Generalized Analysis of Random Common-Mode Rejection Performance of CMOS Current Feedback Instrumentation Amplifiers

The generalized CMRR analysis of the CMOS current feedback instrumentation amplifier (CFIA), in drain and source feedback configurations (termed DCFIA and SCFIA), is developed by focusing on the input stage. The random CMRR for processes (CMRRP) of the CFIAs are derived as a function of the transistor dimensions, small-signal and operating parameters, as well as the process-dependent mismatch factors, to gain insights into the statistical CMRR characteristics, which enables better design optimization. Simplification of the analysis based on practical constraints yields closed-form CMRRP equations that reveal different design considerations for the two CFIA configurations. For example, while the DCFIA relies on the matching of the input transistors, the matching of the drain load transistors is critical for CMRRP enhancement of the SCFIA (this criterion differs from established design practice). The analysis also reveals that unlike the SCFIA, the DCFIA features a direct tradeoff between noise and CMRRP. The integrity of the analysis is validated by comparison with simulations of various published CFIAs designed in a 0.35-mu m CMOS process.