A Novel Technique of Enhancing CMRR of Fully- Differential Instrumentation Amplifier Using High Gain CMFB Loop

In this paper, the enhancement of the common- mode rejection ratio (CMRR) of a fully-differential capacitively-coupled chopper instrumentation amplifier (CCIA) used for biomedical signal acquisition system is proposed. To achieve the gain accuracy of the instrumentation amplifier (INA) high gain multi-stage fully-differential operational amplifier (Op-Amp) has been used as the building block. The enhancement of the CMRR is achieved by using common-mode feedback (CMFB) circuit with very high loop gain. The high CMFB loop gain has been achieved by suitable selection of the CMFB circuit and the connection of the CMFB circuit to the multi-stage Op-Amp. The entire circuit has been designed using 180 nm technology with 1.8 V supply. The designed multi-stage Op-Amp has a low-frequency differential mode (A ) and common mode ( ) gain of 123.5 dB and - 105.02 dB, respectively, using a CMFB loop having a gain of 136.3 dB. Using this Op-Amp the designed CCIA achieves a CMRR as high as 145.1 dB with Ad of 40 dB @3dB bandwidth of 10.38 KHz. Finally, the response of the CCIA has been compared with some of the state-of-art designs.