When analyzing a fully differential circuit, since the two sides are completely symmetrical, we can analyze it as a half-side equivalent circuit.

Take the following differential pair as an example. The source terminal of the MOS transistor pair is connected to a resistor R and a capacitor C.

To separate the circuit into a half-side equivalent circuit, the components connected to the differential pair must be separated.

Currently, the differential pair consists of a resistor R and a capacitor C. We can see that the small-signal voltages across resistor R and capacitor C are opposite in magnitude.

If we view the resistor and capacitor as composed of many segments, there must be a point in the middle where the potential is zero, which is equivalent to ground.

Because the small-signal voltages on both sides are equal in magnitude and opposite in direction, the zero potential is the midpoint of the resistor and capacitor.

This means that resistor R is separated into two 0.5R resistors and capacitor C into two 2C capacitors.

This allows the capacitor and capacitor to be separated from the differential pair, forming the following half-side equivalent circuit.
 
Now we can analyze the circuit's performance using a half-edge equivalent circuit.