11.6 A multi-channel neural-recording amplifier system with 90dB CMRR employing CMOS-inverter-based OTAs with CMFB through supply rails in 65nm CMOS

In addition to minimizing input-referred noise and lowering power consumption, a good multi-channel neural amplifier system should be able to significantly reject common-mode electrical interference (CMI). The dominant source of CMI comes from capacitive coupling of electrical mains supply line or EMGs onto neural tissues and can be as high as 100mV_pp. Thus any neural recording setup needs a total common-mode rejection ratio (TCMRR) of at least 70dB for a minimum detectable neural signal of 5uV_rms. However, multi-channel neural amplifiers are commonly implemented with a shared reference input whose input impedance is several times lower than that of corresponding signal inputs. This results in a large mismatch at the bipolar electrode-amplifier input interface. As analysed in Fig. 11.6.1, the TCMRR is significantly degraded below 70dB, independent of an amplifier´s intrinsic CMRR (ICMRR). In this work, we report a micro-power, low-noise 16-channel neural amplifier that eliminates this impedance mismatch problem by using single-ended CMOS inverter-based LNAs for both the reference and signal inputs. Compared to conventional replica channel works, when operating at 1V supply, the LNAs can accommodate a large input CMI of up to 220mV_pp through the use of a common-mode feedback (CMFB) loop implemented through the supply rails of the CMOS-inverter-based LNAs, which coincidentally leads to a high amplifier ICMRR.