An Experimental Reliability Study of Pseudo-Resistors in Biomedical Applications

This paper presents an extensive study of pseudo-resistors for usage in biomedical applications like implantable brain-machine-interfaces. In order to suppress DC offsets while simultaneously recording signals ranging into the sub-hertz domain, low-frequency high-pass filters are employed in the frontend of neural recording integrated circuitry. With the limited ranges of on-chip passives, no sub-hertz corner frequencies are possible. Switch-capacitor resistances and resistor duty-cycling are infeasible since these techniques cannot provide the required absolute resistance values. Concepts like multi-rate duty-cycled resistors have been successfully demonstrated, but require clock signals, which can lead to noise back-folding, especially in chopped systems. Pseudo-resistors utilize the off-resistance of MOSFETs to form extraordinary large resistors, and are thus capable of providing the required corner frequency in a very area efficient way. The drawbacks are limited linearity, and deviations of 100x over process variation and temperature were reported in prior art. In this brief we show measurement results from more than 160 dice (>5400 recording channels) from 3 production runs to demonstrate that the variations are much less severe in realistic operating conditions, and that sufficient linearity can be achieved. Our experiments let us conclude that the usage of pseudo-resistors in neural recording applications can in fact be used to robustly realize the required HP corner frequency for electrode DC offset blocking.