A micro-power low-noise auto-zeroing CMOS amplifier for cortical neural prostheses

A novel architecture to realize a low-power, low-noise amplifier for cortical neural prostheses is presented. The design consists of a low-noise variable gain amplifier as the first stage, a low-Gm high-pass filter as the second stage, and a low-pass Gm-C amplifier as the last stage. Discrete-time autozeroing is utilized to reduce the offset and noise. The bandwidth and autozeroing frequency of the amplifier is optimized to reduce noise folding. A current division technique is utilized to achieve a low-Gm OTA (Operational Transconductance Amplifier) so that low frequency operation is realized without any external capacitors. All the input pair transistors are biased in sub-threshold operation to reduce power consumption. A cross-couple parallel pair of source degeneration transistors is employed to increase the linearity crucial to neural spike detection. This design achieves variable gain from 470 (55 dB) to 1. In a CMOS 0.18 um process with 1.8 V power supply, the total circuit occupies 0.245 mm² with 26 uW power consumption and 1.8 kHz bandwidth. Total harmonic distortion is less than 1%, while input noise is 4.24 uV_rms within the band of interest.