Unbalanced impedance

The individual ionic activities must be estimated by use of the Debye-HUckel theory. However, Eq. (4.8.3) shows that the cationic and anionic contributions tend to cancel out. Hence, except for a truly unusual situation in which a particular ratio a(B)/a(A) exceeds a factor of ten, and/or where t/ z for a particular species is very large compared to all others, the emf remains well below the value of RT/F = 0.0592 V at room temperature. Thus, junction potentials tend to be small compared to most emfs developed by cells. The effect may be further reduced by use of salt bridges that contain cations and anions of comparable mobility, so as to compensate for the tendency to develop internal emfs. The effect is also attenuated by employing parchment, agar-agar gels, or collodion to impede unbalancing ionic motion across the junction. In any event, junction potentials of the type described here tend to be small. 

In practice, this interference cancellation process works fairly well however, the assumption that the power-line-induced signal is common mode (uniform) over the body does not hold perfectly in all situations. Slight differences in its phase or amplitude over the subject s body when in close proximity to some electric field source or unbalanced electrode impedances can cause this cancellation process to be less than perfect. Some line noise may still pass through into the recording. Usually, improvements in skin electrode preparation can remedy this problem. 

Early active-balanced output circuits used the approach shown in Fig. 10.288. The signal is buffered to provide one phase of the balanced output. This signal then is inverted with another op-amp to provide the other phase of the output signal. The outputs are taken through two resistors, each of which is half of the desired source impedance. Because the load is driven from the two outputs, the maximum output voltage is double that of an unbalanced stage. 

Simple TV antennas are often designed to have Zq values of 300 ohms. For connection to a 75 ohm coax cable without reflections, there should be a special transformer called a "balun." The antenna is usually a "balanced line," while the coax is unbalanced, so this isolation transformer mates them ("balun" means "balanced-unbalanced"), but it also mates the characteristic impedances. 

A component, usually a transformer, which can have a "balanced line" input (no grounds) but an unbalanced output (having one conductor be grounded). Often it also serves to match impedances, especially the "characteristic impedances" that can lead to reflections if they are not matched. 

The proposed formulas are known to have a satisfactory accuracy for planning and implementation studies. An acceptable level of error is introduced by the impedance matrix reduction discussed earlier. Owing to the matrix reduction, unbalanced sheath currents that flow into the earth at earthing joints are not considered in the proposed formulas. 

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