An Analysis of CMRR Degradation in Multi-Channel Biosignal Recording Systems

Multi-channel biosignal recording systems that employ a shared-reference scheme and achieve high common-mode rejection ratio (i.e., CMRR > 80dB) have been reported in recent works. While it is well-understood that a shared-reference scheme causes impedance mismatch at the input terminals of bioamplifier, and thus limits the maximum achievable CMRR, a theoretical study that can provide quantitative assessment of this source of degradation is still lacking. This brief provides an equivalent electrical circuit model of the input interface consisting of an electrode array and bioamplifiers, followed by a complete analysis to formulate the CMRR degradation. Simulation results based on a previously designed and fabricated 32-channel neural recording front-end in a 180nm CMOS process are presented, exhibiting close agreement with the theory.