A CMRR Enhancement Circuit Employing Gₘ-Controllable Output Stages for Capacitively Coupled Instrumentation Amplifiers

The capacitively coupled instrumentation amplifier (CCIA) is broadly used for neural signal acquisition. Although it is suitable for various biopotential recording applications, the CCIA is vulnerable to capacitor mismatches, which degrades the common-mode rejection ratio (CMRR). The neural signals with small amplitude can be easily contaminated by large-amplitude common-mode (CM) interferences due to the degraded CMRR. This brief presents a CMRR enhancement circuit for the CCIA, with which the capacitor mismatches can be compensated in a power-efficient manner. In the proposed CCIA, the gains of a pair of output stages are controlled separately to reject the output signals generated by the CM interferers in the presence of capacitor mismatches. Employing the proposed CMRR enhancement scheme, the implemented CCIA achieves a CMRR over 90 dB from 10 Hz to 1 kHz and 3.4-μVrms integrated input-referred noise (IRN) while only consuming 1.5 μW per channel.