Common mode electronic noise in differential circuits

Abstract

Differential circuits are assumed to reject common-mode noise yet no parameter is available to describethat rejection as the Common Mode Rejection Ratio (CMRR) applies only to deterministic signals, not torandom noise. We propose a model and a method to analyze the contribution of the input common-modeelectronic noise to the output voltage noise of differential circuits. The analysis shows that the sameparameter-matching conditions that improve the CMRR determine common-mode noise rejection butCMRR is more sensitive to mismatching. For example, a low-frequency CMRR as low as 8 dB implies thatmore than 82% of common-mode noise is rejected. Thus, often only differential-mode noise is relevant.However, if the equivalent input current noise sources predominate over equivalent input voltage noisesources, cross-correlation between them partially yields common-mode noise that will also be rejected.We also propose a simple test that yields an estimate of that correlation.