Non-linear model for pseudo CMOS resistors addressing the recovery time in bio-amplifiers

Abstract This work presents a new fast model, based on piecewise linear (PWL) methodology, for pseudo-resistors SPICE simulation, including the non-linear working range, fully supported by experimental data. SPICE simulations with fixed resistances or transistor modelled as pseudo-resistors do not reproduce simultaneously its real behavior for both the linear and the non-linear regions. A complete set of steps which validate the PWL macromodel for bio-amplifiers project is presented. As a case study, this paper addresses the pseudo-resistor characteristics operating in the non-linear region when implemented in a low-frequency bio-potential amplifier with narrow bandwidth, for use in heart rate meters and QRS complex monitoring systems. The use of the pseudo-resistor in this low-voltage low-power topology associates DC offset cancellation with an adequate frequency response. Moreover, this work shows that the non-linear characteristics of these devices can be used for a significant reduction in bio-amplifier recovery time (a.k.a. settling time) without any additional device or mechanism, making the heart rate measurements steadier and more reliable than obtained so far in current systems. The transient recovery time, as well as the response to QRS complex, was widely evaluated through experimental measurements and SPICE simulations, thanks to the proposed model. The bio-potential amplifier design was implemented using GF 8HP 0.13 μ m BiCMOS technology from Global Foundries.