What does an impedance to a fixed potential mean?
It simply means that the impedance or resistance from each input terminal to the common point is equal.







simulate this circuit &ndash; Schematic created using CircuitLab

Figure 1 and 2.


Figure 1 shows a transformer input for a balanced line microphone circuit. Clearly the input side of the circuit is symmetrical and the centre-tap is connected to ground. The impedance seen by each line will be the same and it will present itself as a load connected to a fixed potential, 0 V.
Figure 2 is similar but this time there is a 48 V phantom power supply feeding down the two lines. Again the impedence matches but this time the reference is +48 V.


There are many more cases such as electronic differential amplifiers, etc. The main design intent is to present equal loading to both of the differential signals to prevent unbalancing the circuits which may cause a deterioration in noise and common-mode signal rejection.