquisition as an activity-dependent and nonuniform sampling alternative to conventional fixed-rate digitisation. We demonstrate the applicability to biosignal representation by quantifying the achievable bandwidth saving by nonuniform quantisation to commonly recorded biological signal fragments allowing a compression ratio of <inline-formula><tex-math notation="LaTeX">$\approx$</tex-math> </inline-formula>5 and 26 when applied to electrocardiogram and extracellular action potential signals, respectively. We describe several desirable properties of CT sampling, including bandwidth reduction, elimination/reduction of quantisation error, and describe its impact on aliasing. This is followed by demonstration of a resource-efficient hardware implementation. We propose a novel circuit topology for a charge-based CT analogue-to-digital converter that has been optimized for the acquisition of neural signals. This has been implemented in a commercially available 0.35 <inline-formula><tex-math notation="LaTeX">$\mu \text{m}$</tex-math></inline-formula> CMOS technology occupying a compact footprint of 0.12 mm<sup>2</sup>. Silicon verified measurements demonstrate an 8-bit resolution and a 4 kHz bandwidth with static power consumption of 3.75 <inline-formula> <tex-math notation="LaTeX">$\mu$</tex-math></inline-formula>W from a 1.5 V supply. The dynamic power dissipation is completely activity-dependent, requiring 1.39 pJ energy per conversion.