What is the common mode rejection ratio of a differential signaling scheme and how does it reduce EMI?

The common-mode rejection ratio (CMRR) of a differential signaling scheme measures the system's ability to reject common-mode signals (signals that appear equally on both conductors of the differential pair) while preserving the differential signal (the difference between the two conductors). CMRR = 20 x log(A_differential / A_common_mode) [dB], where A_differential is the gain for differential signals and A_common_mode is the gain for common-mode signals. Differential signaling reduces EMI because: the two conductors carry equal and opposite signals, and their electromagnetic fields tend to cancel in the far field (the net radiation from the pair is proportional to the common-mode current, not the differential current); the cancellation is perfect only if the two conductors are perfectly balanced (identical impedance, identical coupling to the environment, and identical timing). The CMRR determines how much of the differential signal is converted to common-mode signal (through imbalance) and how much reduction in EMI radiation is achieved. For the EMI reduction to be effective, the CMRR of the entire signal path (driver, cable, and receiver) must be high. Typical CMRR values: LVDS drivers/receivers: 50-70 dB (excellent differential signaling with very low common-mode conversion), USB 2.0: 40-50 dB, Ethernet (100BASE-TX): 30-40 dB, and unbalanced twisted pair (no differential driver): 20-30 dB. The EMI reduction compared to single-ended signaling is approximately equal to the CMRR of the weakest link in the signal path.