An Implantable CMOS Front-End System for Nerve-Signal Sensors

An analog front-end dedicated to processing of cuff-recorded human nerve signals is presented in this paper. The system is comprised of a low-noise preamplifier and an A/D converter (ADC) for quantizing the recorded nerve signal. The instrumentation amplifier utilizes CMOS transistors biased in the weak/moderate inversion region at a relatively high current for low thermal noise performance and achieves low flicker noise performance through chopper stabilization.

The resulting measured equivalent input referred thermal noise is 6.6 nV/√Hz at a chopping frequency of 20 kHz. A two-stage design is implemented which achieves a measured amplification of 72.5 dB over a signal bandwidth of 4 kHz. For the ADC, a third order ΣΔ-modulator employing a continuous-time (CT) loopfilter was implemented.

Each of the integrators in the loop-filter are implemented as Gm−C elements. For a sampling frequency of 1.4 MHz, the measured SNDR for the ADC is 62 dB, whereas the dynamic range (DR) is 67 dB over a 4 kHz bandwidth, equivalent to a resolution of 10 bits. The system draws a current of 196 μA from a 1.8 V supply thus consuming approximately 350 μW excluding buffers and bias circuitry.