Continuous-Time Acquisition of Biosignals Using a Charge-Based ADC Topology

Abstract

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 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 CMOS technology occupying a compact footprint of 0.12 mm. Silicon verified measurements demonstrate an 8-bit resolution and a 4 kHz bandwidth with static power consumption of 3.75 W from a 1.5 V supply. The dynamic power dissipation is completely activity-dependent, requiring 1.39 pJ energy per conversion.

A low‐power, low‐data‐rate efficient ADC with hybrid exponential‐linear transfer curve for bio‐potential recording systems

“…Many of the biosignals such as ECG and ENG exhibit short active periods in the form of spikes interrupting potentially long silent periods. This can lead to significant oversampling if those signals are sampled uniformly . To avoid this oversampling, adaptive sampling scheme can be used by increasing the sampling rate whenever activity is detected in the signal or level‐crossing sampling can be used …”

“…The continues‐time level‐crossing ADC (LC‐ADC) or event‐driven sampling is a structure used in acquisition of biosignals . It is an almost new and promising ADC approach, which is based on level‐crossing sampling .…”

“…This can lead to significant oversampling if those signals are sampled uniformly. 18 To avoid this oversampling, adaptive sampling scheme 19 can be used by increasing the sampling rate whenever activity is detected in the signal or level-crossing sampling can be used. 18 The continues-time level-crossing ADC (LC-ADC) or event-driven sampling is a structure used in acquisition of biosignals.…”

“…18 To avoid this oversampling, adaptive sampling scheme 19 can be used by increasing the sampling rate whenever activity is detected in the signal or level-crossing sampling can be used. 18 The continues-time level-crossing ADC (LC-ADC) or event-driven sampling is a structure used in acquisition of biosignals. 15,18 It is an almost new and promising ADC approach, which is based on level-crossing sampling.…”

“…18 The continues-time level-crossing ADC (LC-ADC) or event-driven sampling is a structure used in acquisition of biosignals. 15,18 It is an almost new and promising ADC approach, which is based on level-crossing sampling. 20 In this approach, the samples are generated only when the input signal crosses the threshold levels.…”