It is well-known that a twisted-pair wire (TPW) is very effective for reduction of the inductive coupling noise. The model of reduction of the coupling noise on TPW has been studied by C. R. Paul. In this model, he describes on an effect of reducing the inductive and capacitive coupling noise, and carries out examination on TPW on the premise that the coupling noise has been induced on TPW. He also says that TPW has no effect for reducing the capacitive coupling noise in case that one side of TPW is grounded, and as a result, unbalanced, and that TPW has a great effect of reducing the capacitive coupling noise if TPW is balanced against ground.
However, in practical cases, TPW is not used under balanced condition. And he does not quantitatively verify an effect of reducing the capacitive coupling noise, including a mechanism of the capacitive coupling.
In this paper, we propose a simple capacitive coupling model and the equivalent circuit that are valid for low frequency area. And we study theoretically an effect of reducing the capacitive coupling noise and clarify experimentally its effect, in every case that TPW is balanced, unbalanced and grounded against ground.