Finally，many people believe that perfect signal balance is required to reject noise in balanced interfaces. While signal balance helps maintain headroom， it does not affect the common-mode rejection of the source/receiver system (impedance balance and CMR is what counts to reject hum and noise).
Especially in discrete balanced output stages, some designers trim signal balance by using a pot connected between the two outputs with its wiper grounded. (See Figure 2 .) But，the best CMR of the source/receiver system occurs at a very different setting from that which achieves best signal balance. Of course， this concern can be largely eliminated by turning to the popular integrated solutions， which guarantee high common-mode rejection through careful trimming of the internal resistors.
Output common-mode rejection (OCMR) is defined as the change in differential output voltage due to a change in output common-mode voltage. When measuring OCMR，VIN is grounded and a common-mode voltage，VCM， is applied to the output as shown in Figure 4. Ideally no differential mode signal (VOD) should appear. However， a small mode-conversion effect causes an error signal whose magnitude is quantified by OCMR.
发表于 2023-4-13 13:38
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